ada assignment operator

Ada Advantages

Ara community, ada resource association, news and resource for the ada programming language, by jim rogers.

This is part three (of five) of this article. Click here to return to part two, or click here to go to the article’s table of contents.

Ada Operators

C++ allows extensive overloading of operators. Ada allows a limited overloading of operators. The exception in Ada is that the assignment operator ( := ) cannot be overridden. When you override the equality operator ( = ) you also implicitly override the inequality operator ( /= ). Java does not allow overriding of operators.

If you want to achieve the equivalent of overriding the assignment operator in Ada you must declare your type to inherit from the abstract tagged type Controlled defined in the package Ada.Finalization. Controlled types provide three operations that can be overridden.

• Initialize is called during object construction and allows control over the initialization logic for the controlled type. The Initialize procedure performs much the same function as a default constructor in C++ or Java.
• Adjust is a procedure that is called at the end of assignment. Overriding Adjust lets you control assignment behavior.
• Finalize is called when an object goes out of scope. The Finalize procedure can be overridden to achieve the same purposes as a C++ destructor.

Ada also allows the definition of limited types. Any type declared limited has no predefined operators, including assignment. Use of limited types allows the programer to selectively restrict the available operations on a type. Only those operations specifically provided by the programmer will be available for a limited type.

The package Ada.Finalization defines a second abstract tagged type named Limited_Controlled . Limited_Controlled types do not have an adjust procedure.

Any attempt to assign a value to an object of a limited type will result in a compile time error message.

Operator Examples

This example is divided into three files. The first file is a package specification. This package specification defines two types and the subprograms for those types.

The Days type is an enumeration type containing the names of the days of the week in English. Days has the procedure Print_Message defined for it.

The Daily_Sales type is an array of floats indexed by the values in type Days . Daily_Sales has two functions defined for it: Total and Geometric_Mean .

The package Operator_Examples becomes visible to another compilation unit when that compilation unit names   Operator_Examples in a   with clause. Java does not require this explicit declaration of dependency. Instead, the compiler must scan every line of code to determine external dependencies. The public contents of this package specification become directly visible to a foreign compilation unit when it follows the with clause with a use clause. The Ada use clause is similar to a Java import statement. 

The next file is the package body, or implementation. The package body contains the definitions of all the subprograms declared in the package specification. In this example the package body also declares a dependency upon two packages: Ada.Text_Io and Ada.Numerics.Elementary_Functions . As you can see above, the exponentiation operation is defined as ** . The normal version of this operator takes an integer value as its power. I wanted to pass it a fractional value. I declared a dependency upon Ada.Numerics.Elementary_Functions and then included the same package in a use clause so that I could use the overloaded version of the exponentiation operator that takes a float for the exponent value.

The third file contains the parameterless procedure used as the starting point for a program to exercise the Operator_Examples package. This package declares a dependency upon two different I/O packages. The package Ada.Text_Io defines text file operations and procedures for the input and output of strings and characters. The package Ada.Float_Text_Io defines text file input and output procedures for the type float . Both packages have Put procedures. The package Ada.Float_Text_Io overloads the Put operations defined in Ada.Text_Io .

The output from this program is:

This ends part three of this article. Click here to continue with part four, Loops and other control structures.

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Why are there no (augmented assignment) operators like +=, -= or ++ in Ada? [closed]

I'm wondering why there are no operators like += , -= , ++ , -= , <<= or x ? y : z (not an augmented assignment ...) in Ada? Many other languages (C, C++, C#, Java, Perl) have them.

-- Example (C/C++/...):

-- Example (Ada):

( Example doesn't make sense - only for demonstration )

Is it because ...

• Operator-overloading (but C++ has such a mechanism too)?
• Readability?
• Technical reasons / limitations?
• It's only a trick for making those expressions shorter and not really required for programming?
• The assignment operator in Ada is := and not = (so += -> +=: )?

• 5 While this question does seem to skirt the line between asking a question and advocacy, I think it is a reasonable question (with an answer even. Check out the Rationale), so I don't agree with the close decision. –  T.E.D. Jan 23, 2013 at 16:15
• 3 +1 for T.E.D. While I stand by my answer I would have been interested to see and understand other points of view –  user1818839 Jan 24, 2013 at 11:40
• 1 +1 for ted and Brian; I also want to see other points for this question... –  clx Jan 24, 2013 at 14:55
• 1 @T.E.D. OK, I'll bite ... do you have a link to the right section of the Rationale? –  user1818839 Jan 26, 2013 at 11:05
• 1 Add me to the list of people who would have liked to see this discussed more. –  Shark8 Jan 26, 2013 at 17:07

Because the design of Ada was taken much more closely from mathematics than some other languages... And so...

