User-Defined Functions in C

In the C programming language, user-defined functions are another term for functions that are created and defined by the user, and are useful for writing efficient, clean, modular, and reusable code. In every instance of programming, whether it’s a simple type of program or large-scale software, the benefits of knowing user-defined functions will make your life easier when developing code. In this article, we will define a user-defined function, its structure, syntax, how to call it, how to pass parameters, advantages of using it, and a recursive user-defined function with an example in C.

Table of Contents:

• What is a User-Defined Function in C?
• Structure of User-Defined Function in C
• Methods to Call User-Defined Functions in C
• Passing Parameters to User-Defined Functions in C
• Scope of Variables in User-Defined Functions in C
• Recursive User-Defined Functions in C
• Benefits of User-Defined Functions in C
• Conclusion

What is a User-Defined Function in C?

A user-defined function in C is a function that is created and defined by the user to do a specific programming task. It is a type of function in C. This function helps the user to organize programs in an efficient, modular, and reusable manner. The user-defined functions also make debugging easier and can be used in different parts of the program. It also helps to improve the program’s readability and saves effort and time of the user.

Structure of User-Defined Function in C

In C programming, the structure of a user-defined function typically has three main components:

1. Function Declaration (Prototype)
2. Function Definition
3. Function Call

1. Function Declaration (Prototype) in C

A function declaration is a function prototype in C that tells the compiler about the name, parameters, and return type of the function before it is used in the program. It helps the compiler to make sure that the function is called with the correct number and types of arguments. The function declaration is usually written above the main function in the program.

Syntax:

return_type function_name(parameter_list);

Here,

• return_type: This is the value type that the function returns.
• function_name: This is the name that the user will give the function, but it must also adhere to C naming conventions.
• parameter_list: The list of input variables that the function accepts.

Example:

void greet();
int add(int a, int b);

Explanation: The code shows that the void greet(); declares the greet function that takes no arguments and returns nothing, and int add(int a, int b); declares an add function that takes two integers as input and returns an integer.

2. Function Definition in C

A function definition in C has the actual body of the function, that block of code which executes when it is called. A function definition consists of a return type, the function name, an optional parameter list, and the instructions inside the function braces.

Syntax:

return_type function_name(parameter_list) {
    // Body of the function
    return value;  
}

Here,

• return_type: This is the type of value that will be returned from the function.
• function_name: This is the name the user is going to give the function.
• parameter_list: This is the list of input variables the function accepts.
• Function Body: The code block that performs the required operation.
• Return value: This is the value returned from the function.

Example:

int add(int a, int b) {
    int sum = a + b;
    return sum;
}

Explanation: The code shows that the add function takes two integers, a and b, adds them, stores the result in sum, and then returns the value of sum as an integer.  

3. Function Call in C

In C, a function call is a method by which the user executes a user-defined function. A function call takes control from the calling program to a called function, and when it does this, the function, depending on how it is defined, may have options for passing arguments and receiving return values.

Syntax:

function_name(arguments);

Here,

• function_name: It is the name that will be given by the user to the function to call.
• arguments: These are the values the user will pass to the function’s parameters.

Example:

int result = add(5, 3);

Explanation: The code shows how the add function with arguments 5 and 3 is used to assign the value and return the variable result.

Example of a User-Defined Function in C

Output:

The code shows how a user-defined function, add, is created, which takes two integers, 15 and 25, adds them, and returns the sum. The add function is called in main(), and the result of 40 is printed using printf(). This program also shows how the function is declared, defined, and called. 

Methods to Call User-Defined Functions in C

Below are the four main methods that can be used to call a user-defined function in C.

1. Call by Value

Call by value is the default method of passing arguments to functions in C programming, in which the actual values of the variable are passed to the function. But the function receives a copy of the variable, so if any changes are made inside the function will not affect the original value. It is a safe and most commonly used method for simple data types.

Example:

Output:

The code shows how the modify function changes only the copy of the variable a and not its original value because the function is passed by value.

2. Calling Functions with No Arguments

Calling functions with no arguments is a method that does not take any input parameters and performs the task independently. It may or may not return a value, and is used to display messages. In this method, no data is passed to a function.

Example:

Output:

The code shows how the greet function is called from the main function, which does not take any arguments and only prints the message “Hello, welcome to C programming!” when it is called.

