assignment statement math

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  • 1.1 Getting Started
  • 1.1.1 Preface
  • 1.1.2 About the AP CSA Exam
  • 1.1.3 Transitioning from AP CSP to AP CSA
  • 1.1.4 Java Development Environments
  • 1.1.5 Growth Mindset and Pair Programming
  • 1.1.6 Pretest for the AP CSA Exam
  • 1.1.7 Survey
  • 1.2 Why Programming? Why Java?
  • 1.3 Variables and Data Types
  • 1.4 Expressions and Assignment Statements
  • 1.5 Compound Assignment Operators
  • 1.6 Casting and Ranges of Values
  • 1.7 Unit 1 Summary
  • 1.8 Mixed Up Code Practice
  • 1.9 Toggle Mixed Up or Write Code Practice
  • 1.10 Coding Practice
  • 1.11 Multiple Choice Exercises
  • 1.3. Variables and Data Types" data-toggle="tooltip">
  • 1.5. Compound Assignment Operators' data-toggle="tooltip" >

Time estimate: 90 min.

1.4. Expressions and Assignment Statements ¶

In this lesson, you will learn about assignment statements and expressions that contain math operators and variables.

1.4.1. Assignment Statements ¶

Assignment statements initialize or change the value stored in a variable using the assignment operator = . An assignment statement always has a single variable on the left hand side. The value of the expression (which can contain math operators and other variables) on the right of the = sign is stored in the variable on the left.

../_images/assignment.png

Figure 1: Assignment Statement (variable = expression;) ¶

Instead of saying equals for the = in an assignment statement, say “gets” or “is assigned” to remember that the variable gets or is assigned the value on the right. In the figure above score is assigned the value of the expression 10 times points (which is another variable) plus 5.

The following video by Dr. Colleen Lewis shows how variables can change values in memory using assignment statements.

As we saw in the video, we can set one variable’s value to a copy of the value of another variable like y = x; . This won’t change the value of the variable that you are copying from.

Let’s step through the following code in the Java visualizer to see the values in memory. Click on the Next button at the bottom of the code to see how the values of the variables change. You can run the visualizer on any Active Code in this e-book by just clicking on the Code Lens button at the top of each Active Code.

Activity: CodeLens 1.4.1.2 (asgn_viz1)

exercise

1-4-3: What are the values of x, y, and z after the following code executes? You can step through this code by clicking on this Java visualizer link.

  • x = 0, y = 1, z = 2
  • These are the initial values in the variable, but the values are changed.
  • x = 1, y = 2, z = 3
  • x changes to y's initial value, y's value is doubled, and z is set to 3
  • x = 2, y = 2, z = 3
  • Remember that the equal sign doesn't mean that the two sides are equal. It sets the value for the variable on the left to the value from evaluating the right side.
  • x = 0, y = 0, z = 3

The following has the correct code to ‘swap’ the values in x and y (so that x ends up with y’s initial value and y ends up with x’s initial value), but the code is mixed up and contains one extra block which is not needed in a correct solution. Drag the needed blocks from the left into the correct order on the right. Check your solution by clicking on the Check button. You will be told if any of the blocks are in the wrong order or if you need to remove one or more blocks. After three incorrect attempts you will be able to use the Help Me button to make the problem easier.

1.4.2. Adding 1 to a Variable ¶

If you use a variable to keep score, you would probably increment it (add one to the current value) whenever score should go up. You can do this by setting the variable to the current value of the variable plus one ( score = score + 1 ) as shown below. The formula would look strange in math class, but it makes sense in coding because it is assigning a new value to the variable on the left that comes from evaluating the arithmetic expression on the right. So, the score variable is set to the previous value of score plus 1.

Try the code below to see how score is incremented by 1. Try substituting 2 instead of 1 to see what happens.

1.4.3. Input with Variables ¶

Variables are a powerful abstraction in programming because the same algorithm can be used with different input values saved in variables. The code below ( Java Scanner Input Repl using the Scanner class or Java Console Input Repl using the Console class) will say hello to anyone who types in their name for different name values. Click on run and then type in your name. Then, try run again and type in a friend’s name. The code works for any name: behold, the power of variables!

Although you will not be tested in the AP CSA exam on using the Java input or the Scanner or Console classes, learning how to do input in Java is very useful and fun. For more information on using the Scanner class, go to https://www.w3schools.com/java/java_user_input.asp , and for the newer Console class, https://howtodoinjava.com/java-examples/console-input-output/ .

1.4.4. Operators ¶

Java uses the standard mathematical operators for addition ( + ), subtraction ( - ), and division ( / ). The multiplication operator is written as * , as it is in most programming languages, since the character sets used until relatively recently didn’t have a character for a real multiplication sign, × , and keyboards still don’t have a key for it. Likewise no ÷ .

You may be used to using ^ for exponentiation, either from a graphing calculator or tools like Desmos. Confusingly ^ is an operator in Java, but it has a completely different meaning than exponentiation and isn’t even exactly an arithmetic operator. You will learn how to use the Math.pow method to do exponents in Unit 2.

Arithmetic expressions can be of type int or double . An arithmetic expression consisting only of int values will evaluate to an int value. An arithmetic expression that uses at least one double value will evaluate to a double value. (You may have noticed that + was also used to combine String and other values into new String s. More on this when we talk about String s more fully in Unit 2.)

Java uses the operator == to test if the value on the left is equal to the value on the right and != to test if two items are not equal. Don’t get one equal sign = confused with two equal signs == . They mean very different things in Java. One equal sign is used to assign a value to a variable. Two equal signs are used to test a variable to see if it is a certain value and that returns true or false as you’ll see below. Also note that using == and != with double values can produce surprising results. Because double values are only an approximation of the real numbers even things that should be mathematically equivalent might not be represented by the exactly same double value and thus will not be == . To see this for yourself, write a line of code below to print the value of the expression 0.3 == 0.1 + 0.2 ; it will be false !

coding exercise

Run the code below to see all the operators in action. Do all of those operators do what you expected? What about 2 / 3? Isn’t it surprising that it prints 0? See the note below.

When Java sees you doing integer division (or any operation with integers) it assumes you want an integer result so it throws away anything after the decimal point in the answer. This is called truncating division . If you need a double answer, you should make at least one of the values in the expression a double like 2.0.

With division, another thing to watch out for is dividing by 0. An attempt to divide an integer by zero will result in an ArithmeticException error message. Try it in one of the active code windows above.

Operators can be used to create compound expressions with more than one operator. You can either use a literal value which is a fixed value like 2, or variables in them. When compound expressions are evaluated, operator precedence rules are used, just like when we do math (remember PEMDAS?), so that * , / , and % are done before + and - . However, anything in parentheses is done first. It doesn’t hurt to put in extra parentheses if you are unsure as to what will be done first or just to make it more clear.

In the example below, try to guess what it will print out and then run it to see if you are right. Remember to consider operator precedence . How do the parentheses change the precedence?

1.4.5. The Remainder Operator ¶

The operator % in Java is the remainder operator. Like the other arithmetic operators is takes two operands. Mathematically it returns the remainder after dividing the first number by the second, using truncating integer division. For instance, 5 % 2 evaluates to 1 since 2 goes into 5 two times with a remainder of 1.

While you may not have heard of remainder as an operator, think back to elementary school math. Remember when you first learned long division, before they taught you about decimals, how when you did a long division that didn’t divide evenly, you gave the answer as the number of even divisions and the remainder. That remainder is what is returned by this operator. In the figures below, the remainders are the same values that would be returned by 2 % 3 and 5 % 2 .

../_images/mod-py.png

Figure 1: Long division showing the integer result and the remainder ¶

Sometimes people—including Professor Lewis in the next video—will call % the modulo , or mod , operator. That is not actually correct though the difference between remainder and modulo, which uses Euclidean division instead of truncating integer division, only matters when negative operands are involved and the signs of the operands differ. With positive operands, remainder and mod give the same results. Java does have a method Math.floorMod in the Math class if you need to use modulo instead of remainder, but % is all you need in the AP exam.

Here’s the video .

In the example below, try to guess what it will print out and then run it to see if you are right.

The result of x % y when x is smaller than y is always x. The value y can’t go into x at all (goes in 0 times), since x is smaller than y, so the result is just x. So if you see 2 % 3 the result is 2.

1-4-10: What is the result of 158 % 10?

  • This would be the result of 158 divided by 10. % gives you the remainder.
  • % gives you the remainder after the division.
  • When you divide 158 by 10 you get a remainder of 8.

1-4-11: What is the result of 3 % 8?

  • 8 goes into 3 no times so the remainder is 3. The remainder of a smaller number divided by a larger number is always the smaller number!
  • This would be the remainder if the question was 8 % 3 but here we are asking for the reminder after we divide 3 by 8.
  • What is the remainder after you divide 3 by 8?

1.4.6. Programming Challenge : Dog Years ¶

dog

In this programming challenge, you will calculate your age, and your pet’s age from your birthdates, and your pet’s age in dog years. In the code below, type in the current year, the year you were born, the year your dog or cat was born (if you don’t have one, make one up!) in the variables below. Then write formulas in assignment statements to calculate how old you are, how old your dog or cat is, and how old they are in dog years which is 7 times a human year. Finally, print it all out. If you are pair programming, switch drivers (who has control of the keyboard in pair programming) after every line of code.

Calculate your age and your pet’s age from the birthdates, and then your pet’s age in dog years.

Your teacher may suggest that you use a Java IDE like repl.it for this challenge so that you can use input to get these values using the Scanner class . Here is a repl template that you can use to get started if you want to try the challenge with input.

1.4.7. Summary ¶

Arithmetic expressions include expressions of type int and double .

The arithmetic operators consist of + , - , * , / , and % also known as addition, subtraction, multiplication, division, and remainder.

An arithmetic operation that uses two int values will evaluate to an int value. With integer division, any decimal part in the result will be thrown away.

An arithmetic operation that uses at least one double value will evaluate to a double value.

Operators can be used to construct compound expressions.

During evaluation, operands are associated with operators according to operator precedence to determine how they are grouped. ( * , / , % have precedence over + and - , unless parentheses are used to group those.)

An attempt to divide an integer by zero will result in an ArithmeticException .

The assignment operator ( = ) allows a program to initialize or change the value stored in a variable. The value of the expression on the right is stored in the variable on the left.

During execution, expressions are evaluated to produce a single value.

The value of an expression has a type based on the types of the values and operators used in the expression.

1.4.8. AP Practice ¶

The following is a 2019 AP CSA sample question.

1-4-13: Consider the following code segment.

What is printed when the code segment is executed?

  • 0.666666666666667
  • Don't forget that division and multiplication will be done first due to operator precedence.
  • Yes, this is equivalent to (5 + ((a/b)*c) - 1).
  • Don't forget that division and multiplication will be done first due to operator precedence, and that an int/int gives an int truncated result where everything to the right of the decimal point is dropped.

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2.3: Arithmetic Operations and Assignment Statements

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  • Page ID 206261

  • Robert Belford
  • University of Arkansas at Little Rock

hypothes.is tag:  s20iostpy03ualr Download Assignment:  S2020py03

Learning Objectives

Students will be able to:

  • Explain each Python arithmetic operator
  • Explain the meaning and use of an  assignment statement
  • Explain the use of "+"  and "*" with strings and numbers
  • Use the  int()   and  float()  functions to convert string input to numbers for computation
  • Incorporate numeric formatting into print statements
  • Recognize the four main operations of a computer within a simple Python program
  • Create  input  statements in Python
  • Create  Python  code that performs mathematical and string operations
  • Create  Python  code that uses assignment statements
  • Create  Python   code that formats numeric output

Prior Knowledge

  • Understanding of Python print and input statements
  • Understanding of mathematical operations
  • Understanding of flowchart input symbols

Further Reading

  • https://en.wikibooks.org/wiki/Non-Programmer%27s_Tutorial_for_Python_3/Hello,_World
  • https://en.wikibooks.org/wiki/Non-Programmer%27s_Tutorial_for_Python_3/Who_Goes_There%3F

Model 1: Arithmetic Operators in  Python

Python includes several arithmetic operators: addition, subtraction, multiplication, two types of division, exponentiation and  mod .