Assignment is not an operator

Operators have specific properties - they operate on quantities returning a result - while leaving the quantities themselves unchanged.

This is important - stick rigorously to this understanding of an "operator" and you make a lot of optimisations possible because the semantics are much more predictable. Essentially, operators don't have side effects. You can repeat them or factor out repeated ones, and you have a lot more freedom to reorder expressions without changing their results.

If you mistake assignment for an operator, ... well, basically you're screwed. Just ONE "operator" with side effects means you lose valuable properties for ALL operators ... for what? some notational convenience, a hugely fertile breeding ground for bugs, and no extra performance or efficiency.

Incidentally when I had to poke around inside GCC recently I found a function in its expression analyser that explicitly broke (intermediate representation for) a++ and transformed it internally into (intermediate representation for) a = a + 1; So the shorter form really doesn't appear to be any more efficient!

The same rationale applies (less strictly in Ada than VHDL) to functions - they are just operators in another guise, and pure functions (in VHDL that's every function without the word "impure" in its declaration!) don't have side effects.

Which is also why Ada has both functions and procedures : functions, operators and expressions are essentially similar (and ideally, stateless and side-effect free); procedures, assignments and statements are a separate category (procedure calls and assignments are forms of statement).

Separating the concepts and using the appropriate one for each task goes a long way to making clear programs that you can understand and probably do what you intended...

Oh and Ada-2012 has finally caught up with VHDL-2008 and Algol-W (1963) with if- and case-expressions...

It is obvious that the assignments here are still statements...

Just to make sure:

Ada's designers had an impressive and usually VERY clear grasp of what was possible without compromising integrity and what would just lead to a mess. While newer language features have been added, as compiler techniques developed enough to make them reliable, it has always been a cautious process and the Ada-83 subset is still there virtually intact.

• Thank you for this answer! Haven't seen an answer for this question so far - but this contains all i want to know. –  ollo Jan 23, 2013 at 13:02
• Btw. those if/case asignemts are realy great, haven't seen them before. No need für ?: thereby. –  ollo Jan 23, 2013 at 17:38
• It's always struck me as odd that C had one, but not the other. –  user1818839 Jan 24, 2013 at 11:34

Not the answer you're looking for? Browse other questions tagged operators ada or ask your own question .

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Ada Programming/All Operators

