Functions in C

What is a Function?

A function is a group of statements combined to perform an operation. C supports two types of functions:

• Library Functions – built-in (printf, scanf, strlen, etc.)
• User-defined Functions – created by the programmer

Why Use Functions?

• Makes code modular
• Avoids repeating the same code
• Easier debugging & maintenance
• Improves readability
• Enables teamwork by dividing work

Parts of a Function

• Function Declaration (prototype)
• Function Definition (actual code)
• Function Call (using the function)

Syntax of a Function

return_type function_name(parameter_list) {
    // body of function
}

• return_type – type of value returned (int, float, void, etc.)
• function_name – any valid C identifier
• parameter_list – input values (optional)

Function Declaration (Prototype)

A prototype tells the compiler the function name, return type, and parameters.

int add(int, int);
void display();
float avg(float x, float y);

Function Definition

This is where the function's actual logic is written.

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

Function Call

A function is executed when it is called.

int result = add(5, 7);

Types of Functions Based on Arguments & Return Value

• 1. No arguments, no return value
• 2. Arguments, no return value
• 3. No arguments, return value
• 4. Arguments and return value

Scope of Variables in Functions

• Local Variables – inside function, destroyed after execution
• Global Variables – declared outside, accessible everywhere
• Static Variables – retain value between function calls

Function Advantages

• Less code duplication
• Improved structure
• Better testing
• Easy code distribution among team
• Reusable in large applications

Function Examples

1. Function with No Arguments & No Return Value

#include <stdio.h>

void greet(){
    printf("Hello, Welcome!");
}

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

2. Function with Arguments & No Return Value

#include <stdio.h>

void show(int a, int b){
    printf("Values: %d, %d", a, b);
}

int main(){
    show(10, 20);
    return 0;
}

3. Function with No Arguments & Return Value

#include <stdio.h>

int give(){
    return 100;
}

int main(){
    printf("%d", give());
    return 0;
}

4. Function with Arguments and Return Value

#include <stdio.h>

int add(int x, int y){
    return x + y;
}

int main(){
    printf("Sum = %d", add(5, 9));
    return 0;
}

5. Function to Find Maximum

#include <stdio.h>

int max(int a, int b){
    return (a > b) ? a : b;
}

int main(){
    printf("%d", max(7, 3));
    return 0;
}

6. Function Returning a Character

#include <stdio.h>

char grade(int m){
    if(m >= 90) return 'A';
    return 'B';
}

int main(){
    printf("Grade = %c", grade(92));
    return 0;
}

7. Function to Calculate Square

#include <stdio.h>

int square(int n){
    return n * n;
}

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

8. Function for Multiplication

#include <stdio.h>

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

int main(){
    printf("%d", multiply(4, 9));
    return 0;
}

9. Even or Odd Function

#include <stdio.h>

int isEven(int n){
    return n % 2 == 0;
}

int main(){
    printf("%d", isEven(7));
    return 0;
}

10. Function to Print a Line

#include <stdio.h>

void line(){
    printf("--------------\n");
}

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

11. Function to Swap Two Numbers (Call by Value)

#include <stdio.h>

void swap(int a, int b){
    int temp = a;
    a = b;
    b = temp;
    printf("%d %d", a, b);
}

int main(){
    swap(10, 20);
    return 0;
}

12. Function to Count Digits

#include <stdio.h>

int digits(int n){
    int c = 0;
    while(n != 0){
        c++;
        n /= 10;
    }
    return c;
}

int main(){
    printf("%d", digits(672));
    return 0;
}

13. Function to Reverse a Number

#include <stdio.h>

int reverse(int n){
    int r = 0;
    while(n != 0){
        r = r * 10 + n % 10;
        n /= 10;
    }
    return r;
}

int main(){
    printf("%d", reverse(1234));
    return 0;
}

14. Function to Calculate Factorial

#include <stdio.h>

int fact(int n){
    int f = 1;
    for(int i=1; i<=n; i++)
        f *= i;
    return f;
}

int main(){
    printf("%d", fact(6));
    return 0;
}

15. Function Returning Float

#include <stdio.h>

float average(float a, float b){
    return (a + b)/2;
}

int main(){
    printf("%.2f", average(10, 20));
    return 0;
}

16. Function for Simple Interest

#include <stdio.h>

float si(float p, float r, float t){
    return (p * r * t) / 100;
}

int main(){
    printf("%.2f", si(1000, 5, 2));
    return 0;
}

17. Function with Array Argument

#include <stdio.h>

int sum(int arr[], int n){
    int s = 0;
    for(int i=0; i< n; i++)
        s += arr[i];
    return s;
}

int main(){
    int a[] = {1,2,3,4,5};
    printf("%d", sum(a, 5));
    return 0;
}

18. Function Returning Array Element

#include <stdio.h>

int last(int arr[], int n){
    return arr[n-1];
}

int main(){
    int a[] = {4, 7, 9};
    printf("%d", last(a, 3));
    return 0;
}

19. Function to Count Vowels in String

#include <stdio.h>

int vowels(char *s){
    int c = 0;
    for(int i=0; s[i]; i++){
        char ch = s[i];
        if(ch=='a'||ch=='e'||ch=='i'||ch=='o'||ch=='u'||
           ch=='A'||ch=='E'||ch=='I'||ch=='O'||ch=='U')
            c++;
    }
    return c;
}

int main(){
    printf("%d", vowels("Sourav Sahu"));
    return 0;
}

20. Function Calling Another Function

#include <stdio.h>

int square(int n){
    return n*n;
}

int cube(int n){
    return n * square(n);
}

int main(){
    printf("%d", cube(4));
    return 0;
}

Real Life Uses of Functions

• Menu-driven programs
• Calculators
• ATM operations
• Game modules (movement, attack, score)
• Student record management
• Mathematical computations

Practice Questions

1. What is a function? Explain with syntax.
2. Give differences between library and user-defined functions.
3. Explain the purpose of a function prototype.
4. Write the types of functions based on return type & parameters.
5. Explain scope of local and global variables.

Practice Task