Critical Thinking Questions:

1.  Draw a line between each flowchart symbol and its corresponding line of Python code. Make note of any problems.

2. Execute the print statements in the previous Python program

    a.  Next to each print statement above, write the output.     b.  What is the value of the following line of code?

    c.  Predict the values of 17%3 and 18%3 without using your computer.

3.  Explain the purpose of each arithmetic operation:

a.               +          ____________________________

b.               -           ____________________________

c.               *          ____________________________

d.               **        ____________________________

e.               /           ____________________________

f.                //          ____________________________

g.                %         ____________________________

An  assignment statement  is a line of code that uses a "=" sign. The statement stores the result of an operation performed on the right-hand side of the sign into the variable memory location on the left-hand side.

4.         Enter and execute the following lines of Python code in the editor window of your IDE (e.g. Thonny):

 a.  What are the variables in the above python program?    b.  What does the  assignment statement :  MethaneMolMs = 16  do?    c.  What happens if you replace the comma (,) in the print statements with a plus sign (+) and execute the code again?  Why does this happen?

5.    What is stored in memory after each assignment statement is executed?

variable assignments

Note: Concatenating Strings in python

The "+"  concatenates  the two strings stored in the variables into one string.    "+" can only be used when both operators are strings.

6.         Run the following program in the editor window of your IDE (e.g. Thonny) to see what happens if you try to use the "+" with strings instead of numbers?

   a.  The third line of code contains an assignment statement. What is stored in  fullName   when the line is executed?    b.  What is the difference between the two output lines?    c.  How could you alter your assignment statements so that  print(fullName)  gives the same output as  print(firstName,lastName)    d. Only one of the following programs will work. Which one will work, and why doesn’t the other work? Try doing this without running the programs!

   e.  Run the programs above and see if you were correct.    f.  The program that worked above results in no space between the number and the street name. How can you alter the code so that it prints properly while using a concatenation operator?

7.  Before entering the following code into the Python interpreter (Thonny IDE editor window), predict the output of this program.

Now execute it.  What is the actual output?  Is this what you thought it would do?  Explain.

8.   Let’s take a look at a python program that prompts the user for two numbers and subtracts them. 

            Execute the following code by entering it in the editor window of Thonny.

      a.   What output do you expect?       b.   What is the actual output       c.   Revise the program in the following manner:

  • Between lines two and three add the following lines of code:       num1 = int(firstNumber)      num2 = int(secondNumber)
  • Next, replace the statement:     difference = firstNumber – secondNumber with the statement:     difference = num1 – num2
  • Execute the program again. What output did you get?

     d.  Explain the purpose of the function  int().      e.  Explain how the changes in the program produced the desired output.

Model 3: Formatting Output in  Python

There are multiple ways to format output in python. The old way is to use the string modulo %, and the new way is with a format method function.

9.  Look closely at the output for python program 7.

    a. How do you indicate the number of decimals to display using

the string modulo (%) ______________________________________________________

the format function ________________________________________________________

     b. What happens to the number if you tell it to display less decimals than are in the number, regardless of formatting method used?

     c. What type of code allows you to right justify your numbers?

10.       Execute the following code by entering it in the editor window of Thonny.

a.  Does the output look like standard output for something that has dollars and cents associated with it?

b.  Replace the last line of code with the following:

print("Total cost of laptops: $%.2f" % price)   

print("Total cost of laptops:" ,format(price, '.2f.))

                Discuss the change in the output.

      

c.  Replace the last line of code with the following:

print("Total cost of laptops: $",   format(price,'.2f') print("Total cost of laptops: $" ,format(price, '.2f.))

              Discuss the change in the output.

d.  Experiment with the number ".2" in the ‘0.2f’ of the print above statement by substituting the following numbers and explain the results.

                     .4         ___________________________________________________

                     .0         ___________________________________________________

                     .1         ___________________________________________________

                     .8         ___________________________________________________

e.  Now try the following numbers in the same print statement. These numbers contain a whole number and a decimal. Explain the output for each number.

                     02.5     ___________________________________________________

                     08.2     ___________________________________________________

                     03.1     ___________________________________________________

f.  Explain what each part of the format function:  format(variable,  "%n.nf")  does in a print statement where n.n represents a number.

variable ____________________________           First n _________________________

Second n_______________________                      f    _________________________

g.          Revise the print statement by changing the "f" to "d" and  laptopCost = 600 . Execute the statements and explain the output format.

            print("Total cost of laptops: %2d" % price)             print("Total cost of laptops: %10d" % price)

h.         Explain how the function  format(var,'10d')  formats numeric data.  var  represents a whole number.

11.    Use the following program and output to answer the questions below.

a.   From the code and comments in the previous program, explain how the four main operations are implemented in this program. b.  There is one new function in this sample program.  What is it? From the corresponding output, determine what it does.

Application Questions: Use the Python Interpreter to check your work

  • 8 to the 4 th  power
  • The sum of 5 and 6 multiplied by the quotient of 34 and 7 using floating point arithmetic  
  • Write an assignment statement that stores the remainder obtained from dividing 87 and 8 in the variable  leftover  
  • Assume:  

courseLabel = "CHEM" courseNumber = "3350"

Write a line of Python code that concatenates the label with the number and stores the result in the variable  courseName . Be sure that there is a space between the course label and the course number when they are concatenated.

  • Write one line of Python code that will print the word "Happy!" one hundred times.  
  • Write one line of code that calculates the cost of 15 items and stores the result in the variable  totalCost
  • Write one line of code that prints the total cost with a label, a dollar sign, and exactly two decimal places.  Sample output:  Total cost: $22.5  
  • Assume: 

height1 = 67850 height2 = 456

Use Python formatting to write two print statements that will produce the following output exactly at it appears below:

output

Homework Assignment: s2020py03

Download the assignment from the website, fill out the word document, and upload to your Google Drive folder the completed assignment along with the two python files.

1. (5 pts)  Write a Python program that prompts the user for two numbers, and then gives the sum and product of those two numbers. Your sample output should look like this:

Enter your first number:10 Enter your second number:2 The sum of these numbers is: 12 The product of these two numbers is: 20

  • Your program must contain documentation lines that include your name, the date, a line that states "Py03 Homework question 1" and a description line that indicates what the program is supposed to do. 
  • Paste the code this word document and upload to your Google drive when the assignment is completed, with file name [your last name]_py03_HWQ1
  • Save the program as a python file (ends with .py), with file name [your last name]_py03Q1_program and upload that to the Google Drive.

2. (10 pts) Write a program that calculates the molarity of a solution. Molarity is defined as numbers of moles per liter solvent. Your program will calculate molarity and must ask for the substance name, its molecular weight, how many grams of substance you are putting in solution, and the total volume of the solution. Report your calculated value of molarity to 3 decimal places. Your output should also be separated from the input with a line containing 80 asterixis.

Assuming you are using sodium chloride, your input and output should look like:

clipboard_edfaec3a5372d389c1f48c61ebe904909.png

  • Your program must contain documentation lines that include your name, the date, a line that states "Py03 Homework question 2" and a description line that indicates what the program is supposed to do. 
  • Paste the code to question two below
  • Save the program as a python file (ends with .py), with file name [your last name]_py03Q2_program and upload that to the Google Drive.

3. (4 pts) Make two hypothes.is annotations dealing with external open access resources on formatting with the format function method of formatting.  These need the tag of s20iostpy03ualr .

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4 Variables, expressions and statements

2.2  variable names.

Programmers generally choose names for their variables that are meaningful—they document what the variable is used for.

Variable names can be as long as you like. They can contain both letters and numbers, but they can’t begin with a number. It is legal to use uppercase letters, but it is conventional to use only lower case for variables names.

The underscore character, _ , can appear in a name. It is often used in names with multiple words, such as your_name or airspeed_of_unladen_swallow .

If you give a variable an illegal name, you get a syntax error:

76trombones is illegal because it begins with a number. more@ is illegal because it contains an illegal character, @ . But what’s wrong with class ?

It turns out that class is one of Python’s keywords . The interpreter uses keywords to recognize the structure of the program, and they cannot be used as variable names.

Python 3 has these keywords:

You don’t have to memorize this list. In most development environments, keywords are displayed in a different color; if you try to use one as a variable name, you’ll know.

2.3  Expressions and statements

An expression is a combination of values, variables, and operators. A value all by itself is considered an expression, and so is a variable, so the following are all legal expressions:

When you type an expression at the prompt, the interpreter evaluates it, which means that it finds the value of the expression. In this example, n has the value 17 and n + 25 has the value 42.

A statement is a unit of code that has an effect, like creating a variable or displaying a value.

The first line is an assignment statement that gives a value to n . The second line is a print statement that displays the value of n .

When you type a statement, the interpreter executes it, which means that it does whatever the statement says. In general, statements don’t have values.

2.4  Script mode

So far we have run Python in interactive mode , which means that you interact directly with the interpreter. Interactive mode is a good way to get started, but if you are working with more than a few lines of code, it can be clumsy.

The alternative is to save code in a file called a script and then run the interpreter in script mode to execute the script. By convention, Python scripts have names that end with .py .

If you know how to create and run a script on your computer, you are ready to go. Otherwise I recommend using PythonAnywhere again. I have posted instructions for running in script mode at http://tinyurl.com/thinkpython2e .

Because Python provides both modes, you can test bits of code in interactive mode before you put them in a script. But there are differences between interactive mode and script mode that can be confusing.

For example, if you are using Python as a calculator, you might type

The first line assigns a value to miles , but it has no visible effect. The second line is an expression, so the interpreter evaluates it and displays the result. It turns out that a marathon is about 42 kilometers.

But if you type the same code into a script and run it, you get no output at all. In script mode an expression, all by itself, has no visible effect. Python evaluates the expression, but it doesn’t display the result. To display the result, you need a print statement like this:

This behavior can be confusing at first. To check your understanding, type the following statements in the Python interpreter and see what they do:

Now put the same statements in a script and run it. What is the output? Modify the script by transforming each expression into a print statement and then run it again.

2.5  Order of operations

When an expression contains more than one operator, the order of evaluation depends on the order of operations . For mathematical operators, Python follows mathematical convention. The acronym PEMDAS is a useful way to remember the rules:

  • P arentheses have the highest precedence and can be used to force an expression to evaluate in the order you want. Since expressions in parentheses are evaluated first, 2 * (3-1) is 4, and (1+1)**(5-2) is 8. You can also use parentheses to make an expression easier to read, as in (minute * 100) / 60 , even if it doesn’t change the result.
  • E xponentiation has the next highest precedence, so 1 + 2**3 is 9, not 27, and 2 * 3**2 is 18, not 36.
  • M ultiplication and D ivision have higher precedence than A ddition and S ubtraction. So 2*3-1 is 5, not 4, and 6+4/2 is 8, not 5.
  • Operators with the same precedence are evaluated from left to right (except exponentiation). So in the expression degrees / 2 * pi , the division happens first and the result is multiplied by pi . To divide by 2 π, you can use parentheses or write degrees / 2 / pi .

I don’t work very hard to remember the precedence of operators. If I can’t tell by looking at the expression, I use parentheses to make it obvious.

2.6  String operations

In general, you can’t perform mathematical operations on strings, even if the strings look like numbers, so the following are illegal:

But there are two exceptions, + and * .

The + operator performs string concatenation , which means it joins the strings by linking them end-to-end. For example:

The * operator also works on strings; it performs repetition. For example, 'Spam'*3 is 'SpamSpamSpam' . If one of the values is a string, the other has to be an integer.