• 1.1.1 Logical operators
• 1.1.2 Relational operators
• 1.1.3 Binary adding operators
• 1.1.4 Unary adding operators
• 1.1.5 Multiplying operator
• 1.1.6 Highest precedence operator
• 1.2 Short-circuit control forms
• 1.3.1 Range membership test
• 1.3.2 Subtype membership test
• 1.3.3 Class membership test
• 1.3.4 Range membership test
• 1.3.5 Choice list membership test
• 1.4.1 Wikibook
• 1.4.2 Ada 95 Reference Manual
• 1.4.3 Ada 2005 Reference Manual
• 1.4.4 Ada Quality and Style Guide
• 2.1.1 Concatenating arrays
• 2.2.1 Concatenating strings
• 2.3.1 Wikibook
• 2.3.2 Ada 95 Reference Manual
• 2.3.3 Ada 2005 Reference Manual
• 3.1.1.1.1 Usage
• 3.1.1.1.2 Working Example
• 3.2.1 Wikibook
• 3.2.2 Ada 95 Reference Manual
• 3.2.3 Ada 2005 Reference Manual
• 4.1.1.1.1 Usage
• 4.1.1.1.2 Working Example
• 4.1.2.1.1 Usage
• 4.1.2.1.2 Working Example
• 4.1.2.2.1 Usage
• 4.1.2.2.2 Working Example
• 4.2.1 Wikibook
• 4.2.2 Ada 95 Reference Manual
• 4.2.3 Ada 2005 Reference Manual
• 5.1.1.1.1 Usage
• 5.1.1.2.1 Usage
• 5.2.1 Wikibook
• 5.2.2 Ada 95 Reference Manual
• 5.2.3 Ada 2005 Reference Manual
• 6.1 Operator
• 6.2.1 Wikibook
• 6.2.2 Ada 95 Reference Manual
• 6.2.3 Ada 2005 Reference Manual
• 7.1.1.1.1 Usage
• 7.2.1 Wikibook
• 7.2.2 Ada Reference Manual
• 8.1 Operator
• 8.2.1 Wikibook
• 8.2.2 Ada 95 Reference Manual
• 8.2.3 Ada 2005 Reference Manual
• 9.1.1 Wikibook
• 9.1.2 Ada 95 Reference Manual
• 9.1.3 Ada 2005 Reference Manual
• 9.1.4 Ada Quality and Style Guide
• 10.1.1 Boolean operator
• 10.1.2 Boolean shortcut operator
• 10.1.3 Boolean operator on arrays
• 10.1.4 Bitwise operator
• 10.2 Adding interfaces to tagged types
• 10.3.1 Wikibook
• 10.3.2.1 Ada 2005 Reference Manual
• 10.3.3 Ada Quality and Style Guide
• 11.1 Operator
• 11.2.1 Wikibook
• 11.2.2 Ada 95 Reference Manual
• 11.2.3 Ada 2005 Reference Manual
• 12.1 Operator
• 12.2.1 Wikibook
• 12.2.2 Ada 95 Reference Manual
• 12.2.3 Ada 2005 Reference Manual
• 13.1.1 Wikibook
• 13.1.2 Ada 95 Reference Manual
• 13.1.3 Ada 2005 Reference Manual
• 13.1.4 Ada Quality and Style Guide
• 14.1 Operator
• 14.2.1 Wikibook
• 14.2.2 Ada 95 Reference Manual
• 14.2.3 Ada 2005 Reference Manual
• 15.1 Operator
• 15.2.1 Wikibook
• 15.2.2 Ada 95 Reference Manual
• 15.2.3 Ada 2005 Reference Manual
• 16.1.1 Wikibook
• 16.1.2 Ada 95 Reference Manual
• 16.1.3 Ada 2005 Reference Manual
• 16.1.4 Ada Quality and Style Guide
• 17.1.1 Wikibook
• 17.1.2 Ada 95 Reference Manual
• 17.1.3 Ada 2005 Reference Manual
• 17.1.4 Ada Quality and Style Guide
• 18.1.1 Boolean operator
• 18.1.2 Boolean shortcut operator
• 18.1.3 Boolean operator on arrays
• 18.1.4 Bitwise operator
• 18.2.1 alternative
• 18.2.2 delay
• 18.3.1 Wikibook
• 18.3.2 Ada 95 Reference Manual
• 18.3.3 Ada 2005 Reference Manual
• 18.3.4 Ada Quality and Style Guide
• 19.1.1.1 Arithmetic Addition
• 19.1.1.2.1 Usage
• 19.1.2.1 Type Conversion
• 19.2.1 Wikibook
• 19.2.2 Ada 95 Reference Manual
• 19.2.3 Ada 2005 Reference Manual
• 20.1 Operator rem
• 20.2.1 Wikibook
• 20.2.2 Ada 95 Reference Manual
• 20.2.3 Ada 2005 Reference Manual
• 20.2.4 Ada Quality and Style Guide
• 21.1.1 Boolean operator
• 21.1.2 Boolean operator on arrays
• 21.1.3 Bitwise operator
• 21.2.1 Wikibook
• 21.2.2 Ada 95 Reference Manual
• 21.2.3 Ada 2005 Reference Manual
• 21.2.4 Ada Quality and Style Guide
• 22.1 0. PREAMBLE
• 22.2 1. APPLICABILITY AND DEFINITIONS
• 22.3 2. VERBATIM COPYING
• 22.4 3. COPYING IN QUANTITY
• 22.5 4. MODIFICATIONS
• 22.6 5. COMBINING DOCUMENTS
• 22.7 6. COLLECTIONS OF DOCUMENTS
• 22.8 7. AGGREGATION WITH INDEPENDENT WORKS
• 22.9 8. TRANSLATION
• 22.10 9. TERMINATION
• 22.11 10. FUTURE REVISIONS OF THIS LICENSE
• 22.12 11. RELICENSING
• 23 How to use this License for your documents

Standard operators

Ada allows operator overloading for all standard operators and so the following summaries can only describe the suggested standard operations for each operator. It is quite possible to misuse any standard operator to perform something unusual.

