How to Use Functions in C - Explained With Examples

Valentine Gatwiri

Functions are an essential component of the C programming language. They help you divide bigger problems into smaller, more manageable chunks of code, making it simpler to create and run programs.

We'll look at functions in C, their syntax, and how to use them successfully in this article.

What is a Function in C?

A function is a block of code that executes a particular task in programing. It is a standalone piece of code that can be called from anywhere in the program.

A function can take in parameters, run computations, and output a value. A function in C can be created using this syntax:

The return_type specifies the type of value that the function will return. If the function does not return anything, the return_type will be void .

The function_name is the name of the function, and the parameter list specifies the parameters that the function will take in.

How to Declare a Function in C

Declaring a function in C informs the compiler about the presence of a function without giving implementation details. This enables the function to be called by other sections of the software before it is specified or implemented.

A function declaration usually contains the function name , return type , and the parameter types. The following is the syntax for defining a function in C:

Here, return_type is the data type of the value that the function returns. function_name is the name of the function, and parameter_list is the list of parameters that the function takes as input.

For example, suppose we have a function called add that takes two integers as input and returns their sum. We can declare the function as follows:

This tells the compiler that there is a function called add that takes two integers as input and returns an integer as output.

It's worth noting that function declarations do not include the function body, which includes the actual code that runs when the function is invoked.

The body of the function is defined independently of the function statement, usually in a separate block of code called the function definition.

Here's an example:

In this example, the add function is declared with a function statement at the top of the file, which specifies its name, return type ( int ), and parameters ( a and b , both int s).

The actual code for the add function is defined in the function definition. Here, the function simply adds its two parameters and returns the result.

The main function calls the add function with arguments 2 and 3 , and stores the result in the result variable. Finally, it prints the result using the printf function.

How to Use a Function in Multiple Source Files

If you want to use a function in numerous source files, you must include a function declaration (also known as a function prototype) in the header file and the definition in one source file.

when you build, you first compile the source files to object files, and then you link the object files into the final executable.

Let's create a header file called myfunctions.h :

In this header file, we declare a function add using a function statement.

Next, let's create a source file called myfunctions.c , which defines the add function:

In this file, we include the myfunctions.h header file using quotes, and we define the add function.

Finally, let's create a source file called main.c , which uses the add function:

In this file, we include both the stdio.h header file and our myfunctions.h header file using angle brackets and quotes, respectively. We then call the add function, passing in values a and b and storing the result in sum . Finally, we print the result using printf .

The way you create it is heavily influenced by your environment. If you are using an IDE (such as Visual Studio), you must position all files in the proper locations in the project.

If you are creating from the command line e.g Linux. To compile this program, you would need to compile both myfunctions.c and main.c and link them together as shown below:

The -c option instructs the compiler to create an object file with the same name as the source file but with a .o suffix. The final instruction joins the two object files to create the final executable, which is named program (the -o option specifies the name of the output file).

What Happens if You Call a Function Before Its Declaration in C?

In this instance, the computer believes the usual return type is an integer. If the function gives a different data type, it throws an error.

If the return type is also an integer, it will function properly. But some cautions may be generated:

In this code, the function function() is  called before it is declared. This returns an error:

Screenshot-from-2023-04-05-14-03-36

How to Define a Function in C

Assuming you want to create a code that accepts two integers and returns their sum, you can define a function that does that this way:

In this example, the function sum takes in two integer parameters – num1 and num2 . The function calculates their sum and returns the result. The return type of the function is int .

Where Should a Function Be Defined?

In C, a function can be defined anywhere in the program, as long as it is defined before it is used. But it is a good practice to define functions at the beginning of the file or in a separate file to make the code more readable and organized.

Here's an example code showing how to define a function in C:

In this example, the function add() is defined after its declaration (or prototype) within the same file.

Another approach is to define the function in a separate header file, which is then included in the main file using the #include directive. For example:

In this approach, the function declaration (or prototype) is included in the header file math.h , which is then included in the main file main.c using the #include directive. The function implementation is defined in a separate file math.c .