3. Calling Functions with No Return Value

Calling functions with no return value is a method in which the function performs a task but does not return any value when it is called. The function returns nothing, thus, its return type is void. In this method, the function may or may not take arguments. It is used to display output or perform operations that do not need a result.

Example:

Output:

The code shows how the displaySum() function is called, which takes two integers, 10 and 20, and prints their sum, 30, to the console, but it does not return any value to the calling function.

4. Calling Functions with Parameters and Return Value

Calling functions with parameters and a return value is a method in which the function takes input parameters and then returns a value to the calling function. It is most commonly used and a flexible type of user-defined function. The return type of the function can be anything, e.g., int, float, char, etc.

Example:

Output:

The code shows how the multiply function takes two integers, 6 and 7, as input, calculates the product, returns the value to the main function when it is called, and then the product 42 is stored in the result and gets printed to the console.

Passing Parameters to User-Defined Functions in C

Parameters in C can be passed to user-defined functions to provide input values in two ways: Pass by Value and Pass by Reference.

1. Pass by Value in C

Pass by value is a method in which a copy of the actual value is passed to the function. The function works on this copy, so any changes made inside the function do not affect the original variable in the calling function. It is a default method in C. It is suitable for protecting original values, thus the original values remain unchanged.

Example:

Output:

The code shows how the modify function changes only the copy of the variable a and not the original value, because the parameters are passed by value. Thus, the value of a remains the same outside the function and is then printed to the console.

2. Pass by Reference in C

Pass by reference is the method of passing the address of a variable to a function, so the function can directly change the value of the original variable. In C programming, it is done by using pointers. By passing by reference, the function receives the address of the variable. It is used when there is a need to change multiple values in the function.

Example:

Output:

The code shows how the modify function uses a pointer to change the value of the variable a directly, since the function receives the address of the variable a. Thus, the change in the value is also done outside the function, and then printed to the console.

Scope of Variables in User-Defined Functions in C

The area of the program where a variable in C can be accessed is known as its scope. Knowing the range of variables in a user-defined function is crucial for controlling data flow and memory.

1. Local Variables

Local variables are declared in a user-defined function in C and can only be used in the function itself. Local variables are created when the function is called and destroyed when the function returns.

Example:

Output:

The code shows how the variable x is local to the show() function and cannot be used outside the function, and it only exists until the function is doing the operation.

2. Global Variables

The global variables in C are declared outside the function and can be accessed and modified from any function in the same program. The scope of global variables is the entire program, which enables their use for sharing multiple functions.

Example:

Output:

The code shows how the global variable x is accessible in both the functions main and display, and thus the variable is shared across the functions, and then printed to the console. 

3. Static Variables

The static variables are declared by using the static keyword in C, which allows them to hold their value between function calls. But it still has a scope that is local to the function in which it was declared.

Example:

Output:

The code shows how the counters are printed to the console after the static variable count maintains its value throughout several calls to the counter() function.

Recursive User-Defined Functions in C

A user-defined function that calls itself to perform a specific task is known as a recursive user-defined function in C. The tasks are divided into smaller, more straightforward, and related subtasks using recursion. To prevent infinite recursion or a stack overflow error, a recursive function needs a base case. Most often, this function is used in divide-and-conquer or mathematical problems.

Syntax:

return_type function_name(parameters) {
    if (base_condition) {
        return result;
    } else {
        return function_name(modified_parameters);  // Recursive call
    }
}

Example:

Output:

The code shows how the factorial function calls itself until it reaches the base case, where n == 0, and then multiplies the result during the return phase to return 120.

Benefits of User-Defined Functions in C

• The user-defined functions divide the program into smaller and manageable parts, thus making the code modular.
• It helps to organize the code in such a way that each function can handle a particular task.
• User-defined functions can be written once and reused multiple times in the program by the user.
• The errors in a user-defined function can be found and fixed easily.
• The user-defined functions also help to improve the readability of the program.
• These functions also reduce the code repetition in the program.
• In big projects, different functions can be written and tested by the users independently according to their needs.

Conclusion

User-defined functions in C are essential for producing code that is efficient, organized, and maintainable. They divide the complex programs into smaller, reusable, manageable modules for understanding and debugging. With the helpful function declaration, function definition, and function call in C programming, user-defined functions can be created in a simple way. Therefore, it is important to understand the user-defined functions in C because user-defined functions can assist in simple or complex calculations or programming.

User-Defined Functions in C – FAQs