This use of + and * makes sense by analogy with addition and multiplication. Just as 4*3 is equivalent to 4+4+4 , we expect 'Spam'*3 to be the same as 'Spam'+'Spam'+'Spam' , and it is. On the other hand, there is a significant way in which string concatenation and repetition are different from integer addition and multiplication. Can you think of a property that addition has that string concatenation does not?

2.7  Comments

As programs get bigger and more complicated, they get more difficult to read. Formal languages are dense, and it is often difficult to look at a piece of code and figure out what it is doing, or why.

For this reason, it is a good idea to add notes to your programs to explain in natural language what the program is doing. These notes are called comments , and they start with the # symbol:

In this case, the comment appears on a line by itself. You can also put comments at the end of a line:

Everything from the # to the end of the line is ignored—it has no effect on the execution of the program.

Comments are most useful when they document non-obvious features of the code. It is reasonable to assume that the reader can figure out what the code does; it is more useful to explain why .

This comment is redundant with the code and useless:

This comment contains useful information that is not in the code:

Good variable names can reduce the need for comments, but long names can make complex expressions hard to read, so there is a tradeoff.

2.8  Debugging

Three kinds of errors can occur in a program: syntax errors, runtime errors, and semantic errors. It is useful to distinguish between them in order to track them down more quickly.

2.9  Glossary

2.10  exercises.

Repeating my advice from the previous chapter, whenever you learn a new feature, you should try it out in interactive mode and make errors on purpose to see what goes wrong.

  • We’ve seen that n = 42 is legal. What about 42 = n ?
  • How about x = y = 1 ?
  • In some languages every statement ends with a semi-colon, ; . What happens if you put a semi-colon at the end of a Python statement?
  • What if you put a period at the end of a statement?
  • In math notation you can multiply x and y like this: x y . What happens if you try that in Python?

Practice using the Python interpreter as a calculator:

  • The volume of a sphere with radius r is 4/3 π r 3 . What is the volume of a sphere with radius 5?
  • Suppose the cover price of a book is $24.95, but bookstores get a 40% discount. Shipping costs $3 for the first copy and 75 cents for each additional copy. What is the total wholesale cost for 60 copies?
  • If I leave my house at 6:52 am and run 1 mile at an easy pace (8:15 per mile), then 3 miles at tempo (7:12 per mile) and 1 mile at easy pace again, what time do I get home for breakfast?

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Python Numerical Methods

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< 2.0 Variables and Basic Data Structures | Contents | 2.2 Data Structure - Strings >

Variables and Assignment ¶

When programming, it is useful to be able to store information in variables. A variable is a string of characters and numbers associated with a piece of information. The assignment operator , denoted by the “=” symbol, is the operator that is used to assign values to variables in Python. The line x=1 takes the known value, 1, and assigns that value to the variable with name “x”. After executing this line, this number will be stored into this variable. Until the value is changed or the variable deleted, the character x behaves like the value 1.

TRY IT! Assign the value 2 to the variable y. Multiply y by 3 to show that it behaves like the value 2.

A variable is more like a container to store the data in the computer’s memory, the name of the variable tells the computer where to find this value in the memory. For now, it is sufficient to know that the notebook has its own memory space to store all the variables in the notebook. As a result of the previous example, you will see the variable “x” and “y” in the memory. You can view a list of all the variables in the notebook using the magic command %whos .

TRY IT! List all the variables in this notebook

Note that the equal sign in programming is not the same as a truth statement in mathematics. In math, the statement x = 2 declares the universal truth within the given framework, x is 2 . In programming, the statement x=2 means a known value is being associated with a variable name, store 2 in x. Although it is perfectly valid to say 1 = x in mathematics, assignments in Python always go left : meaning the value to the right of the equal sign is assigned to the variable on the left of the equal sign. Therefore, 1=x will generate an error in Python. The assignment operator is always last in the order of operations relative to mathematical, logical, and comparison operators.

TRY IT! The mathematical statement x=x+1 has no solution for any value of x . In programming, if we initialize the value of x to be 1, then the statement makes perfect sense. It means, “Add x and 1, which is 2, then assign that value to the variable x”. Note that this operation overwrites the previous value stored in x .

There are some restrictions on the names variables can take. Variables can only contain alphanumeric characters (letters and numbers) as well as underscores. However, the first character of a variable name must be a letter or underscores. Spaces within a variable name are not permitted, and the variable names are case-sensitive (e.g., x and X will be considered different variables).

TIP! Unlike in pure mathematics, variables in programming almost always represent something tangible. It may be the distance between two points in space or the number of rabbits in a population. Therefore, as your code becomes increasingly complicated, it is very important that your variables carry a name that can easily be associated with what they represent. For example, the distance between two points in space is better represented by the variable dist than x , and the number of rabbits in a population is better represented by nRabbits than y .

Note that when a variable is assigned, it has no memory of how it was assigned. That is, if the value of a variable, y , is constructed from other variables, like x , reassigning the value of x will not change the value of y .

EXAMPLE: What value will y have after the following lines of code are executed?

WARNING! You can overwrite variables or functions that have been stored in Python. For example, the command help = 2 will store the value 2 in the variable with name help . After this assignment help will behave like the value 2 instead of the function help . Therefore, you should always be careful not to give your variables the same name as built-in functions or values.

TIP! Now that you know how to assign variables, it is important that you learn to never leave unassigned commands. An unassigned command is an operation that has a result, but that result is not assigned to a variable. For example, you should never use 2+2 . You should instead assign it to some variable x=2+2 . This allows you to “hold on” to the results of previous commands and will make your interaction with Python must less confusing.

You can clear a variable from the notebook using the del function. Typing del x will clear the variable x from the workspace. If you want to remove all the variables in the notebook, you can use the magic command %reset .

In mathematics, variables are usually associated with unknown numbers; in programming, variables are associated with a value of a certain type. There are many data types that can be assigned to variables. A data type is a classification of the type of information that is being stored in a variable. The basic data types that you will utilize throughout this book are boolean, int, float, string, list, tuple, dictionary, set. A formal description of these data types is given in the following sections.

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Learning Python by doing

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Variables, Expressions, and Assignments

Variables, expressions, and assignments 1 #, introduction #.

In this chapter, we introduce some of the main building blocks needed to create programs–that is, variables, expressions, and assignments. Programming related variables can be intepret in the same way that we interpret mathematical variables, as elements that store values that can later be changed. Usually, variables and values are used within the so-called expressions. Once again, just as in mathematics, an expression is a construct of values and variables connected with operators that result in a new value. Lastly, an assignment is a language construct know as an statement that assign a value (either as a constant or expression) to a variable. The rest of this notebook will dive into the main concepts that we need to fully understand these three language constructs.

Values and Types #

A value is the basic unit used in a program. It may be, for instance, a number respresenting temperature. It may be a string representing a word. Some values are 42, 42.0, and ‘Hello, Data Scientists!’.

Each value has its own type : 42 is an integer ( int in Python), 42.0 is a floating-point number ( float in Python), and ‘Hello, Data Scientists!’ is a string ( str in Python).

The Python interpreter can tell you the type of a value: the function type takes a value as argument and returns its corresponding type.

Observe the difference between type(42) and type('42') !

Expressions and Statements #

On the one hand, an expression is a combination of values, variables, and operators.

A value all by itself is considered an expression, and so is a variable.

When you type an expression at the prompt, the interpreter evaluates it, which means that it calculates the value of the expression and displays it.

In boxes above, m has the value 27 and m + 25 has the value 52 . m + 25 is said to be an expression.

On the other hand, a statement is an instruction that has an effect, like creating a variable or displaying a value.

The first statement initializes the variable n with the value 17 , this is a so-called assignment statement .

The second statement is a print statement that prints the value of the variable n .

The effect is not always visible. Assigning a value to a variable is not visible, but printing the value of a variable is.

Assignment Statements #

We have already seen that Python allows you to evaluate expressions, for instance 40 + 2 . It is very convenient if we are able to store the calculated value in some variable for future use. The latter can be done via an assignment statement. An assignment statement creates a new variable with a given name and assigns it a value.

The example in the previous code contains three assignments. The first one assigns the value of the expression 40 + 2 to a new variable called magicnumber ; the second one assigns the value of π to the variable pi , and; the last assignment assigns the string value 'Data is eatig the world' to the variable message .

Programmers generally choose names for their variables that are meaningful. In this way, they document what the variable is used for.

Do It Yourself!

Let’s compute the volume of a cube with side \(s = 5\) . Remember that the volume of a cube is defined as \(v = s^3\) . Assign the value to a variable called volume .

Well done! Now, why don’t you print the result in a message? It can say something like “The volume of the cube with side 5 is \(volume\) ”.

Beware that there is no checking of types ( type checking ) in Python, so a variable to which you have assigned an integer may be re-used as a float, even if we provide type-hints .

Names and Keywords #

Names of variable and other language constructs such as functions (we will cover this topic later), should be meaningful and reflect the purpose of the construct.

In general, Python names should adhere to the following rules:

It should start with a letter or underscore.

It cannot start with a number.

It must only contain alpha-numeric (i.e., letters a-z A-Z and digits 0-9) characters and underscores.

They cannot share the name of a Python keyword.

If you use illegal variable names you will get a syntax error.

By choosing the right variables names you make the code self-documenting, what is better the variable v or velocity ?

The following are examples of invalid variable names.

These basic development principles are sometimes called architectural rules . By defining and agreeing upon architectural rules you make it easier for you and your fellow developers to understand and modify your code.

If you want to read more on this, please have a look at Code complete a book by Steven McConnell [ McC04 ] .

Every programming language has a collection of reserved keywords . They are used in predefined language constructs, such as loops and conditionals . These language concepts and their usage will be explained later.

The interpreter uses keywords to recognize these language constructs in a program. Python 3 has the following keywords:

False class finally is return

None continue for lambda try

True def from nonlocal while

and del global not with

as elif if or yield

assert else import pass break

except in raise

Reassignments #

It is allowed to assign a new value to an existing variable. This process is called reassignment . As soon as you assign a value to a variable, the old value is lost.

The assignment of a variable to another variable, for instance b = a does not imply that if a is reassigned then b changes as well.

You have a variable salary that shows the weekly salary of an employee. However, you want to compute the monthly salary. Can you reassign the value to the salary variable according to the instruction?

Updating Variables #

A frequently used reassignment is for updating puposes: the value of a variable depends on the previous value of the variable.

This statement expresses “get the current value of x , add one, and then update x with the new value.”

Beware, that the variable should be initialized first, usually with a simple assignment.

Do you remember the salary excercise of the previous section (cf. 13. Reassignments)? Well, if you have not done it yet, update the salary variable by using its previous value.

Updating a variable by adding 1 is called an increment ; subtracting 1 is called a decrement . A shorthand way of doing is using += and -= , which stands for x = x + ... and x = x - ... respectively.

Order of Operations #

Expressions may contain multiple operators. The order of evaluation depends on the priorities of the operators also known as rules of precedence .

For mathematical operators, Python follows mathematical convention. The acronym PEMDAS is a useful way to remember the rules:

Parentheses have the highest precedence and can be used to force an expression to evaluate in the order you want. Since expressions in parentheses are evaluated first, 2 * (3 - 1) is 4 , and (1 + 1)**(5 - 2) is 8 . You can also use parentheses to make an expression easier to read, even if it does not change the result.

Exponentiation has the next highest precedence, so 1 + 2**3 is 9 , not 27 , and 2 * 3**2 is 18 , not 36 .

Multiplication and division have higher precedence than addition and subtraction . So 2 * 3 - 1 is 5 , not 4 , and 6 + 4 / 2 is 8 , not 5 .

Operators with the same precedence are evaluated from left to right (except exponentiation). So in the expression degrees / 2 * pi , the division happens first and the result is multiplied by pi . To divide by 2π, you can use parentheses or write: degrees / 2 / pi .

In case of doubt, use parentheses!

Let’s see what happens when we evaluate the following expressions. Just run the cell to check the resulting value.

Floor Division and Modulus Operators #

The floor division operator // divides two numbers and rounds down to an integer.