This approach allows for better code organization and modularity, as the function implementation can be separated from the main program code.

How to Call a Function in C

We can call a function from anywhere in the program once we've defined it. We use the function name followed by the argument list in parentheses to call a function. For example, we can use the following code to call the sum function that we defined earlier:

In this code, we are calling the sum function with a and b as its parameters. The function returns the sum of a and b , which is then stored in the variable c .

How to Pass Parameters to a Function

There are two methods of passing parameters (also called arguments) to a function in C: by value and by reference.

When we pass a parameter by value, the method receives a copy of the parameter's value. Changes to the parameter within the code have no effect on the initial variable outside the function.

When we pass a parameter by reference, the method receives a link to the parameter's memory location. Any modifications to the parameter within the code will have an impact on the initial variable outside the function.

Consider the following examples of passing parameters by value and by reference. Assuming we want to create a function that accepts an integer and multiplies it by two, the function can be defined as follows:

In this example, the function doubleValue takes in an integer parameter num by value. It doubles the value of num and assigns it back to num . However, this change will not affect the original value of num outside the function.

Here's another example that shows how you can pass a single parameter by value:

In this example, we define a function called square that takes an integer parameter num by value. Inside the function, we calculate the square of num and print the result. We then call the function with the argument 5 .

Now, let's look at an example of passing a parameter by reference:

In this example, we define a function square that takes an integer pointer parameter num by reference. Inside the function, we reference the pointer and calculate the square of the value pointed to by num .

We then call the function with the address of the integer variable x . After calling the function, the value of x is modified to be the square of its original value, which we then print in the main function.

In conclusion, functions are an essential component of C programming. You can use them to divide large problems into smaller, more manageable pieces of code.

You can declare and define functions in C, and pass parameters either by value or by reference. It's a good practice to declare all functions before using them, and to define them at the beginning of the file or in a separate file for better code organization and modularity.

By using functions effectively, you can write cleaner, more readable code that is easier to debug and maintain.

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cppreference.com

Std:: function.

Class template std::function is a general-purpose polymorphic function wrapper. Instances of std::function can store, copy, and invoke any CopyConstructible Callable target -- functions (via pointers thereto), lambda expressions , bind expressions , or other function objects, as well as pointers to member functions and pointers to data members.

The stored callable object is called the target of std::function . If a std::function contains no target, it is called empty . Invoking the target of an empty std::function results in std::bad_function_call exception being thrown.

std::function satisfies the requirements of CopyConstructible and CopyAssignable .

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C Functions

C structures, c function parameters, parameters and arguments.

Information can be passed to functions as a parameter. Parameters act as variables inside the function.

Parameters are specified after the function name, inside the parentheses. You can add as many parameters as you want, just separate them with a comma:

The following function that takes a string of characters with name as parameter. When the function is called, we pass along a name, which is used inside the function to print "Hello" and the name of each person.

When a parameter is passed to the function, it is called an argument . So, from the example above: name is a parameter , while Liam , Jenny and Anja are arguments .

Multiple Parameters

Inside the function, you can add as many parameters as you want:

Note that when you are working with multiple parameters, the function call must have the same number of arguments as there are parameters, and the arguments must be passed in the same order.

Pass Arrays as Function Parameters

You can also pass arrays to a function:

Example Explained

The function ( myFunction ) takes an array as its parameter ( int myNumbers[5] ), and loops through the array elements with the for loop.

When the function is called inside main() , we pass along the myNumbers array, which outputs the array elements.

Note that when you call the function, you only need to use the name of the array when passing it as an argument myFunction(myNumbers) . However, the full declaration of the array is needed in the function parameter ( int myNumbers[5] ).

Return Values

The void keyword, used in the previous examples, indicates that the function should not return a value. If you want the function to return a value, you can use a data type (such as int or float , etc.) instead of void , and use the return keyword inside the function:

This example returns the sum of a function with two parameters :

You can also store the result in a variable:

Real-Life Example

To demonstrate a practical example of using functions, let's create a program that converts a value from fahrenheit to celsius:

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Pass arrays to a function in C

In C programming, you can pass an entire array to functions. Before we learn that, let's see how you can pass individual elements of an array to functions.

