Lesson 08·C Programming

Functions in C

Modular code blocks, parameter passing mechanisms, and recursion

What is a Function?

A function is a self-contained block of code designed to perform a specific task. C does not execute functions automatically; they must be invoked or called from within the program.

A function prototype informs the compiler about the function name, return type, and expected parameter list prior to its definition or invocation.

Types of Functions

1. Built-in (Library) Functions

Provided by standard C libraries and header files. Examples include: scanf(), printf(), strcpy(), strcmp(), strlen(), strcat().

2. User Defined Functions

Created by the programmer to meet specific program requirements. They can take parameters and optionally return values.

C Example

#include <stdio.h>

void add(int x, int y) {
    int result = x + y;
    printf("Sum of %d and %d is %d.\n", x, y, result);
}

void main() {
    add(10, 15);
    add(55, 64);
    add(168, 325);
}

Program Output

Sum of 10 and 15 is 25.
Sum of 55 and 64 is 119.
Sum of 168 and 325 is 493.

Passing Parameters to a Function

There are two primary mechanisms for passing arguments into functions in C:

• Pass by Value: Copies the actual values into the function parameters. Changes made inside the function do not affect the caller’s original variables.

• Pass by Reference: Passes the memory addresses of variables using pointers. Changes made inside the function directly modify the original values in the caller.

Example: Pass by Value

C Example

#include <stdio.h>

void swap(int p1, int p2) {
    int t = p2;
    p2 = p1;
    p1 = t;
    printf("Inside swap: p1 = %d, p2 = %d\n", p1, p2);
}

int main() {
    int a = 10, b = 20;

    printf("Before: Value of a = %d and value of b = %d\n", a, b);
    swap(a, b);
    printf("After: Value of a = %d and value of b = %d\n", a, b);

    return 0;
}

Program Output

Before: Value of a = 10 and value of b = 20
Inside swap: p1 = 20, p2 = 10
After: Value of a = 10 and value of b = 20

Note

Notice the values of a and b remain completely unchanged in the main() function after returning from swap.

Example: Pass by Reference

C Example

#include <stdio.h>

void swap(int *p1, int *p2) {
    int t = *p2;
    *p2 = *p1;
    *p1 = t;
    printf("Inside swap: *p1 = %d, *p2 = %d\n", *p1, *p2);
}

int main() {
    int a = 10, b = 20;

    printf("Before: Value of a = %d and value of b = %d\n", a, b);
    swap(&a, &b);
    printf("After: Value of a = %d and value of b = %d\n", a, b);

    return 0;
}

Program Output

Before: Value of a = 10 and value of b = 20
Inside swap: *p1 = 20, *p2 = 10
After: Value of a = 20 and value of b = 10

Note

Because memory addresses were passed using pointers, the values of a and b were successfully exchanged in the caller.

Advantages of Functions

Reduces program size and eliminates duplicate code.
Facilitates modular design and structured programming.
Makes debugging, testing, and maintenance significantly easier.
Enhances code readability and logical separation of concerns.

Recursion (Recursive Functions)