For example, suppose that driving to the south of France takes 555 minutes. You might want to know how long that is in hours.

Conventional division returns a floating-point number.

Hours are normally not represented with decimal points. Floor division returns the integer number of hours, dropping the fraction part.

You spend around 225 minutes every week on programming activities. You want to know around how many hours you invest to this activity during a month. Use the \(//\) operator to give the answer.

The modulus operator % works on integer values. It computes the remainder when dividing the first integer by the second one.

The modulus operator is more useful than it seems.

For example, you can check whether one number is divisible by another—if x % y is zero, then x is divisible by y .

String Operations #

In general, you cannot perform mathematical operations on strings, even if the strings look like numbers, so the following operations are illegal: '2'-'1' 'eggs'/'easy' 'third'*'a charm'

But there are two exceptions, + and * .

The + operator performs string concatenation, which means it joins the strings by linking them end-to-end.

The * operator also works on strings; it performs repetition.

Speedy Gonzales is a cartoon known to be the fastest mouse in all Mexico . He is also famous for saying “Arriba Arriba Andale Arriba Arriba Yepa”. Can you use the following variables, namely arriba , andale and yepa to print the mentioned expression? Don’t forget to use the string operators.

Asking the User for Input #

The programs we have written so far accept no input from the user.

To get data from the user through the Python prompt, we can use the built-in function input .

When input is called your whole program stops and waits for the user to enter the required data. Once the user types the value and presses Return or Enter , the function returns the input value as a string and the program continues with its execution.

Try it out!

You can also print a message to clarify the purpose of the required input as follows.

The resulting string can later be translated to a different type, like an integer or a float. To do so, you use the functions int and float , respectively. But be careful, the user might introduce a value that cannot be converted to the type you required.

We want to know the name of a user so we can display a welcome message in our program. The message should say something like “Hello \(name\) , welcome to our hello world program!”.

Script Mode #

So far we have run Python in interactive mode in these Jupyter notebooks, which means that you interact directly with the interpreter in the code cells . The interactive mode is a good way to get started, but if you are working with more than a few lines of code, it can be clumsy. The alternative is to save code in a file called a script and then run the interpreter in script mode to execute the script. By convention, Python scripts have names that end with .py .

Use the PyCharm icon in Anaconda Navigator to create and execute stand-alone Python scripts. Later in the course, you will have to work with Python projects for the assignments, in order to get acquainted with another way of interacing with Python code.

This Jupyter Notebook is based on Chapter 2 of the books Python for Everybody [ Sev16 ] and Think Python (Sections 5.1, 7.1, 7.2, and 5.12) [ Dow15 ] .

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Learn to Program with Python 3 pp 11–49 Cite as

Variables and Assignment Statements

  • Irv Kalb 2  
  • First Online: 23 August 2018

5949 Accesses

This chapter covers the following topics:

A sample Python program

Building blocks of programming

Four types of data

What a variable is

Rules for naming variables

Giving a variable a value with an assignment statement

A good way to name variables

Special Python keywords

Case sensitivity

More complicated assignment statements

Print statements

Basic math operators

Order of operations and parentheses

A few small sample programs

Additional naming conventions

How to add comments in a program

Use of “whitespace”

Errors in programs

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7. Simple statements ¶

A simple statement is comprised within a single logical line. Several simple statements may occur on a single line separated by semicolons. The syntax for simple statements is:

7.1. Expression statements ¶

Expression statements are used (mostly interactively) to compute and write a value, or (usually) to call a procedure (a function that returns no meaningful result; in Python, procedures return the value None ). Other uses of expression statements are allowed and occasionally useful. The syntax for an expression statement is:

An expression statement evaluates the expression list (which may be a single expression).

In interactive mode, if the value is not None , it is converted to a string using the built-in repr() function and the resulting string is written to standard output on a line by itself (except if the result is None , so that procedure calls do not cause any output.)

7.2. Assignment statements ¶

Assignment statements are used to (re)bind names to values and to modify attributes or items of mutable objects:

(See section Primaries for the syntax definitions for attributeref , subscription , and slicing .)

An assignment statement evaluates the expression list (remember that this can be a single expression or a comma-separated list, the latter yielding a tuple) and assigns the single resulting object to each of the target lists, from left to right.

Assignment is defined recursively depending on the form of the target (list). When a target is part of a mutable object (an attribute reference, subscription or slicing), the mutable object must ultimately perform the assignment and decide about its validity, and may raise an exception if the assignment is unacceptable. The rules observed by various types and the exceptions raised are given with the definition of the object types (see section The standard type hierarchy ).

Assignment of an object to a target list, optionally enclosed in parentheses or square brackets, is recursively defined as follows.

If the target list is a single target with no trailing comma, optionally in parentheses, the object is assigned to that target.

If the target list contains one target prefixed with an asterisk, called a “starred” target: The object must be an iterable with at least as many items as there are targets in the target list, minus one. The first items of the iterable are assigned, from left to right, to the targets before the starred target. The final items of the iterable are assigned to the targets after the starred target. A list of the remaining items in the iterable is then assigned to the starred target (the list can be empty).

Else: The object must be an iterable with the same number of items as there are targets in the target list, and the items are assigned, from left to right, to the corresponding targets.

Assignment of an object to a single target is recursively defined as follows.

If the target is an identifier (name):

If the name does not occur in a global or nonlocal statement in the current code block: the name is bound to the object in the current local namespace.

Otherwise: the name is bound to the object in the global namespace or the outer namespace determined by nonlocal , respectively.

The name is rebound if it was already bound. This may cause the reference count for the object previously bound to the name to reach zero, causing the object to be deallocated and its destructor (if it has one) to be called.

If the target is an attribute reference: The primary expression in the reference is evaluated. It should yield an object with assignable attributes; if this is not the case, TypeError is raised. That object is then asked to assign the assigned object to the given attribute; if it cannot perform the assignment, it raises an exception (usually but not necessarily AttributeError ).

Note: If the object is a class instance and the attribute reference occurs on both sides of the assignment operator, the right-hand side expression, a.x can access either an instance attribute or (if no instance attribute exists) a class attribute. The left-hand side target a.x is always set as an instance attribute, creating it if necessary. Thus, the two occurrences of a.x do not necessarily refer to the same attribute: if the right-hand side expression refers to a class attribute, the left-hand side creates a new instance attribute as the target of the assignment:

This description does not necessarily apply to descriptor attributes, such as properties created with property() .

If the target is a subscription: The primary expression in the reference is evaluated. It should yield either a mutable sequence object (such as a list) or a mapping object (such as a dictionary). Next, the subscript expression is evaluated.

If the primary is a mutable sequence object (such as a list), the subscript must yield an integer. If it is negative, the sequence’s length is added to it. The resulting value must be a nonnegative integer less than the sequence’s length, and the sequence is asked to assign the assigned object to its item with that index. If the index is out of range, IndexError is raised (assignment to a subscripted sequence cannot add new items to a list).

If the primary is a mapping object (such as a dictionary), the subscript must have a type compatible with the mapping’s key type, and the mapping is then asked to create a key/value pair which maps the subscript to the assigned object. This can either replace an existing key/value pair with the same key value, or insert a new key/value pair (if no key with the same value existed).

For user-defined objects, the __setitem__() method is called with appropriate arguments.

If the target is a slicing: The primary expression in the reference is evaluated. It should yield a mutable sequence object (such as a list). The assigned object should be a sequence object of the same type. Next, the lower and upper bound expressions are evaluated, insofar they are present; defaults are zero and the sequence’s length. The bounds should evaluate to integers. If either bound is negative, the sequence’s length is added to it. The resulting bounds are clipped to lie between zero and the sequence’s length, inclusive. Finally, the sequence object is asked to replace the slice with the items of the assigned sequence. The length of the slice may be different from the length of the assigned sequence, thus changing the length of the target sequence, if the target sequence allows it.

CPython implementation detail: In the current implementation, the syntax for targets is taken to be the same as for expressions, and invalid syntax is rejected during the code generation phase, causing less detailed error messages.

Although the definition of assignment implies that overlaps between the left-hand side and the right-hand side are ‘simultaneous’ (for example a, b = b, a swaps two variables), overlaps within the collection of assigned-to variables occur left-to-right, sometimes resulting in confusion. For instance, the following program prints [0, 2] :

The specification for the *target feature.

7.2.1. Augmented assignment statements ¶

Augmented assignment is the combination, in a single statement, of a binary operation and an assignment statement:

(See section Primaries for the syntax definitions of the last three symbols.)

An augmented assignment evaluates the target (which, unlike normal assignment statements, cannot be an unpacking) and the expression list, performs the binary operation specific to the type of assignment on the two operands, and assigns the result to the original target. The target is only evaluated once.

An augmented assignment expression like x += 1 can be rewritten as x = x + 1 to achieve a similar, but not exactly equal effect. In the augmented version, x is only evaluated once. Also, when possible, the actual operation is performed in-place , meaning that rather than creating a new object and assigning that to the target, the old object is modified instead.

Unlike normal assignments, augmented assignments evaluate the left-hand side before evaluating the right-hand side. For example, a[i] += f(x) first looks-up a[i] , then it evaluates f(x) and performs the addition, and lastly, it writes the result back to a[i] .

With the exception of assigning to tuples and multiple targets in a single statement, the assignment done by augmented assignment statements is handled the same way as normal assignments. Similarly, with the exception of the possible in-place behavior, the binary operation performed by augmented assignment is the same as the normal binary operations.

For targets which are attribute references, the same caveat about class and instance attributes applies as for regular assignments.

7.2.2. Annotated assignment statements ¶

Annotation assignment is the combination, in a single statement, of a variable or attribute annotation and an optional assignment statement:

The difference from normal Assignment statements is that only a single target is allowed.

For simple names as assignment targets, if in class or module scope, the annotations are evaluated and stored in a special class or module attribute __annotations__ that is a dictionary mapping from variable names (mangled if private) to evaluated annotations. This attribute is writable and is automatically created at the start of class or module body execution, if annotations are found statically.

For expressions as assignment targets, the annotations are evaluated if in class or module scope, but not stored.

If a name is annotated in a function scope, then this name is local for that scope. Annotations are never evaluated and stored in function scopes.

If the right hand side is present, an annotated assignment performs the actual assignment before evaluating annotations (where applicable). If the right hand side is not present for an expression target, then the interpreter evaluates the target except for the last __setitem__() or __setattr__() call.

The proposal that added syntax for annotating the types of variables (including class variables and instance variables), instead of expressing them through comments.

The proposal that added the typing module to provide a standard syntax for type annotations that can be used in static analysis tools and IDEs.

Changed in version 3.8: Now annotated assignments allow the same expressions in the right hand side as regular assignments. Previously, some expressions (like un-parenthesized tuple expressions) caused a syntax error.

7.3. The assert statement ¶

Assert statements are a convenient way to insert debugging assertions into a program:

The simple form, assert expression , is equivalent to

The extended form, assert expression1, expression2 , is equivalent to

These equivalences assume that __debug__ and AssertionError refer to the built-in variables with those names. In the current implementation, the built-in variable __debug__ is True under normal circumstances, False when optimization is requested (command line option -O ). The current code generator emits no code for an assert statement when optimization is requested at compile time. Note that it is unnecessary to include the source code for the expression that failed in the error message; it will be displayed as part of the stack trace.

Assignments to __debug__ are illegal. The value for the built-in variable is determined when the interpreter starts.

7.4. The pass statement ¶

pass is a null operation — when it is executed, nothing happens. It is useful as a placeholder when a statement is required syntactically, but no code needs to be executed, for example:

7.5. The del statement ¶

Deletion is recursively defined very similar to the way assignment is defined. Rather than spelling it out in full details, here are some hints.

Deletion of a target list recursively deletes each target, from left to right.

Deletion of a name removes the binding of that name from the local or global namespace, depending on whether the name occurs in a global statement in the same code block. If the name is unbound, a NameError exception will be raised.

