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User-Defined Functions in C

Example 1: 
#include <stdio.h>
void printCode(); //func declaration

void main(){
    printf("Hello ");
    printCode(); //func call
}
void printCode(){  //func definition
    printf("Wrote code");  
}

Output:

Hello Wrote code

There are 3 things to remember when creating user-defined functions:

1. Function Declaration.
2. Function Call.
3. Function definition

Example 1: Function with No Arguments & No Return Value
#include <stdio.h>

// Function declaration + definition
void greetings() {
    printf("Hello! Welcome to C programming.\n");
}

int main() {
    // Function call
    greetings();  
    return 0;
}

Output:

Hello! Welcome to C programming.

Use:

In real life, it is useful for displaying messages, logging information, or performing simple actions without needing input or output values.

Example 2: Functions with Arguments but No Returns a Value
#include <stdio.h>

void addition(int num1, int num2) {  // parameters
    int sum = num1 + num2;
    printf("The sum is %d", sum);
}

int main() {
    int num1 = 10, num2 = 20;

    // Function call with arguments
    addition(num1, num2);

    return 0;
}

Output:

The sum is 30

Use:

When we want to perform a task (like printing, modifying global variables, etc.) but don’t need a return value.

Example 3. Functions with No Arguments but Returns a Value
#include <stdio.h>

int addition() {
    int num1, num2;
    printf("Enter the value of num1: ");
    scanf("%d",&num1);
    printf("Enter the value of num2: ");
    scanf("%d",&num2);
    // int sum = num1 + num2;
    // return sum;
    return num1 + num2;
}

int main() {
    
    int sum = addition();
    printf("The sum is %d",sum);

    return 0;
}

Output:

Enter the value of num1: 12
Enter the value of num2: 9
The sum is 21

Use:

When we don’t need to pass any data but need to retrieve a result from the function.
Example: Getting user input, generating a random number, reading sensor values in embedded systems, etc.

Example 4. Function with Arguments & Returns a Value
#include <stdio.h>

int addition(int num1, int num2) {
    return num1 + num2;  //return sum
}

int main() {
    int num1, num2;
    printf("Enter the value of num1: ");
    scanf("%d",&num1);
    printf("Enter the value of num2: ");
    scanf("%d",&num2);
    
    int sum = addition(num1,num2);  //passing arguments
    printf("The sum is %d",sum);

    return 0;
}

Output: 

Enter the value of num1: 12
Enter the value of num2: 9
The sum is 21

Use:

When a function requires input values and needs to return a computed result.

Examples:
Banking System: Function to calculate interest.
Game Development: Function to compute player scores.
E-commerce: Function to calculate total cart price with tax.