Pass Individual Array Elements

Passing array elements to a function is similar to passing variables to a function .

Example 1: Pass Individual Array Elements

Here, we have passed array parameters to the display() function in the same way we pass variables to a function.

We can see this in the function definition, where the function parameters are individual variables:

Example 2: Pass Arrays to Functions

To pass an entire array to a function, only the name of the array is passed as an argument.

However, notice the use of [] in the function definition.

This informs the compiler that you are passing a one-dimensional array to the function.

Pass Multidimensional Arrays to a Function

To pass multidimensional arrays to a function, only the name of the array is passed to the function (similar to one-dimensional arrays).

Example 3: Pass two-dimensional arrays

Notice the parameter int num[2][2] in the function prototype and function definition:

This signifies that the function takes a two-dimensional array as an argument. We can also pass arrays with more than 2 dimensions as a function argument.

When passing two-dimensional arrays, it is not mandatory to specify the number of rows in the array. However, the number of columns should always be specified.

For example,

Recommended Reading: Call by Reference in C

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Assigning function to a variable in C++

In C++ , assigning a function to a variable and using that variable for calling the function as many times as the user wants, increases the code reusability. Below is the syntax for the same:

Program 1: Below is the C++ program to implement a function assigned to a variable:

Program 2: Below is the C++ program to implement a parameterized function assigned to a variable:

Program 3: Below is the C++ program to implement a function assigned to a variable that returns a value:

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What kind of Experience do you want to share?

Is this way of type-checking possible?

Let’s say that we have a function that clones an instance in replicatedstorage and returns it. These are the function’s inputs and outputs.

Input: (instance_name: string) --the name of the instance to be copied Output: (cloned_instance: Instance)

Is there a way to give the outputted instance a type that is somehow calculated properly? By that, I mean one instance in replicated_storage could hypothetically be a BasePart, but another is a TextButton. Is there a way to assign the correct types to the specific object through the output?

If anybody is confused, please let me know and I can try to rephrase.

Thanks for the reply. I think I used “type checking” wrongly. Let me try to use an example of how this would work.

The type checker gives “Text” as an optionable property since the cloned_instance output of the function somehow spits out the correct type. I’m really sorry, this is hard to explain.

Right now, I am stuck with having to put types in every variable here.

image

I’m wondering if there is a way for the function to do it automatically.

@TheLazyRaven

If that’s what you mean.

Sorry. I’m talking about Roblox’s autocomplete and type checker system and getting it to provide the proper properties to the output instance.

ooo no clue. Sorry good luck sounds like a pain

If this returns a TextButton , you could do:

No, as cloning an Instance could return any Instance. Your function will have to return the broad type of Instance and then your code which calls it will have to refine or cast the result afterwards.

Its not possible as studio will not look at the arguments you provided when type checking, the best thing you could do is to have separate functions for all the types.

Yeah. I’m just trying to see if it can be done more automatedly. Also, what if the cloned instance has children?

If you want studio to be able to see the children of the instance you are trying to clone (here we are talking specifically if you are in studio editing, not in-game) you shouldn’t specify the type of the instance you are cloning, as when you do studio focuses only on that instance and ignores the children. To add onto my other post, the closest to what you are trying to achieve would be something like:

This would work the way you want it to, however we are creating way too many functions for the simple task of cloning. To avoid using all those functions we could just have table of references:

However this wouldn’t be able to automatically parent the newly cloned instance somewhere, instead you would have to do it yourself. So simply put you can’t have both simpler process of cloning and not having to repeat yourself, as creating a function that will handle every instance removes the ability to type check, whilst other way you have to constantly type the same code.

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Is Bird Flu Coming to People Next? Are We Ready?

Unlike the coronavirus, the H5N1 virus has been studied for years. Vaccines and treatments are available should they ever become necessary.