Deletion of attribute references, subscriptions and slicings is passed to the primary object involved; deletion of a slicing is in general equivalent to assignment of an empty slice of the right type (but even this is determined by the sliced object).

Changed in version 3.2: Previously it was illegal to delete a name from the local namespace if it occurs as a free variable in a nested block.

7.6. The return statement ¶

return may only occur syntactically nested in a function definition, not within a nested class definition.

If an expression list is present, it is evaluated, else None is substituted.

return leaves the current function call with the expression list (or None ) as return value.

When return passes control out of a try statement with a finally clause, that finally clause is executed before really leaving the function.

In a generator function, the return statement indicates that the generator is done and will cause StopIteration to be raised. The returned value (if any) is used as an argument to construct StopIteration and becomes the StopIteration.value attribute.

In an asynchronous generator function, an empty return statement indicates that the asynchronous generator is done and will cause StopAsyncIteration to be raised. A non-empty return statement is a syntax error in an asynchronous generator function.

7.7. The yield statement ¶

A yield statement is semantically equivalent to a yield expression . The yield statement can be used to omit the parentheses that would otherwise be required in the equivalent yield expression statement. For example, the yield statements

are equivalent to the yield expression statements

Yield expressions and statements are only used when defining a generator function, and are only used in the body of the generator function. Using yield in a function definition is sufficient to cause that definition to create a generator function instead of a normal function.

For full details of yield semantics, refer to the Yield expressions section.

7.8. The raise statement ¶

If no expressions are present, raise re-raises the exception that is currently being handled, which is also known as the active exception . If there isn’t currently an active exception, a RuntimeError exception is raised indicating that this is an error.

Otherwise, raise evaluates the first expression as the exception object. It must be either a subclass or an instance of BaseException . If it is a class, the exception instance will be obtained when needed by instantiating the class with no arguments.

The type of the exception is the exception instance’s class, the value is the instance itself.

A traceback object is normally created automatically when an exception is raised and attached to it as the __traceback__ attribute. You can create an exception and set your own traceback in one step using the with_traceback() exception method (which returns the same exception instance, with its traceback set to its argument), like so:

The from clause is used for exception chaining: if given, the second expression must be another exception class or instance. If the second expression is an exception instance, it will be attached to the raised exception as the __cause__ attribute (which is writable). If the expression is an exception class, the class will be instantiated and the resulting exception instance will be attached to the raised exception as the __cause__ attribute. If the raised exception is not handled, both exceptions will be printed:

A similar mechanism works implicitly if a new exception is raised when an exception is already being handled. An exception may be handled when an except or finally clause, or a with statement, is used. The previous exception is then attached as the new exception’s __context__ attribute:

Exception chaining can be explicitly suppressed by specifying None in the from clause:

Additional information on exceptions can be found in section Exceptions , and information about handling exceptions is in section The try statement .

Changed in version 3.3: None is now permitted as Y in raise X from Y .

New in version 3.3: The __suppress_context__ attribute to suppress automatic display of the exception context.

Changed in version 3.11: If the traceback of the active exception is modified in an except clause, a subsequent raise statement re-raises the exception with the modified traceback. Previously, the exception was re-raised with the traceback it had when it was caught.

7.9. The break statement ¶

break may only occur syntactically nested in a for or while loop, but not nested in a function or class definition within that loop.

It terminates the nearest enclosing loop, skipping the optional else clause if the loop has one.

If a for loop is terminated by break , the loop control target keeps its current value.

When break passes control out of a try statement with a finally clause, that finally clause is executed before really leaving the loop.

7.10. The continue statement ¶

continue may only occur syntactically nested in a for or while loop, but not nested in a function or class definition within that loop. It continues with the next cycle of the nearest enclosing loop.

When continue passes control out of a try statement with a finally clause, that finally clause is executed before really starting the next loop cycle.

7.11. The import statement ¶

The basic import statement (no from clause) is executed in two steps:

find a module, loading and initializing it if necessary

define a name or names in the local namespace for the scope where the import statement occurs.

When the statement contains multiple clauses (separated by commas) the two steps are carried out separately for each clause, just as though the clauses had been separated out into individual import statements.

The details of the first step, finding and loading modules, are described in greater detail in the section on the import system , which also describes the various types of packages and modules that can be imported, as well as all the hooks that can be used to customize the import system. Note that failures in this step may indicate either that the module could not be located, or that an error occurred while initializing the module, which includes execution of the module’s code.

If the requested module is retrieved successfully, it will be made available in the local namespace in one of three ways:

If the module name is followed by as , then the name following as is bound directly to the imported module.

If no other name is specified, and the module being imported is a top level module, the module’s name is bound in the local namespace as a reference to the imported module

If the module being imported is not a top level module, then the name of the top level package that contains the module is bound in the local namespace as a reference to the top level package. The imported module must be accessed using its full qualified name rather than directly

The from form uses a slightly more complex process:

find the module specified in the from clause, loading and initializing it if necessary;

for each of the identifiers specified in the import clauses:

check if the imported module has an attribute by that name

if not, attempt to import a submodule with that name and then check the imported module again for that attribute

if the attribute is not found, ImportError is raised.

otherwise, a reference to that value is stored in the local namespace, using the name in the as clause if it is present, otherwise using the attribute name

If the list of identifiers is replaced by a star ( '*' ), all public names defined in the module are bound in the local namespace for the scope where the import statement occurs.

The public names defined by a module are determined by checking the module’s namespace for a variable named __all__ ; if defined, it must be a sequence of strings which are names defined or imported by that module. The names given in __all__ are all considered public and are required to exist. If __all__ is not defined, the set of public names includes all names found in the module’s namespace which do not begin with an underscore character ( '_' ). __all__ should contain the entire public API. It is intended to avoid accidentally exporting items that are not part of the API (such as library modules which were imported and used within the module).

The wild card form of import — from module import * — is only allowed at the module level. Attempting to use it in class or function definitions will raise a SyntaxError .

When specifying what module to import you do not have to specify the absolute name of the module. When a module or package is contained within another package it is possible to make a relative import within the same top package without having to mention the package name. By using leading dots in the specified module or package after from you can specify how high to traverse up the current package hierarchy without specifying exact names. One leading dot means the current package where the module making the import exists. Two dots means up one package level. Three dots is up two levels, etc. So if you execute from . import mod from a module in the pkg package then you will end up importing pkg.mod . If you execute from ..subpkg2 import mod from within pkg.subpkg1 you will import pkg.subpkg2.mod . The specification for relative imports is contained in the Package Relative Imports section.

importlib.import_module() is provided to support applications that determine dynamically the modules to be loaded.

Raises an auditing event import with arguments module , filename , sys.path , sys.meta_path , sys.path_hooks .

7.11.1. Future statements ¶

A future statement is a directive to the compiler that a particular module should be compiled using syntax or semantics that will be available in a specified future release of Python where the feature becomes standard.

The future statement is intended to ease migration to future versions of Python that introduce incompatible changes to the language. It allows use of the new features on a per-module basis before the release in which the feature becomes standard.

A future statement must appear near the top of the module. The only lines that can appear before a future statement are:

the module docstring (if any),

blank lines, and

other future statements.

The only feature that requires using the future statement is annotations (see PEP 563 ).

All historical features enabled by the future statement are still recognized by Python 3. The list includes absolute_import , division , generators , generator_stop , unicode_literals , print_function , nested_scopes and with_statement . They are all redundant because they are always enabled, and only kept for backwards compatibility.

A future statement is recognized and treated specially at compile time: Changes to the semantics of core constructs are often implemented by generating different code. It may even be the case that a new feature introduces new incompatible syntax (such as a new reserved word), in which case the compiler may need to parse the module differently. Such decisions cannot be pushed off until runtime.

For any given release, the compiler knows which feature names have been defined, and raises a compile-time error if a future statement contains a feature not known to it.

The direct runtime semantics are the same as for any import statement: there is a standard module __future__ , described later, and it will be imported in the usual way at the time the future statement is executed.

The interesting runtime semantics depend on the specific feature enabled by the future statement.

Note that there is nothing special about the statement:

That is not a future statement; it’s an ordinary import statement with no special semantics or syntax restrictions.

Code compiled by calls to the built-in functions exec() and compile() that occur in a module M containing a future statement will, by default, use the new syntax or semantics associated with the future statement. This can be controlled by optional arguments to compile() — see the documentation of that function for details.

A future statement typed at an interactive interpreter prompt will take effect for the rest of the interpreter session. If an interpreter is started with the -i option, is passed a script name to execute, and the script includes a future statement, it will be in effect in the interactive session started after the script is executed.

The original proposal for the __future__ mechanism.

7.12. The global statement ¶

The global statement is a declaration which holds for the entire current code block. It means that the listed identifiers are to be interpreted as globals. It would be impossible to assign to a global variable without global , although free variables may refer to globals without being declared global.

Names listed in a global statement must not be used in the same code block textually preceding that global statement.

Names listed in a global statement must not be defined as formal parameters, or as targets in with statements or except clauses, or in a for target list, class definition, function definition, import statement, or variable annotation.

CPython implementation detail: The current implementation does not enforce some of these restrictions, but programs should not abuse this freedom, as future implementations may enforce them or silently change the meaning of the program.

Programmer’s note: global is a directive to the parser. It applies only to code parsed at the same time as the global statement. In particular, a global statement contained in a string or code object supplied to the built-in exec() function does not affect the code block containing the function call, and code contained in such a string is unaffected by global statements in the code containing the function call. The same applies to the eval() and compile() functions.

7.13. The nonlocal statement ¶

The nonlocal statement causes the listed identifiers to refer to previously bound variables in the nearest enclosing scope excluding globals. This is important because the default behavior for binding is to search the local namespace first. The statement allows encapsulated code to rebind variables outside of the local scope besides the global (module) scope.

Names listed in a nonlocal statement, unlike those listed in a global statement, must refer to pre-existing bindings in an enclosing scope (the scope in which a new binding should be created cannot be determined unambiguously).

Names listed in a nonlocal statement must not collide with pre-existing bindings in the local scope.

The specification for the nonlocal statement.

7.14. The type statement ¶

The type statement declares a type alias, which is an instance of typing.TypeAliasType .

For example, the following statement creates a type alias:

This code is roughly equivalent to:

annotation-def indicates an annotation scope , which behaves mostly like a function, but with several small differences.

The value of the type alias is evaluated in the annotation scope. It is not evaluated when the type alias is created, but only when the value is accessed through the type alias’s __value__ attribute (see Lazy evaluation ). This allows the type alias to refer to names that are not yet defined.

Type aliases may be made generic by adding a type parameter list after the name. See Generic type aliases for more.

type is a soft keyword .

New in version 3.12.

Introduced the type statement and syntax for generic classes and functions.

Table of Contents

  • 7.1. Expression statements
  • 7.2.1. Augmented assignment statements
  • 7.2.2. Annotated assignment statements
  • 7.3. The assert statement
  • 7.4. The pass statement
  • 7.5. The del statement
  • 7.6. The return statement
  • 7.7. The yield statement
  • 7.8. The raise statement
  • 7.9. The break statement
  • 7.10. The continue statement
  • 7.11.1. Future statements
  • 7.12. The global statement
  • 7.13. The nonlocal statement
  • 7.14. The type statement

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6. Expressions

8. Compound statements

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Different Forms of Assignment Statements in Python

  • Assignment creates object references instead of copying the objects.
  • Python creates a variable name the first time when they are assigned a value.
  • Names must be assigned before being referenced.
  • There are some operations that perform assignments implicitly.

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2.4: Expressions and Operator Precedence

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Expressions

An expression is a combination of variables, data elements (like numbers and strings), operations (like + or *) and functions (like ). We’ve seen a number of expressions throughout this chapter so far like

In short, writing things in julia will consist of writing expressions (and slightly more complicated structures).

