How to create and call a function in C

Syntax of C function

Before defining a function, it needs to be declared (if the function is defined after 'main' function). It is a good practice to declare a function before defining it. The function declaration tells the compiler about the function name, return type, and parameters.

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return_type function_name(data_type parameter);

• return_type: The 'return_type' refers to return type of the function. It may be any data type or void(for example int).
• function_name();: The 'function_name' will be replaced by a meaningful name for the function. The function name will be followed by a parentheses '()' (for example sumNumber()).
• (data_type parameter);: Inside the parentheses '()', parameter with data type is placed (for example (int a, int b)). Parameters are seperated by comma (,).

Complete example of function declaration: int sumNumber(int a, int b);

A function must be defined following the syntax of function definition, if the function is defined earlier. (Otherwise, a function can be defined as per user requirement.)

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return_type function_name(data_type parameter) {

    // function body code

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    return value; // if the return type is not void

}

​

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• return_type function_name: The 'return_type' refers to return type of the function. It may be any data type or void. The 'function_name' will be replaced by a meaningful name for the function. The function name will be followed by a parentheses '()'. For example: int sumNumber()
• data_type parameter: Inside the parentheses '()', parameters are placed. Data type of parameter must be defined before the name of the parameter. For example: (int a, int b)
• // function body code: Here goes the code block of the function where function task will be specified.
• return value;: It returns the value of the return type data as specified in the function definition. If the function return type is void, this code is not necessary in the function. For example: return a + b;

To use a function, it needs to be called. A function can be called from the main function or other functions.

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function_name(parameter_argument);

• function_name(parameter_argument);: The code calls the 'function_name' function with an argument in parentheses '()' named 'parameter_argument'. For example: sumNumber(4, 5);

Create and Call a Function

A function named 'sumNumber' will be created and called in the following example:

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#include <stdio.h>

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// Function declaration

int sumNumber(int a, int b);

​

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int main() {

    // Function call

    int result = sumNumber(5, 9);

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    printf("The result is: %d\n", result);

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    return 0;

}

​

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// Function definition

int sumNumber(int a, int b) {

    return a + b;

}

​

The result is: 14

Explanation of code:

• int sumNumber(int a, int b);: The line declares a function named 'sumNumber' having two integer parameters (a and b) with integer return type.
• int sumNumber(int a, int b){return a + b;}: The line defines a function named 'sumNumber' with two integer parameters 'int a' and 'int b'. return a + b; returns the sum of two integer variables 'a' and 'b'. The function returns integer type.
• int result = sumNumber(5, 9);: The line of code calls the 'sumNumber' function with two integer arguments 5 and 9 in the main function. The result is assigned to the integer variable 'result'.
• printf("The result is: %d\n", result);: The line prints the result of the function. The '%d' is the format specifier in C program.

Create and call a function without declaring the function

If you define a function before the 'main' function, you need not to declare the function first. A function will be created and called in the following example:

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#include <stdio.h>

​

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    // Function definition

    int sumNumber(int a, int b) {

        return a + b;

    }

​

int main() {

    // Function call

    int result = sumNumber(5, 9);

​

    printf("The result is: %d\n", result);

​

    return 0;

}

​

​

​

The result is: 14

Explanation of code:

• int sumNumber(int a, int b){return a + b;}: The line defines a function named 'sumNumber' with two integer parameters 'int a' and 'int b'. The function returns integer type. The function is defined before the 'main' function.
• return a + b;: This is the function body that returns the sum of two integer variables 'a' and 'b'.
• int result = sumNumber(5, 9);: In the 'main' function, the code calls the 'sumNumber' function with two integer arguments 5 and 9. The result is assigned to the integer variable 'result'.
• printf("The result is: %d\n", result);: The line prints the result of the function. The '%d' is the format specifier in C program.