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A young chicken in its coop on a farm.

By Apoorva Mandavilli

Bird flu outbreaks among dairy cows in multiple states, and at least one infection in farmworker in Texas, have incited fears that the virus may be the next infectious threat to people.

The influenza virus, called H5N1, is highly pathogenic, meaning it has the ability to cause severe disease and death. But while its spread among cows was unexpected, people can catch the virus only from close contact with infected animals, not from one another, federal officials said.

“It’s really about folks who are in environments where they may be interacting with cattle that are infected with this virus,” said Dr. Demetre Daskalakis, the director of the National Center for Immunization and Respiratory Diseases at the Centers for Disease Control and Prevention.

“The risk for most everyone else is very low,” he added. “Right now, our risk assessment hasn’t changed, but if it does change, we’re going be pretty quick and pretty transparent about that.”

Avian influenza is often fatal in birds, but none of the infected cows have died so far. The only symptom in the patient in Texas was conjunctivitis, or pink eye, which was also reported in people infected during other bird flu outbreaks.

The C.D.C. and other agencies in the United States and elsewhere have tracked H5N1 for years to monitor its evolution. Federal agencies have stockpiled vaccines and drugs to be used in a possible bird flu outbreak.

“We are more ready for an influenza pandemic than probably any other outbreak that could occur, any other pathogen,” said Rick Bright, the chief executive of Bright Global Health, a consulting company that focuses on improving responses to public health emergencies.

Dr. Bright led influenza preparedness at the Biomedical Advanced Research and Development Authority, or BARDA, the federal agency that supports research on vaccines and drugs for emergencies, for several years before he served as the agency’s director from 2016 to 2020.

Here’s what you need to know about the H5N1 virus:

Is a human pandemic inevitable?

Among birds and animals, the H5N1 bird flu is already a pandemic, or a panzootic, with infections observed on every continent except Australia. To date, the virus has not evolved into a form that can spread easily from one person to another, and it may never do so.

As its name indicates, H5N1 has primarily been a problem in birds. But it has now spread to a wide range of species, from sea birds and small scavengers like foxes to large mammals, like bears and cows.

There have been sporadic infections in people since 1997, when a cluster of cases appeared in Hong Kong. But most patients worldwide have been in very close contact with infected animals, and generally they did not pass the virus to other people.

To become adept at transmission between people, H5N1 would need to pick up several additional mutations and shift its shape. The strain that was isolated from the infected farmworker in Texas carries one of those mutations, but that change has appeared before — in people, foxes and seals, among others — without further consequences.

The infections in people so far “fortunately are all still single-time cross-species transmission,” said Vincent Munster, a virologist at the National Institute of Allergy and Infectious Diseases who has studied the mutations needed for H5N1 to adapt to people.

History suggests that even if the virus changes enough to begin widespread transmission between people, it may have to give something up in return, Dr. Munster said. For example, when other flu viruses have adapted to humans , they have lost much of their virulence, causing only mild symptoms.

How will we know if the virus spreads to people?

H5N1 is an influenza virus that is followed by extensive scientific networks that track flu viruses worldwide.

“We’ve got our eye on it, and we’ve had our eye on it for years,” Dr. Daskalakis said.

These surveillance networks tracked H5N1 even before its surge in birds and animals over the past two years. Now they are on high alert. Scientists are watching for mutations that could make H5N1 more likely to infect people or resistant to the vaccines and drugs available to counter it.

The World Health Organization, the C.D.C. and other global health organizations routinely share information and genetic sequences to monitor which flu strains are spreading and where.

In the current outbreak, the Department of Agriculture has shared genetic sequences from infected cows with the C.D.C., which analyzes the sequences and ensures that the stockpiled vaccines and drugs are still effective.

Do we have a vaccine for bird flu?

BARDA has enough building blocks for vaccines — including adjuvants, substances that can enhance a vaccine’s strength — to make millions of doses in weeks. Mass production could also ramp up quickly if needed, federal officials said.