Each operator is either a keyword or a delimiter —hence all operator pages are redirects to the appropriate keyword or delimiter .

Operators have arguments which in the RM are called Left and Right for binary operators, Right for unary operators (indicating the position with respect to the operator symbol).

The list is sorted from lowest precedence to highest precedence.

Logical operators

Relational operators

Binary adding operators

Unary adding operators

Multiplying operator

Highest precedence operator

Short-circuit control forms

These are not operators and thus cannot be overloaded.

Membership tests

The Membership Tests also cannot be overloaded because they are not operators.

Range membership test

Subtype membership test, class membership test, choice list membership test.

This language feature has been introduced in Ada 2012 .

Ada 2012 extended the membership tests to include the union (short-circuit or) of several range or value choices.

• Ada Programming

Ada 95 Reference Manual

• 4.5 Operators and Expression Evaluation ( Annotated )

Ada 2005 Reference Manual

Ada quality and style guide.

• 2.1.3 Alignment of Operators
• 5.7.4 Overloaded Operators
• 5.7.5 Overloading the Equality Operator

Operators: &

As operator, concatenating arrays.

Any array type (including fixed Strings) can be concatenated using the & operator. You can also append a single element to an array.

Common non-standard operations

Concatenating strings.

The & operator is also defined for Bounded_String and Unbounded_String.

• Ada Programming/Delimiters
• Ada Programming/Operators
• 4.4 Expressions ( Annotated )
• 4.5.3 Binary Adding Operators ( Annotated )
• A.4.4 Bounded-Length String Handling ( Annotated )
• A.4.5 Unbounded-Length String Handling ( Annotated )

Operators: **

Standard operations, arithmetic power of.

The "**" operator is defined as arithmetic power of for all numeric types.

Working Example

• 4.5.6 Highest Precedence Operators ( Annotated )

Operators: *

Arithmetic multiplication.

The "*" operator is defined as arithmetic multiplication for all numeric types.

Common Non-Standard Operations

Character replication.

A String is created where a single character is replicated n-times.

In addition to standard Strings this operator is also defined for Bounded_String and Unbounded_String.

The character replication operator is part of the Ada.Strings.Fixed package . You need to with and use the package to make the operator visible.

String replication

A String is created where a source string is replicated n-times.

In addition to standard fixed strings this operator is also defined for Bounded_String and Unbounded_String.

The string replication operator is part of the Ada.Strings.Fixed package . You need to with and use the package to make the operator visible.

• 4.5.5 Multiplying Operators ( Annotated )
• A.4.3 Fixed-Length String Handling ( Annotated )

Operators: -

Arithmetic subtraction.

The "-" operator is defined as arithmetic subtraction for all numeric types.

The "-" unary operator is defined as arithmetic negative sign for all numeric types.

• Ada Programming/Mathematical calculations
• 4.5.4 Unary Adding Operators ( Annotated )

Operators: /=

The operator /= compares two values on inequality. It is predefined for all non limited types . The operator will also be defined if a suitable operator = is available.

• 4.5.2 Relational Operators and Membership Tests ( Annotated )

Operators: /

Standard operations, arithmetic division.

The "/" operator is defined as arithmetic division for all numeric types.

Ada Reference Manual

Operators: =.

The operator = compares two values on equality. It is predefined for all non limited types .

Operators: abs

This keyword is used for the operator that gets the absolute value of a number.

• Ada Programming/Keywords
• 2.9 Reserved Words ( Annotated )
• Annex P (informative) Syntax Summary ( Annotated )
• 3.1.3 Capitalization
• 5.5.3 Parenthetical Expressions

Operators: and

Logical operator, boolean operator, boolean shortcut operator.

Shortcut operators are used to make the evaluation of parts of boolean expressions conditional: and then , or else . This should never be done to speed up the evaluation (with modern optimizing compilers, it will possibly not have that effect). The correct use is to prevent the evaluation of expressions known to raise an exception.

In the example above, G (Dog) is only called when the pointer Dog is not null , i.e. it actually points to something.

