Functions

Functions

Functions are the building blocks of C programs, enabling modularity, code reuse, and abstraction. This section covers function fundamentals, parameter passing, recursion, function pointers, and advanced function-related concepts.

1. What is a Function in C?

A function is a block of code that performs a specific task. It runs only when it is called.

Why do we use functions?

• To avoid code repetition
• To make programs modular
• To improve readability
• To simplify debugging and testing

Example: Simple Function

#include <stdio.h>

void greet() {
    printf("Hello, Welcome to C Programming!\n");
}

int main() {
    greet();
    return 0;
}

Explanation: The function greet() prints a message. It is called inside main().

2. What are the Parts of a Function?

A function in C has three main parts:

• Declaration (Prototype)
• Definition
• Function Call

Example:

#include <stdio.h>

// Function Declaration
int add(int, int);

int main() {
    int result = add(5, 3);  // Function Call
    printf("Sum = %d", result);
    return 0;
}

// Function Definition
int add(int a, int b) {
    return a + b;
}

Explanation:

• Declaration tells the compiler about the function.
• Definition contains the actual logic.
• Call executes the function.

3. Types of Functions in C

Functions are mainly of two types:

1. Library Functions

Built-in functions provided by C standard library.

Example: printf(), scanf(), sqrt()

2. User-defined Functions

Functions created by the programmer.

4. Types of User-Defined Functions (Based on Arguments & Return Type)

Case 1: No Arguments, No Return Value

#include <stdio.h>

void showMessage() {
    printf("This function has no arguments and no return value.");
}

int main() {
    showMessage();
    return 0;
}

Case 2: Arguments but No Return Value

#include <stdio.h>

void printSum(int a, int b) {
    printf("Sum = %d", a + b);
}

int main() {
    printSum(4, 6);
    return 0;
}

Case 3: No Arguments but Return Value

#include <stdio.h>

int getNumber() {
    return 10;
}

int main() {
    int num = getNumber();
    printf("Number = %d", num);
    return 0;
}

Case 4: Arguments and Return Value

#include <stdio.h>

int multiply(int x, int y) {
    return x * y;
}

int main() {
    int result = multiply(5, 4);
    printf("Multiplication = %d", result);
    return 0;
}

5. What is the Difference Between Call by Value and Call by Reference?

Call by Value

Copy of variable is passed. Original value does not change.

#include <stdio.h>

void change(int x) {
    x = 100;
}

int main() {
    int a = 10;
    change(a);
    printf("Value of a = %d", a);
    return 0;
}

Output: Value of a = 10

Call by Reference (Using Pointers)

Address is passed. Original value changes.

#include <stdio.h>

void change(int *x) {
    *x = 100;
}

int main() {
    int a = 10;
    change(&a);
    printf("Value of a = %d", a);
    return 0;
}

Output: Value of a = 100

6. What is Recursion in C?

Recursion is when a function calls itself.

Example: Factorial Using Recursion

#include <stdio.h>

int factorial(int n) {
    if(n == 0)
        return 1;
    else
        return n * factorial(n - 1);
}

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

Explanation: Function keeps calling itself until base condition (n == 0) is met.

7. What are Inline Functions?

Inline functions suggest the compiler to insert function code directly at the call location to reduce overhead.

#include <stdio.h>

inline int square(int x) {
    return x * x;
}

int main() {
    printf("Square = %d", square(6));
    return 0;
}

8. What is a Static Function?

A static function is visible only within the same source file.

#include <stdio.h>

static void display() {
    printf("Static function example");
}

int main() {
    display();
    return 0;
}

9. What is a Function Pointer?

A function pointer stores the address of a function.

#include <stdio.h>

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

int main() {
    int (*ptr)(int, int);
    ptr = add;

    printf("Sum = %d", ptr(3, 4));
    return 0;
}

10. Practice Questions (Important for Exams & Interviews)

• Write a function to check whether a number is prime.
• Write a function to reverse a number.
• Write a recursive function to calculate Fibonacci series.
• Write a function to swap two numbers using call by reference.
• Write a function to find the largest among three numbers.