The C.D.C. already has two candidate viruses that can be used to make vaccines. As the virus changes — gaining mutations that make it resistant to the current vaccines and drugs, for example — federal researchers may create newer candidates.

Three pharmaceutical companies can be called on to make vaccines for bird flu, but those vaccines would be manufactured on the same production lines that are used to make seasonal flu vaccines. Before embarking on large-scale manufacturing, federal officials would have to consider the implications of disrupting seasonal production, said David Boucher, the infectious disease director at the federal Administration for Strategic Preparedness and Response.

The pharmaceutical companies don’t all use egg-based methods to produce vaccines, an important consideration because of the potential for bird flu outbreaks to derail the nation’s egg supply. BARDA is also looking to add mRNA to the list of technologies that can be used to make bird flu vaccines. (The Covid-19 vaccines made by Pfizer and Moderna relied on the method.)

What about treatments?

At least four antiviral medications are available to treat people who may become sick with bird flu, including the widely available generic drug oseltamivir, sometimes marketed as Tamiflu.

Unlike the vaccines, which are stockpiled by the federal government, the antiviral drugs are available commercially. Generic versions of oseltamivir are made by many manufacturers worldwide.

The federal government has a stockpile of tens of millions of doses of oseltamivir, Dr. Boucher said. The government is in close communication with manufacturers that could quickly ramp up the production of oseltamivir, as it has in the past during some bad flu seasons.

All of these preparations are in place for a worst-case scenario, but “we’re not there yet,” Dr. Boucher said. “Our job here is to prepare for the worst and get ready for it in case it does come.”

Apoorva Mandavilli is a reporter focused on science and global health. She was a part of the team that won the 2021 Pulitzer Prize for Public Service for coverage of the pandemic. More about Apoorva Mandavilli

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COMMENTS

  1. ::assign

    Assigns a new value to the string, replacing its current contents. (1) string Copies str. (2) substring Copies the portion of str that begins at the character position subpos and spans sublen characters (or until the end of str, if either str is too short or if sublen is string::npos). (3) c-string Copies the null-terminated character sequence (C-string) pointed by s.

  2. vector :: assign() in C++ STL

    vector:: assign () is an STL in C++ which assigns new values to the vector elements by replacing old ones. It can also modify the size of the vector if necessary. The syntax for assigning constant values: vectorname.assign(int size, int value) Parameters: size - number of values to be assigned.

  3. c

    foo and &foo values are equivalent in C and have same type. The & operator here is correct but redundant. Note that assigning a function pointer to a void * is not valid in C. void *fp1 = foo; // invalid int (*fp2)() = foo; // valid int (*fp3)() = &foo; // valid (These are actually declarations but the constraints of the assignment operator apply.)

  4. std::vector<T,Allocator>::assign

    std::vector<T,Allocator>:: assign. Replaces the contents of the container. 1) Replaces the contents with count copies of value value. 2) Replaces the contents with copies of those in the range [first,last). The behavior is undefined if either argument is an iterator into *this . This overload has the same effect as overload (1) if InputIt is an ...

  5. ::assign

    Parameters fn Either a function object of the same type, or some function, function pointer, pointer to member, or function object, as forwarded to function's constructor. Return value none Data races The object is modified (including both its target and its allocator). If fn is an rvalue reference, the function may modify fn. Exception safety If fn is a function pointer or a reference_wrapper ...

  6. How to Use Functions in C

    For example, we can use the following code to call the sum function that we defined earlier: int a = 5; int b = 10; int c = sum(a, b); In this code, we are calling the sum function with a and b as its parameters. The function returns the sum of a and b, which is then stored in the variable c.

  7. std::function<R(Args...)>::assign

    function::assign (until C++17) function::operator bool. function::operator() function::target_type. function::target. Non-member functions: operator== operator!= ... callable function to initialize the target with alloc - allocator to use to allocate memory for the internal data structures Return value (none)

  8. std::string::assign() in C++

    std::string::assign () in C++. The member function assign () is used for the assignments, it assigns a new value to the string, replacing its current contents. Syntax 1: Assign the value of string str. string& string::assign (const string& str) str : is the string to be assigned. Returns : *this.