Actually and then and or else are not operators in the sense of the reference manual, they are called 'Short-circuit Control Forms'. The difference is that (true) operators can be redefined (i.e. overloaded), whereas these cannot. They are however defined for any boolean type.

Since Ada allows parallel evaluation of the arguments for an expression, shortcut operators are not the standard way of evaluating boolean expressions. In any case where the final result of the evaluation is guaranteed to be the same, the compiler is allowed to use a shortcut evaluation.

Boolean operator on arrays

The and operator is applied to each pair of boolean elements from the left and right arrays. The result has the same bounds as the left operand.

Bitwise operator

The operator and could be used with modular types to perform bitwise operations.

Adding interfaces to tagged types

This language feature is only available from Ada 2005 on.

• Ada Programming/Keywords/interface
• 3.9.4 Interface Types ( Annotated )

Operators: >=

The operator >= compares two values on greater than or equal to. It is predefined for all discrete types.

Operators: >

The operator > compares two values on being greater. It is predefined for all discrete types.

• Ada Programming/Delimiters/<
• Ada Programming/Delimiters/>>

Operators: in

This keyword is used in:

• The in and in out mode of subprograms parameters.
• membership tests
• quantified expressions
• 6.1 Subprogram Declarations ( Annotated )

Operators: <=

The operator <= compares two values on less than or equal to. It is predefined for all discrete types.

Operators: <

The operator < compares two values on less than. It is predefined for all discrete types.

• Ada Programming/Delimiters/>
• Ada Programming/Delimiters/<<

Operators: mod

This keyword is used in the mod operator and in the declaration of modular types .

Operators: not

• Logical negation operator
• Negative membership test : not in

Operators: or

In the below example the function G is only called when F(X) returns the value False .

This shortcut operator is sometimes used to speed up the evaluation of boolean expressions, but the Ada Style Guide recommends to compare the performance of both forms before switching one to the other. In general, it is good idea to use or else in sake of performance only when the second expression involves a function call.

The or else form is also used when the second expression is known to raise an exception unless the first expression is False .

Unlike C/C++, Ada short-cut operators are not the standard way to evaluate boolean expressions. This is because Ada is designed to do by default what is generally safer, but lets the programmer request a different behaviour.

The or operator is applied to each pair of boolean elements from the left and right arrays. The result has the same bounds as the left operand.

The operator or could be used with modular types to perform bitwise operations.

Select statement

Alternative.

See Ada Programming/Tasking#Selective waiting .

See Ada Programming/Tasking#Timeout .

• 4.5.1 Logical Operators and Short-circuit Control Forms ( Annotated )
• 5.5.5 Short Circuit Forms of the Logical Operators
• 10.5.2 Short-Circuit Operators
• 10.6.3 Bit Operations on Modular Types

Operators: +

Arithmetic addition.

The "+" operator is defined as arithmetic addition for all numeric types.

The "+" operator is defined as arithmetic plus sign for all numeric types.

Type Conversion

The operator plus sign is often used to create a type conversion operator:

Operators: rem

Operator rem.

The rem keyword is used as the remainder operator, that is, the remainder of the signed integer division. The following formula applies:

Operators: xor

The xor operation is applied to each boolean inside the array .

The operator xor could be used with modular types and also with boolean arrays to perform bitwise operations.

GNU Free Documentation License

Version 1.3, 3 November 2008 Copyright (C) 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc. < http://fsf.org/ >

Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.

0. PREAMBLE

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1. APPLICABILITY AND DEFINITIONS

This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The "Document", below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as "you". You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law.

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The "Invariant Sections" are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License. If a section does not fit the above definition of Secondary then it is not allowed to be designated as Invariant. The Document may contain zero Invariant Sections. If the Document does not identify any Invariant Sections then there are none.

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In the combination, you must combine any sections Entitled "History" in the various original documents, forming one section Entitled "History"; likewise combine any sections Entitled "Acknowledgements", and any sections Entitled "Dedications". You must delete all sections Entitled "Endorsements".

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You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.

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Ada implements the vast majority of programming concepts that you're accustomed to in C++ and Java: classes, inheritance, templates (generics), etc. Its syntax might seem peculiar, though. It's not derived from the popular C style of notation with its ample use of brackets; rather, it uses a more expository syntax coming from Pascal. In many ways, Ada is a simpler language — its syntax favors making it easier to conceptualize and read program code, rather than making it faster to write in a cleverly condensed manner. For example, full words like begin and end are used in place of curly braces. Conditions are written using if , then , elsif , else , and end if . Ada's assignment operator does not double as an expression, smoothly eliminating any frustration that could be caused by = being used where == should be.