Operator Precedence

When we type out an expression like 11+2*(4+3)^3 , it is important to understand the order in which operators are performed. For mathematics, the PEMDAS pnemonic is helpful to rememember in that the order is:

Parentheses : The expression inside the ( ) are done first. For the example above, the  4+3  is the first operation done.

Exponentials : The ^ is done next. Raise the 7 from above to the power of 3 resulting in 343 .

Multiplication and Division : In this example, the  2*(343)  is done next

Addition and Subtraction : Lastly add 11 to the result and the result is 697 .

In any computing language, there are other operators as well and there is order to that precedence, so we will see that there are other things to think about. For example, the assignment operator, has the lowest precedence. That is when assigning something to a variable, all calculations are done on the right side of the = before the assignment.

Details on all this can be found on the JULIA DOCUMENTATION ON OPERATOR PRECEDENCE

A comment in computer code is sequences of characters which are ignored. The purpose of a comment is to alert a human on what is going on. You may have been told to write comments so that someone else who reads your code understands what you are doing. However, I have found that the person mostly like to read your code is you at a later date. You should add comments for yourself.

In julia, a comment is anything to the right of a , pound sign or hash tag. For example:

Both lines 1 and 3 have comments. On line 1, the entire line is ignore since the line starts with # . On line 3, everything after the 2 (the power) is ignored. Also, notice that there are two hash tags on line 1 and 1 on line 3. This is simply different style. Since anything after a single is a comment, everything after the first one is ignored.

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Comma-Separated Lists

What is a comma-separated list.

When you type in a series of numbers separated by commas, MATLAB ® creates a comma-separated list and returns each value individually.

When used with large and more complex data structures like MATLAB structures and cell arrays, comma-separated lists can help simplify your code.

Generating a Comma-Separated List

You can generate a comma-separated list from either a cell array or a MATLAB structure.

Generating a List from a Cell Array

When you extract multiple elements from a cell array, the result is a comma-separated list. Define a 4-by-6 cell array.

Extract the fifth column to generate a comma-separated list.

This is the same as explicitly typing the list.

Generating a List from a Structure

When you extract a field of a structure array across one of its dimensions, the result is a comma-separated list.

Start by converting the cell array used above into a 4-by-1 MATLAB structure with six fields: f1 through f6 . Read field f5 for all rows, and MATLAB returns a comma-separated list.

Assigning Output from a Comma-Separated List

You can assign any or all consecutive elements of a comma-separated list to variables with a simple assignment statement. Define the cell array C and assign the first row to variables c1 through c6 . C = cell(4,6); for k = 1:24 C{k} = k*2; end [c1,c2,c3,c4,c5,c6] = C{1,1:6}; c5 c5 = 34 When you specify fewer output variables than the number of outputs returned by the expression, MATLAB assigns the first N outputs to those N variables and ignores any remaining outputs. In this example, MATLAB assigns C{1,1:3} to the variables c1 , c2 , and c3 and ignores C{1,4:6} . [c1,c2,c3] = C{1,1:6}; You can assign structure outputs in the same manner. S = cell2struct(C,{ 'f1' , 'f2' , 'f3' , 'f4' , 'f5' , 'f6' },2); [sf1,sf2,sf3] = S.f5; sf3 sf3 = 38 You also can use the deal function for this purpose.

Assigning to a Comma-Separated List

The simplest way to assign multiple values to a comma-separated list is to use the deal function. This function distributes its input arguments to the elements of a comma-separated list.

This example uses deal to overwrite each element in a comma-separated list. First initialize a two-element list. This step is necessary because you cannot use comma-separated list assignment with an undefined variable when using : as an index. See Comma-Separated List Assignment to an Undefined Variable for more information. c{1} = []; c{2} = []; c{:} ans = [] ans = []

Use deal to overwrite each element in the list. [c{:}] = deal([10 20],[14 12]); c{:} ans = 10 20 ans = 14 12

This example works in the same way, but with a comma-separated list of vectors in a structure field. s(1).field1 = [[]]; s(2).field1 = [[]]; s.field1 ans = [] ans = []

Use deal to overwrite the structure fields. [s.field1] = deal([10 20],[14 12]); s.field1 ans = 10 20 ans = 14 12

How to Use Comma-Separated Lists

Common uses for comma-separated lists are:

Constructing Arrays

Displaying arrays, concatenation, function call arguments, function return values.

These sections provide examples of using comma-separated lists with cell arrays. Each of these examples applies to structures as well.

You can use a comma-separated list to enter a series of elements when constructing a matrix or array. When you specify a list of elements with C{:,5} , MATLAB inserts the four individual elements.

When you specify the C cell itself, MATLAB inserts the entire cell array.

Use a list to display all or part of a structure or cell array.

Putting a comma-separated list inside square brackets extracts the specified elements from the list and concatenates them.

When writing the code for a function call, you enter the input arguments as a list with each argument separated by a comma. If you have these arguments stored in a structure or cell array, then you can generate all or part of the argument list from the structure or cell array instead. This can be especially useful when passing in variable numbers of arguments.

This example passes several name-value arguments to the plot function.

MATLAB functions can also return more than one value to the caller. These values are returned in a list with each value separated by a comma. Instead of listing each return value, you can use a comma-separated list with a structure or cell array. This becomes more useful for functions that have variable numbers of return values.

This example returns three values to a cell array.

Fast Fourier Transform Example

The fftshift function swaps the left and right halves of each dimension of an array. For the vector [0 2 4 6 8 10] , the output is [6 8 10 0 2 4] . For a multidimensional array, fftshift performs this swap along each dimension.

fftshift uses vectors of indices to perform the swap. For the vector shown above, the index [1 2 3 4 5 6] is rearranged to form a new index [4 5 6 1 2 3] . The function then uses this index vector to reposition the elements. For a multidimensional array, fftshift constructs an index vector for each dimension. A comma-separated list makes this task much simpler.

Here is the fftshift function.

The function stores the index vectors in cell array idx . Building this cell array is relatively simple. For each of the N dimensions, determine the size of that dimension and find the integer index nearest the midpoint. Then, construct a vector that swaps the two halves of that dimension.

By using a cell array to store the index vectors and a comma-separated list for the indexing operation, fftshift shifts arrays of any dimension using just a single operation: y = x(idx{:}) . If you use explicit indexing, you need to write one if statement for each dimension you want the function to handle.

Another way to handle this without a comma-separated list is to loop over each dimension, converting one dimension at a time and moving data each time. With a comma-separated list, you move the data just once. A comma-separated list makes it easy to generalize the swapping operation to any number of dimensions.

Troubleshooting Operations with Comma-Separated Lists

Some common MATLAB operations and indexing techniques do not work directly on comma-separated lists. This section details several errors you can encounter when working with comma-separated lists and explains how to resolve the underlying issues.

Intermediate Indexing Produced a Comma-Separated List

Compound indexing expressions with braces or dots can produce comma-separated lists. You must index into the individual elements of the list to access them.

For example, create a 1-by-2 cell array that contains two 3-by-3 matrices of doubles.

Use brace indexing to display both elements.

Indexing into A this way produces a comma-separated list that includes both matrices contained by the cell array. You cannot use parentheses indexing to retrieve the entries at (1,2) in both matrices in the list.

To retrieve the entries at (1,2) in both of the matrices in the cell array, index into the cells individually.

Expression Produced a Comma-Separated List Instead of a Single Value

Arguments for conditional statements, logical operators, loops, and switch statements cannot be comma-separated lists. For example, you cannot directly loop through the contents of a comma-separated list using a for loop.

Create a cell array of the first three prime numbers.

A{:} produces a comma-separated list of the three values.

Using for to loop through the comma-separated list generated by A{:} errors.

To loop over the contents of A , enclose A{:} in square brackets to concatenate the values into a vector.

Assigning Multiple Elements Using Simple Assignment

Unlike with arrays, using simple assignment to assign values to multiple elements of a comma-separated list errors. For example, define a 2-by-3 cell array.

Assigning a value of 5 to all cells of the array using : as an index for B errors.

One way to accomplish this assignment is to enclose B{:} in square brackets and use the deal function.

Comma-Separated List Assignment to an Undefined Variable

You cannot assign a comma-separated list to an undefined variable using : as an index. In the example in Assigning to a Comma-Separated List , the variable x is defined as a comma-separated list with explicit indices before assigning new values to it using : as an index.

Performing the same assignment with a variable that has not been initialized errors.

To solve this problem, initialize y in the same way as x , or create y using enough explicit indices to accommodate the number of values produced by the deal function.

cell | deal | struct

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CS105: Introduction to Python

Variables and assignment statements.

Computers must be able to remember and store data. This can be accomplished by creating a variable to house a given value. The assignment operator = is used to associate a variable name with a given value. For example, type the command:

in the command line window. This command assigns the value 3.45 to the variable named a . Next, type the command:

in the command window and hit the enter key. You should see the value contained in the variable a echoed to the screen. This variable will remember the value 3.45 until it is assigned a different value. To see this, type these two commands:

You should see the new value contained in the variable a echoed to the screen. The new value has "overwritten" the old value. We must be careful since once an old value has been overwritten, it is no longer remembered. The new value is now what is being remembered.

Although we will not discuss arithmetic operations in detail until the next unit, you can at least be equipped with the syntax for basic operations: + (addition), - (subtraction), * (multiplication), / (division)

For example, entering these command sequentially into the command line window:

would result in 12.32 being echoed to the screen (just as you would expect from a calculator). The syntax for multiplication works similarly. For example:

would result in 35 being echoed to the screen because the variable b has been assigned the value a * 5 where, at the time of execution, the variable a contains a value of 7.

After you read, you should be able to execute simple assignment commands using integer and float values in the command window of the Repl.it IDE. Try typing some more of the examples from this web page to convince yourself that a variable has been assigned a specific value.

In programming, we associate names with values so that we can remember and use them later. Recall Example 1. The repeated computation in that algorithm relied on remembering the intermediate sum and the integer to be added to that sum to get the new sum. In expressing the algorithm, we used th e names current and sum .

In programming, a name that refers to a value in this fashion is called a variable . When we think of values as data stored somewhere i n the computer, we can have a mental image such as the one below for the value 10 stored in the computer and the variable x , which is the name we give to 10. What is most important is to see that there is a binding between x and 10.

The term variable comes from the fact that values that are bound to variables can change throughout computation. Bindings as shown above are created, and changed by assignment statements . An assignment statement associates the name to the left of the symbol = with the value denoted by the expression on the right of =. The binding in the picture is created using an assignment statemen t of the form x = 10 . We usually read such an assignment statement as "10 is assigned to x" or "x is set to 10".

If we want to change the value that x refers to, we can use another assignment statement to do that. Suppose we execute x = 25 in the state where x is bound to 10.Then our image becomes as follows:

Choosing variable names

Suppose that we u sed the variables x and y in place of the variables side and area in the examples above. Now, if we were to compute some other value for the square that depends on the length of the side , such as the perimeter or length of the diagonal, we would have to remember which of x and y , referred to the length of the side because x and y are not as descriptive as side and area . In choosing variable names, we have to keep in mind that programs are read and maintained by human beings, not only executed by machines.

Note about syntax

In Python, variable identifiers can contain uppercase and lowercase letters, digits (provided they don't start with a digit) and the special character _ (underscore). Although it is legal to use uppercase letters in variable identifiers, we typically do not use them by convention. Variable identifiers are also case-sensitive. For example, side and Side are two different variable identifiers.

There is a collection of words, called reserved words (also known as keywords), in Python that have built-in meanings and therefore cannot be used as variable names. For the list of Python's keywords See 2.3.1 of the Python Language Reference.