  9. Assignment operators

    for assignments to class type objects, the right operand could be an initializer list only when the assignment is defined by a user-defined assignment operator. removed user-defined assignment constraint. CWG 1538. C++11. E1 ={E2} was equivalent to E1 = T(E2) ( T is the type of E1 ), this introduced a C-style cast. it is equivalent to E1 = T{E2}

  10. Strings in C (With Examples)

    C Programming Strings. In C programming, a string is a sequence of characters terminated with a null character \0. For example: char c[] = "c string"; When the compiler encounters a sequence of characters enclosed in the double quotation marks, it appends a null character \0 at the end by default. Memory Diagram.

  11. std::function

    Class template std::function is a general-purpose polymorphic function wrapper. Instances of std::function can store, copy, and invoke any CopyConstructible Callable target-- functions (via pointers thereto), lambda expressions, bind expressions, or other function objects, as well as pointers to member functions and pointers to data members.. The stored callable object is called the target of ...

  12. C Function Parameters

    The function ( myFunction) takes an array as its parameter ( int myNumbers[5] ), and loops through the array elements with the for loop. When the function is called inside main(), we pass along the myNumbers array, which outputs the array elements. Note that when you call the function, you only need to use the name of the array when passing it ...

  13. ::assign

    In the range version (1), the new contents are elements constructed from each of the elements in the range between first and last, in the same order. In the fill version (2), the new contents are n elements, each initialized to a copy of val. In the initializer list version (3), the new contents are copies of the values passed as initializer list, in the same order.

  14. Pass arrays to a function in C

    To pass an entire array to a function, only the name of the array is passed as an argument. result = calculateSum(num); However, notice the use of [] in the function definition. float calculateSum(float num[]) {. ... .. } This informs the compiler that you are passing a one-dimensional array to the function.

  15. list assign() function in C++ STL

    list assign () function in C++ STL. The list::assign () is a built-in function in C++ STL which is used to assign values to a list. It can also be used to copy elements from one list to another. To assign elements to a list. Syntax: list_name.assign(count, value) Parameters: This function accepts two mandatory parameters as shown in the above ...

  16. Assigning function to a variable in C++

    Assigning function to a variable in C++. In C++, assigning a function to a variable and using that variable for calling the function as many times as the user wants, increases the code reusability. Below is the syntax for the same: Syntax: C++. auto fun = [&]() {. cout << "inside function".

  17. Is this way of type-checking possible?

    Let me try to use an example of how this would work. local button = cloner.clone_instance("my_text_button") button.Text = "Hello world". ^. |. The type checker gives "Text" as an optionable property since the cloned_instance output of the function somehow spits out the correct type. I'm really sorry, this is hard to explain.

  18. c++

    The built-in assignment operator =. is right-associative which means it groups to right, e.g. a = b = c means a = (b = c), is an lvalue expression that refers to the left hand side.. Note that in C an assignment produces a pure value, while in C++ it produces a reference (in the common language meaning of referring).. This means that you can assign a value to multiple variables:

  19. Is Bird Flu Coming to People Next? Are We Ready?

    April 5, 2024, 1:23 p.m. ET. Bird flu outbreaks among dairy cows in multiple states, and at least one infection in farmworker in Texas, have incited fears that the virus may be the next infectious ...

  20. c++

    Can we assign values to function: To answer the actual title of your question: Yes, we can, using function pointers: return x + 1; return x * 2; int(*f)(int) = foo; // int the function pointed to returns an int. // (*f) f is the name of the function pointer. // (int) the function pointed to takes on int as paramter.

  21. c++

    I need to assign different methods (member functions) with different return types to a variable defined as auto variable. So the following code that returns the same data type is working as expected: #include <iostream> // Function pointers int add(int a, int b) { return a + b; } int subtract(int a, int b) { return a - b; } // Define a type for ...