All languages provide one or more ways to express comments. In Ada, two consecutive hyphens -- mark the start of a comment that continues to the end of the line. This is exactly the same as using // for comments in C++ and Java. There is no equivalent of /* ... /* block comments in Ada; use multiple -- lines instead.

Ada compilers are stricter with type and range checking than most C++ and Java programmers are used to. Most beginning Ada programmers encounter a variety of warnings and error messages when coding more creatively, but this helps detect problems and vulnerabilities at compile time — early on in the development cycle. In addition, dynamic checks (such as array bounds checks) provide verification that could not be done at compile time. Dynamic checks are performed at run time, similar to what is done in Java.

Ada identifiers and reserved words are case insensitive. The identifiers VAR , var and VaR are treated as the same; likewise begin , BEGIN , Begin , etc. Language-specific characters, such as accents, Greek or Russian letters, and Asian alphabets, are acceptable to use. Identifiers may include letters, digits, and underscores, but must always start with a letter. There are 73 reserved keywords in Ada that may not be used as identifiers, and these are:

abort else null select abs elsif of separate abstract end or some accept entry others subtype access exception out synchronized aliased exit overriding tagged all for package task and function pragma terminate array generic private then at goto procedure type begin if protected until body in raise use case interface range when constant is record while declare limited rem with delay loop renames xor delta mod requeue digits new return do not reverse

Ada is designed to be portable. Ada compilers must follow a precisely defined international (ISO) standard language specification with clearly documented areas of vendor freedom where the behavior depends on the implementation. It's possible, then, to write an implementation-independent application in Ada and to make sure it will have the same effect across platforms and compilers.

Ada is truly a general purpose, multiple paradigm language that allows the programmer to employ or avoid features like run-time contract checking, tasking, object oriented programming, and generics. Efficiently programmed Ada is employed in device drivers, interrupt handlers, and other low-level functions. It may be found today in devices with tight limits on processing speed, memory, and power consumption. But the language is also used for programming larger interconnected systems running on workstations, servers, and supercomputers.

The Craft of Coding

Musings on programming, coding ada: datatypes and assignment.

In Ada you will notice a couple of differences in the way things are coded (as compared to C).

data types (and types, and subtypes)

Ada has integers (I mean does any language not have integers?) They use the classic operators +, –, * and / for arithmetic, and ** for exponentiation. In addition there is rem for remainder and mod for modulus. Here is a typical integer declaration:

Dig a little deeper, and Ada makes it quite easy for the user to define their own types. For example:

Unfortunately Ada consider this to be a new type, unrelated to any other type (even if it is an integer). So you can’t add a variable with is of type eightbit to an integer. You can also create subtypes.  For example:

This creates a subtype positive which spans all positive integer values. Truth be told you can do a lot of different things with types, and subtypes. You could also create eightbit as a subtype.

To perform integer I/O you will need the package  ada.Integer_Text_IO .

Then of course there are real types. Here is an example of a float:

To perform float I/O you will need the package  Ada.Float_Text_IO .

To print out floats in a nice way, the put() function uses a number of optional parameters. The default is to display a real with a 3-digit exponent. So 1234.5678 would be displayed as 1.2345678E+003. The parameter Fore specifies how many characters before the decimal point, and Aft specifies after the decimal point. The parameter Exp specifies the number of digits in the exponent. Here is an example:

displays the output  ”   1234.57″ , with 7 spaces before the decimal point, 2 after, and no exponent.

There are of course more than one type of integer and float in Ada:

STANDARD LIBRARIES

A list of all the standard Ada95 libraries can be found here .

Assignment statements

Assignment is performed using the classic operator :=  . This of course is not that uncommon in languages which evolved before C. So an assignment is of the form:

This means that the equals operator, = ,  is used for testing equality in expressions. For example in an if statement:

If you try to use == in an if statement you will get a compiler error of the form:

Which stops assignments happening in decision statement logicals.