Syntax and Sema ntic Errors

Now that we know how to write arithmetic expressions and assignment statements in Python, we can pause and think about what Python does if we write something that the Python interpreter cannot interpret. Python informs us about such problems by giving an error message. Broadly speaking there are two categories for Python errors:

  • Syntax errors: These occur when we write Python expressions or statements that are not well-formed according to Python's syntax. For example, if we attempt to write an assignment statement such as 13 = age , Python gives a syntax error. This is because Python syntax says that for an assignment statement to be well-formed it must contain a variable on the left hand side (LHS) of the assignment operator "=" and a well-formed expression on the right hand side (RHS), and 13 is not a variable.
  • Semantic errors: These occur when the Python interpreter cannot evaluate expressions or execute statements because they cannot be associated with a "meaning" that the interpreter can use. For example, the expression age + 1 is well-formed but it has a meaning only when age is already bound to a value. If we attempt to evaluate this expression before age is bound to some value by a prior assignment then Python gives a semantic error.

Even though we have used numerical expressions in all of our examples so far, assignments are not confined to numerical types. They could involve expressions built from any defined type. Recall the table that summarizes the basic types in Python.

The following video shows execution of assignment statements involving strings. It also introduces some commonly used operators on strings. For more information see the online documentation. In the video below, you see the Python shell displaying "=> None" after the assignment statements. This is unique to the Python shell presented in the video. In most Python programming environments, nothing is displayed after an assignment statement. The difference in behavior stems from version differences between the programming environment used in the video and in the activities, and can be safely ignored.

Distinguishing Expressions and Assignments

So far in the module, we have been careful to keep the distinction between the terms expression and statement because there is a conceptual difference between them, which is sometimes overlooked. Expressions denote values; they are evaluated to yield a value. On the other hand, statements are commands (instructions) that change the state of the computer. You can think of state here as some representation of computer memory and the binding of variables and values in the memory. In a state where the variable side is bound to the integer 3, and the variable area is yet unbound, the value of the expression side + 2 is 5. The assignment statement side = side + 2 , changes the state so that value 5 is bound to side in the new state. Note that when you type an expression in the Python shell, Python evaluates the expression and you get a value in return. On the other hand, if you type an assignment statement nothing is returned. Assignment statements do not return a value. Try, for example, typing x = 100 + 50 . Python adds 100 to 50, gets the value 150, and binds x to 150. However, we only see the prompt >>> after Python does the assignment. We don't see the change in the state until we inspect the value of x , by invoking x .

What we have learned so far can be summarized as using the Python interpreter to manipulate values of some primitive data types such as integers, real numbers, and character strings by evaluating expressions that involve built-in operators on these types. Assignments statements let us name the values that appear in expressions. While what we have learned so far allows us to do some computations conveniently, they are limited in their generality and reusability. Next, we introduce functions as a means to make computations more general and reusable.

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  • Assignment Statement

An Assignment statement is a statement that is used to set a value to the variable name in a program .

Assignment statement allows a variable to hold different types of values during its program lifespan. Another way of understanding an assignment statement is, it stores a value in the memory location which is denoted by a variable name.

Assignment Statement Method

The symbol used in an assignment statement is called as an operator . The symbol is ‘=’ .

Note: The Assignment Operator should never be used for Equality purpose which is double equal sign ‘==’.

The Basic Syntax of Assignment Statement in a programming language is :

variable = expression ;

variable = variable name

expression = it could be either a direct value or a math expression/formula or a function call

Few programming languages such as Java, C, C++ require data type to be specified for the variable, so that it is easy to allocate memory space and store those values during program execution.

data_type variable_name = value ;

In the above-given examples, Variable ‘a’ is assigned a value in the same statement as per its defined data type. A data type is only declared for Variable ‘b’. In the 3 rd line of code, Variable ‘a’ is reassigned the value 25. The 4 th line of code assigns the value for Variable ‘b’.

Assignment Statement Forms

This is one of the most common forms of Assignment Statements. Here the Variable name is defined, initialized, and assigned a value in the same statement. This form is generally used when we want to use the Variable quite a few times and we do not want to change its value very frequently.

Tuple Assignment

Generally, we use this form when we want to define and assign values for more than 1 variable at the same time. This saves time and is an easy method. Note that here every individual variable has a different value assigned to it.

(Code In Python)

Sequence Assignment

(Code in Python)

Multiple-target Assignment or Chain Assignment

In this format, a single value is assigned to two or more variables.

Augmented Assignment

In this format, we use the combination of mathematical expressions and values for the Variable. Other augmented Assignment forms are: &=, -=, **=, etc.

Browse more Topics Under Data Types, Variables and Constants

  • Concept of Data types
  • Built-in Data Types
  • Constants in Programing Language 
  • Access Modifier
  • Variables of Built-in-Datatypes
  • Declaration/Initialization of Variables
  • Type Modifier

Few Rules for Assignment Statement

Few Rules to be followed while writing the Assignment Statements are:

  • Variable names must begin with a letter, underscore, non-number character. Each language has its own conventions.
  • The Data type defined and the variable value must match.
  • A variable name once defined can only be used once in the program. You cannot define it again to store other types of value.
  • If you assign a new value to an existing variable, it will overwrite the previous value and assign the new value.

FAQs on Assignment Statement

Q1. Which of the following shows the syntax of an  assignment statement ?

  • variablename = expression ;
  • expression = variable ;
  • datatype = variablename ;
  • expression = datatype variable ;

Answer – Option A.

Q2. What is an expression ?

  • Same as statement
  • List of statements that make up a program
  • Combination of literals, operators, variables, math formulas used to calculate a value
  • Numbers expressed in digits

Answer – Option C.

Q3. What are the two steps that take place when an  assignment statement  is executed?

  • Evaluate the expression, store the value in the variable
  • Reserve memory, fill it with value
  • Evaluate variable, store the result
  • Store the value in the variable, evaluate the expression.

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Congratulations to the 84th Annual Putnam Competition Winners!

Announcement of Winners of the 84th Putnam Competition

The William Lowell Putnam Mathematical Competition is the preeminent mathematics competition for undergraduate college students in the United States and Canada. The competition consists of two 3-hour sessions, during each session, participants work individually on 6 challenging mathematical problems.

Students Ranked in the Top 200

Theodore Lysek, Noah Mart, Jianzhi Wang

Students Ranked in the Top 500

Yash Aggarwal, Daniel Ao, Aditya Baireddy, Ralph Cao, Preston Fu, Andrew Huang, Aathreya Kadambi, Andrew Lee, Bradley Louie Saito, Justin Park, Youngmin Park, Shreyas Ramamurthy, Robin Sharif, Brian Sui, Dylan Xu, Robert Yang

For a detailed list of the winning teams, Putnam Fellows, participants earning honorable mentions, teams earning honorable mentions, and the Elizabeth Lowell Putnam Prize winner, please visit  https://maa.org/math-competitions/william-lowell-putnam-mathematical-com... .

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Expressions and Assignment Statements ¶

In this lesson, you will learn about assignment statements and expressions that contain math operators and variables.

Assignment Statements ¶

Remember that a variable holds a value that can change or vary. Assignment statements initialize or change the value stored in a variable using the assignment operator = . An assignment statement always has a single variable on the left hand side of the = sign. The value of the expression on the right hand side of the = sign (which can contain math operators and other variables) is copied into the memory location of the variable on the left hand side.

Assignment statement

Figure 1: Assignment Statement (variable = expression) ¶

Instead of saying equals for the = operator in an assignment statement, say “gets” or “is assigned” to remember that the variable on the left hand side gets or is assigned the value on the right. In the figure above, score is assigned the value of 10 times points (which is another variable) plus 5.

The following video by Dr. Colleen Lewis shows how variables can change values in memory using assignment statements.

As we saw in the video, we can set one variable to a copy of the value of another variable like y = x;. This won’t change the value of the variable that you are copying from.

coding exercise

Run the code in the VariableAssignment class and see how the values of the variables change.

The CalculateMoney program is supposed to figure out the total money value given the number of dimes, quarters and nickels. There is an error in the calculation of the total. Fix the error to compute the correct amount. The SalaryExample program is supposed to calculate and print the total pay given the weekly salary and the number of weeks worked. Use string concatenation with the totalPay variable to produce the output Total Pay = $3000 . Don’t hardcode the number 3000 in your print statement.

exercise

Incrementing the value of a variable ¶

If you use a variable to keep score you would probably increment it (add one to the current value) whenever score should go up. You can do this by setting the variable to the current value of the variable plus one (score = score + 1) as shown below. The formula looks a little crazy in math class, but it makes sense in coding because the variable on the left is set to the value of the arithmetic expression on the right. So, the score variable is set to the previous value of score + 1.

Run the code in UpdateScore and see how the score value changes.

1-4-3: What is the value of b after the following code executes?

  • It sets the value for the variable on the left to the value from evaluating the right side. What is 5 * 2?
  • Correct. 5 * 2 is 10.

1-4-4: What are the values of x, y, and z after the following code executes?

  • x = 0, y = 1, z = 2
  • These are the initial values in the variable, but the values are changed.
  • x = 1, y = 2, z = 3
  • x changes to y's initial value, y's value is doubled, and z is set to 3
  • x = 2, y = 2, z = 3
  • Remember that the equal sign doesn't mean that the two sides are equal. It sets the value for the variable on the left to the value from evaluating the right side.
  • x = 1, y = 0, z = 3

Operators ¶

Java uses the standard mathematical operators for addition ( + ), subtraction ( - ), multiplication ( * ), and division ( / ). Arithmetic expressions can be of type int or double. An arithmetic operation that uses two int values will evaluate to an int value. An arithmetic operation that uses at least one double value will evaluate to a double value. (You may have noticed that + was also used to put text together in the input program above – more on this when we talk about strings.)

Java uses the operator == to test if the value on the left is equal to the value on the right and != to test if two items are not equal. Don’t get one equal sign = confused with two equal signs == ! They mean different things in Java. One equal sign is used to assign a value to a variable. Two equal signs are used to test a variable to see if it is a certain value and that returns true or false as you’ll see below. Use == and != only with int values and not doubles because double values are an approximation and 3.3333 will not equal 3.3334 even though they are very close.

Run the code in OperatorExample to see all the operators in action. Do all of those operators do what you expected? What about 2 / 3 ? Isn’t surprising that it prints 0 ? See the note below.

When Java sees you doing integer division (or any operation with integers) it assumes you want an integer result so it throws away anything after the decimal point in the answer, essentially rounding down the answer to a whole number. If you need a double answer, you should make at least one of the values in the expression a double like 2.0.

With division, another thing to watch out for is dividing by 0. An attempt to divide an integer by zero will result in an ArithmeticException error message. Try it in one of the active code windows above.

Operators can be used to create compound expressions with more than one operator. You can either use a literal value which is a fixed value like 2, or variables in them. When compound expressions are evaluated, operator precedence rules are used, so that *, /, and % are done before + and -. However, anything in parentheses is done first. It doesn’t hurt to put in extra parentheses if you are unsure as to what will be done first.

Open the TestCompound.java file, try to guess what it will print out and then run it to see if you are right. Remember to consider operator precedence .

1-4-5: Consider the following code segment. Be careful about integer division.

What is printed when the code segment is executed?

  • 0.666666666666667
  • Don't forget that division and multiplication will be done first due to operator precedence.
  • Yes, this is equivalent to (5 + ((a/b)*c) - 1).
  • Don't forget that division and multiplication will be done first due to operator precedence, and that an int/int gives an int result where it is rounded down to the nearest int.

1-4-6: Consider the following code segment.

What is the value of the expression?

  • Dividing an integer by an integer results in an integer
  • Correct. Dividing an integer by an integer results in an integer
  • The value 5.5 will be rounded down to 5

1-4-7: Consider the following code segment.

  • Correct. Dividing a double by an integer results in a double
  • Dividing a double by an integer results in a double

1-4-8: Consider the following code segment.

  • Correct. Dividing an integer by an double results in a double
  • Dividing an integer by an double results in a double

The Modulo Operator ¶

The percent sign operator ( % ) is the mod (modulo) or remainder operator. The mod operator ( x % y ) returns the remainder after you divide x (first number) by y (second number) so 5 % 2 will return 1 since 2 goes into 5 two times with a remainder of 1. Remember long division when you had to specify how many times one number went into another evenly and the remainder? That remainder is what is returned by the modulo operator.

