A user-defined function is a custom function written by the programmer to perform a specific task. Functions help in dividing a large program into smaller modules, making it easy to understand, test, and reuse.

1. Functions with No Arguments and No Return Value

• No input parameters

• Doesn’t return any value

• Used for simple tasks like printing messages.

Example:

#include <stdio.h>

// Function declaration
void greet();

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

// Function definition
void greet() {
    printf("Hello, World!\n");
}

Output:

Hello, World!

2. Functions with Arguments but No Return Value

• Takes input parameters

• Performs operations but doesn’t return a value

• Used for calculations where the result is printed inside the function.

Example:

#include <stdio.h>

// Function declaration
void add(int a, int b);

int main() {
    int x = 5, y = 3;
    add(x, y);  // Function call with arguments
    return 0;
}

// Function definition
void add(int a, int b) {
    printf("Sum: %d\n", a + b);
}

Output:

Sum: 8

3. Functions with Arguments and Return Value

• Takes input parameters

• Returns a computed value

• Most commonly used for reusable calculations.

Example:

#include <stdio.h>

// Function declaration
int multiply(int a, int b);

int main() {
    int x = 4, y = 5;
    int result = multiply(x, y);  // Function call
    printf("Product: %d\n", result);
    return 0;
}

// Function definition
int multiply(int a, int b) {
    return a * b;
}

Output:

Product: 20

4. Functions with No Arguments but Return a Value

• No input parameters

• Returns a value

• Often used for getting user input or generating values.

Example:

#include <stdio.h>

// Function declaration
int getNumber();

int main() {
    int num = getNumber();  // Function call
    printf("You entered: %d\n", num);
    return 0;
}

// Function definition
int getNumber() {
    int n;
    printf("Enter a number: ");
    scanf("%d", &n);
    return n;
}

Output:

Enter a number: 7
You entered: 7

5. Recursive Functions

• Calls itself until a base condition is met

• Used for problems that can be broken into smaller sub-problems

• Examples: Factorial, Fibonacci series.

Example (Factorial Calculation):

#include <stdio.h>

// Function declaration
int factorial(int n);

int main() {
    int num = 5;
    printf("Factorial of %d: %d\n", num, factorial(num));
    return 0;
}

// Function definition
int factorial(int n) {
    if (n == 0 || n == 1)
        return 1;
    else
        return n * factorial(n - 1);  // Recursive call
}

Output:

Factorial of 5: 120