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Ada Operators and Conditional Expressions

• Operators (highest precedence first):
• **, abs, not
• *, /, mod, rem
• +, - (unary)
• +, -, & (binary)
• =, /=, <, <=, >, >=
• and, or, xor
• Notes on the above:
• The above operators can be redefined by the user.
• Parentheses required in some situations (eg a and b or c)
• rem and mod differ for negative numbers
• & is string concatenation
• Other operator-like items:
• and then, or else (short circuit - only evaluate second operand if needed)
• if a and then b then -- b only evaluated if a is true
• if a or else b then -- b only evaluated if a is false
• in, not in (examples: if i in Natural, if i not in 1 .. 10 | 20 .. 30)
• Conditional Expressions:
• Evaluates to one of several values
• Two forms: if and case
• x := (if a = b then 3 else 4);
• put(case day of Mon|Wed|Fri => 2, Tu|Th => 1, others => 0);
• Must be immediately surrounded by parentheses

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Is it correct to call the assignment symbol an "operator" when it is actually a statement?

In some languages (C++, Java, Ruby, etc.) an assignment returns a value and can be used in an expression:

In other languages (Ada, VHDL), an assignment is a proper statement and cannot be used in as expression:

However, in both cases, it is possible to find teaching material where the assignment symbol is called the "assignment operator".

My question is, is it technically correct to call the assignment symbol an "operator" in the second case, where it is actually a statement, does not return a value, and cannot be used in the middle of an expression?

• terminology

• 2 define technically correct –  gnat May 13, 2014 at 17:22
• @gnat, good point. I thought this meant as specified in a technical document - for instance, in a language reference manual or technical standard. It could also mean a textbook definition of what an operator is. –  rick May 13, 2014 at 17:35

The assignment statement is made up of three parts:

• The target , or lvalue.
• The assignment operator .
• The value to be assigned, or rvalue.

The rvalue can be a constant or an expression that returns a value of the correct (or not necessarily correct, in some languages) type. The requirement is that the value to be assigned itself forms a valid rvalue under the rules of the specific language.

As you point out, some languages make an assignment statement return the assigned value. C, C++, C#, Java, and a whole slew of others do it that way, some with restrictions. This is a potentially dangerous practice, but it is an incredibly useful shorthand notation in the hands of those who know how to use it. As we know, that means people will misuse it, either deliberately or because they don't understand the finer details of the syntax involved. The technical way to express this is that the assignment statement does form a rvalue.

So some other languages make assignment statements valid only as stand-alone statements, and dictate that the assignment operator is only valid inside a valid assignment statement. In other words, in these languages a complete assignment statement is not a valid rvalue.

Both are valid ways to design a language, depending on one's goals. Always keep in mind that C was designed to be able to do basically anything assembly language can do, only in a portable and preferably more readable fashion, and many C-like languages derive many features directly from C, even if the syntax is slightly different. Ada on the other hand was designed to make it extremely hard to write programs that do anything but exactly what is expected.

It follows from the above reasoning that = in C, C++, C#, Java, ...; := in Pascal, Ada, ...; and so on, can all properly be called the assignment operator , which forms one part of a complete assignment statement. Whether a complete assignment statement forms a valid rvalue is a different matter and really has nothing to do with the status of the character sequence as such which is used to indicate assignment to a lvalue.

• Thanks for the nice background, but what does it take for a symbol to be called an "operator"? I thought it needed to return a value. In your answer, if we replace "assignment operator" with "assignment symbol" everything would match what I expected, but as it is now I don't see a basis to call such symbol an operator. Could you elaborate on that? –  rick May 13, 2014 at 17:43
• @rick The difference is whether or not the complete assignment statement forms a valid rvalue . The people who made Ada decided that the assignment statement is not a rvalue (which makes x := (y := z); invalid, because the value side of an assignment by definition must be a rvalue), while the people who designed C decided that an assignment statement is a valid rvalue (making x = (y = z); valid). I've re-read your question and honestly don't see how this (most of which is already in the answer) plus the topmost part of the answer does not answer your question. –  user May 13, 2014 at 17:57
• The operator itself forms a part of the statement, just as the binary + (the addition) operator forms one part of a statement. –  user May 13, 2014 at 17:58
• @rick Anyway, I have edited my answer to try to make it more clear. Does it make more sense now? –  user May 13, 2014 at 18:03

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Overloading the assignment operator