../_images/mod-py.png

Figure 2: Long division showing the whole number result and the remainder ¶

In ModExample program, try to guess what it will print out and then run it to see if you are right.

The result of x % y when x is smaller than y is always x . The value y can’t go into x at all (goes in 0 times), since x is smaller than y , so the result is just x . So if you see 2 % 3 the result is 2 .

1-4-10: What is the result of 158 % 10?

  • This would be the result of 158 divided by 10. modulo gives you the remainder.
  • modulo gives you the remainder after the division.
  • When you divide 158 by 10 you get a remainder of 8.

1-4-11: What is the result of 3 % 8?

  • 8 goes into 3 no times so the remainder is 3. The remainder of a smaller number divided by a larger number is always the smaller number!
  • This would be the remainder if the question was 8 % 3 but here we are asking for the reminder after we divide 3 by 8.
  • What is the remainder after you divide 3 by 8?

FlowCharting ¶

Assume you have 16 pieces of pizza and 5 people. If everyone gets the same number of slices, how many slices does each person get? Are there any leftover pieces?

In industry, a flowchart is used to describe a process through symbols and text. A flowchart usually does not show variable declarations, but it can show assignment statements (drawn as rectangle) and output statements (drawn as rhomboid).

The flowchart in figure 3 shows a process to compute the fair distribution of pizza slices among a number of people. The process relies on integer division to determine slices per person, and the mod operator to determine remaining slices.

Flow Chart

Figure 3: Example Flow Chart ¶

A flowchart shows pseudo-code, which is like Java but not exactly the same. Syntactic details like semi-colons are omitted, and input and output is described in abstract terms.

Complete the program PizzaCalculator based on the process shown in the Figure 3 flowchart. Note the first line of code declares all 4 variables as type int. Add assignment statements and print statements to compute and print the slices per person and leftover slices. Use System.out.println for output.

Storing User Input in Variables ¶

Variables are a powerful abstraction in programming because the same algorithm can be used with different input values saved in variables.

Program input and output

Figure 4: Program input and output ¶

A Java program can ask the user to type in one or more values. The Java class Scanner is used to read from the keyboard input stream, which is referenced by System.in . Normally the keyboard input is typed into a console window, but since this is running in a browser you will type in a small textbox window displayed below the code. The code String name = scan.nextLine() gets the string value you enter as program input and then stores the value in a variable.

Run the NameReader program a few times, typing in a different name. The code works for any name: behold, the power of variables!

The Scanner class has several useful methods for reading user input. A token is a sequence of characters separated by white space.

Run the AgeReader program to read in an integer from the input stream. You can type a different integer value in the input window shown below the code.

A rhomboid (slanted rectangle) is used in a flowchart to depict data flowing into and out of a program. The previous flowchart in Figure 3 used a rhomboid to indicate program output. A rhomboid is also used to denote reading a value from the input stream.

Flow Chart

Figure 5: Flow Chart Reading User Input ¶

Figure 5 contains an updated version of the pizza calculator process. The first two steps have been altered to initialize the pizzaSlices and numPeople variables by reading two values from the input stream. In Java this will be done using a Scanner object and reading from System.in.

Complete the PizzaCalculatorInput program based on the process shown in the Figure 5 flowchart. The program should scan two integer values to initialize pizzaSlices and numPeople. Run the program a few times to experiment with different values for input. What happens if you enter 0 for the number of people? The program will bomb due to division by zero! We will see how to prevent this in a later lesson.

The SumInput program reads two integer values from the input stream and attempts to print the sum. Unfortunately there is a problem with the last line of code that prints the sum.

Run the program and look at the result. When the input is 5 and 7 , the output is Sum is 57 . Both of the + operators in the print statement are performing string concatenation. While the first + operator should perform string concatenation, the second + operator should perform addition. You can force the second + operator to perform addition by putting the arithmetic expression in parentheses ( num1 + num2 ) .

More information on using the Scanner class can be found here https://www.w3schools.com/java/java_user_input.asp

Programming Challenge : Dog Years ¶

In this programming challenge, you will calculate your age, and your pet’s age from your birthdates, and your pet’s age in dog years. In the DogAgeChallenge` program, type in the current year, the year you were born, the year your dog or cat was born (if you don’t have one, make one up!) in the variables below. Then write formulas in assignment statements to calculate how old you are, how old your dog or cat is, and how old they are in dog years which is 7 times a human year. Finally, print it all out.

Arithmetic expressions include expressions of type int and double.

The arithmetic operators consist of +, -, * , /, and % (modulo for the remainder in division).

An arithmetic operation that uses two int values will evaluate to an int value. With integer division, any decimal part in the result will be thrown away, essentially rounding down the answer to a whole number.

An arithmetic operation that uses at least one double value will evaluate to a double value.

Operators can be used to construct compound expressions.

During evaluation, operands are associated with operators according to operator precedence to determine how they are grouped. (*, /, % have precedence over + and -, unless parentheses are used to group those.)

An attempt to divide an integer by zero will result in an ArithmeticException to occur.

The assignment operator (=) allows a program to initialize or change the value stored in a variable. The value of the expression on the right is stored in the variable on the left.

During execution, expressions are evaluated to produce a single value.

The value of an expression has a type based on the evaluation of the expression.

IMAGES

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COMMENTS

  1. 1.4. Expressions and Assignment Statements

    An assignment statement always has a single variable on the left hand side. The value of the expression (which can contain math operators and other variables) on the right of the = sign is stored in the variable on the left. Figure 1: Assignment Statement (variable = expression;) ¶

  2. 2.3: Arithmetic Operations and Assignment Statements

    An assignment statement is a line of code that uses a "=" sign. The statement stores the result of an operation performed on the right-hand side of the sign into the variable memory location on the left-hand side. 4. Enter and execute the following lines of Python code in the editor window of your IDE (e.g. Thonny):

  3. Variables, expressions and statements

    An assignment statement creates a new variable and gives. it a value: >>> message = 'And now for something completely different'. >>> n = 17. >>> pi = 3.1415926535897932. This example makes three assignments. The first assigns a string. to a new variable named message; the second gives the integer 17 to n; the third.

  4. Operators and Expressions in Python

    The Assignment Operator and Statements. The assignment operator is one of the most frequently used operators in Python. The operator consists of a single equal sign (=), and it operates on two operands. The left-hand operand is typically a variable, while the right-hand operand is an expression.

  5. PDF 1. The Assignment Statement and Types

    The Assignment Statement and Types Topics: Python's Interactive Mode Variables Expressions Assignment Strings, Ints, and Floats The Python Interactive Shell Python can be used in a way that reminds you of a calculator. In the ``command shell of your system simply type python and you will be met with a prompt... >

  6. Variables and Assignment

    The assignment operator, denoted by the "=" symbol, is the operator that is used to assign values to variables in Python. The line x=1 takes the known value, 1, and assigns that value to the variable with name "x". After executing this line, this number will be stored into this variable.

  7. 3 ASSIGNMENT STATEMENTS and ARITHMETIC OPERATIONS

    an assignment statement could be used to assign a value to the memory location containing the value for the remaining portion of carbon 14. First a variable for this purpose would need to be typed as a REAL It is also possible to use a to initialize variables. Two variables can also store the same value.

  8. PDF Expressions, Statements, Variables, Assignments, Types

    The Assignment Operator •In Python, we call the = sign the assignment operator. •The assignment operator looks the same, but is not the same as the = sign in math. •First difference: -in math, "a = 5" is the same as "5 = a". -In Python, "a = 5" is a valid piece of code assigning value 5 to variable a. "5 = a" is not valid code. 25

  9. Variables, Expressions, and Assignments

    Lastly, an assignment is a language construct know as an statement that assign a value (either as a constant or expression) to a variable. The rest of this notebook will dive into the main concepts that we need to fully understand these three language constructs. Values and Types A value is the basic unit used in a program.

  10. What is the difference between an expression and a statement in Python

    An expression statement evaluates an expression and discards the result. (Any expression can be used; there are no other distinguishing features of an expression statement.) An assignment statement evaluates an expression and assigns its value to one or more targets. It is identified by the use of = outside of the context of a function call.

  11. Assignment Expressions: The Walrus Operator

    In Python 3.8, you can combine those two statements and do a single statement using the walrus operator. So inside of print(), you could say walrus, the new object, and use the operator, the assignment expression :=, and a space, and then say True. That's going to do two things. Most notably, in reverse order, it returned the value True.

  12. Variables and Assignment Statements

    In an assignment statement, everything on the right of the equals sign is calculated, and the result is assigned to the variable on the left. Whenever you see an assignment statement, you can read or think of the equals sign as meaning any of the following: "is assigned". "is given the value of". "is set to". "becomes".

  13. Python's Assignment Operator: Write Robust Assignments

    Unlike in math equations, in Python assignments, the left operand must be a variable rather than an expression or a value. For example, the following construct is illegal, and Python flags it as invalid syntax: ... The first statement in this example is what you can call an annotated assignment statement in Python.

  14. 7. Simple statements

    Assignment statements are used to (re)bind names to values and to modify attributes or items of mutable objects: assignment_stmt ::= ( target_list "=")+ ( starred_expression | yield_expression ) target_list ::= target ("," target )* [","] target ::= identifier | "(" [ target_list] ")" | "[" [ target_list] "]"

  15. 2.2 Assignment Statement and Variables

    For example, 2*y. -2. looks up the value of y (which is -1) and substitutes that value in and multiplies the result by 2. As you can see the result is -2. If we change one of the values, like. y = y+5. this means that the right hand side is evaluated by looking up the value of y and the result is 4.

  16. Different Forms of Assignment Statements in Python

    Assignment statement forms :- 1. Basic form: This form is the most common form. 2. Tuple assignment: When we code a tuple on the left side of the =, Python pairs objects on the right side with targets on the left by position and assigns them from left to right.

  17. 1.1: Statements and Conditional Statements

    In mathematics, a statement is a declarative sentence that is either true or false but not both. A statement is sometimes called a proposition. The key is that there must be no ambiguity. To be a statement, a sentence must be true or false, and it cannot be both.

  18. 2.4: Expressions and Operator Precedence

    Operator Precedence. When we type out an expression like 11+2* (4+3)^3, it is important to understand the order in which operators are performed. For mathematics, the PEMDAS pnemonic is helpful to rememember in that the order is: Parentheses: The expression inside the ( ) are done first. For the example above, the 4+3 is the first operation done.

  19. Comma-Separated Lists

    You can assign any or all consecutive elements of a comma-separated list to variables with a simple assignment statement. Define the cell array Cand assign the first row to variables c1 through c6. C = cell ... Arguments for conditional statements, logical operators, loops, and switch statements cannot be comma-separated lists.

  20. CS105: Variables and Assignment Statements

    The assignment operator = is used to associate a variable name with a given value. For example, type the command: a=3.45 in the command line window. This command assigns the value 3.45 to the variable named a. Next, type the command: a in the command window and hit the enter key.

  21. What are Assignment Statement: Definition, Assignment Statement ...

    An Assignment statement is a statement that is used to set a value to the variable name in a program. Assignment statement allows a variable to hold different types of values during its program lifespan. Another way of understanding an assignment statement is, it stores a value in the memory location which is denoted by a variable name. Syntax

  22. Congratulations to the 84th Annual Putnam Competition Winners!

    The William Lowell Putnam Mathematical Competition is the preeminent mathematics competition for undergraduate college students in the United States and Canada. The competition consists of two 3-hour sessions, during each session, participants work individually on 6 challenging mathematical problems. Students Ranked in the Top 200

  23. Expressions and Assignment Statements

    An assignment statement always has a single variable on the left hand side of the = sign. The value of the expression on the right hand side of the = sign (which can contain math operators and other variables) is copied into the memory location of the variable on the left hand side. Figure 1: Assignment Statement (variable = expression) ¶.