Loops in C (for, while, do-while)

Why Loops?

• Repeat actions without writing code again and again
• Process lists, tables, and patterns
• Reduce time and code complexity
• Useful for calculations and automation

Types of Loops in C

1. for loop
2. while loop
3. do-while loop

1. for Loop

Used when the number of iterations is known.

for(initialization; condition; update){
    // code
}

Example – Print 1 to 5

#include <stdio.h>

int main() {

    for(int i = 1; i <= 5; i++){
        printf("%d\n", i);
    }

    return 0;
}

Example – Sum of First 10 Numbers

#include <stdio.h>

int main() {

    int sum = 0;

    for(int i = 1; i <= 10; i++){
        sum += i;
    }

    printf("Sum = %d", sum);

    return 0;
}

Example – Print Even Numbers (1-20)

#include <stdio.h>

int main() {

    for(int i = 2; i <= 20; i += 2){
        printf("%d ", i);
    }

    return 0;
}

2. while Loop

Used when the number of iterations is not known in advance.

while(condition){
    // code
}

Example – Print 1 to 5

#include <stdio.h>

int main() {

    int i = 1;

    while(i <= 5){
        printf("%d\n", i);
        i++;
    }

    return 0;
}

Example – Countdown from 10 to 1

#include <stdio.h>

int main() {

    int n = 10;

    while(n >= 1){
        printf("%d\n", n);
        n--;
    }

    return 0;
}

Example – Read Until Negative Number

#include <stdio.h>

int main() {

    int n;

    printf("Enter numbers (negative to stop):\n");

    while(1){
        scanf("%d", &n);
        if(n < 0)
            break;
        printf("You entered: %d\n", n);
    }

    return 0;
}

3. do-while Loop

Special loop: code executes **at least once**, even if condition is false.

do{
    // code
} while(condition);

Example – Print 1 to 5

#include <stdio.h>

int main() {

    int i = 1;

    do{
        printf("%d\n", i);
        i++;
    }while(i <= 5);

    return 0;
}

Example – Ask for Password Until Correct

#include <stdio.h>

int main() {

    int pass;

    do{
        printf("Enter password: ");
        scanf("%d", &pass);
    }while(pass != 1234);

    printf("Access Granted!");

    return 0;
}

Example – Always Executes Once

#include <stdio.h>

int main() {

    int x = 10;

    do{
        printf("This will print even if x > 5\n");
    }while(x < 5);

    return 0;
}

Infinite Loops

A loop that never ends unless a break is used.

while(1){
    printf("Running...\n");
}

For Loop – Example 1 (Print squares 1 to 10)

#include <stdio.h>

int main() {

    for(int i = 1; i <= 10; i++){
        printf("%d squared = %d\n", i, i*i);
    }

    return 0;
}

For Loop – Example 2 (Reverse order 50 to 1)

#include <stdio.h>

int main() {

    for(int i = 50; i >= 1; i--){
        printf("%d ", i);
    }

    return 0;
}

For Loop – Example 3 (Print table of 7)

#include <stdio.h>

int main() {

    for(int i = 1; i <= 10; i++){
        printf("7 x %d = %d\n", i, 7*i);
    }

    return 0;
}

For Loop – Example 4 (Count even & odd numbers)

#include <stdio.h>

int main() {

    int even = 0, odd = 0;

    for(int i = 1; i <= 20; i++){
        if(i % 2 == 0)
            even++;
        else
            odd++;
    }

    printf("Even: %d, Odd: %d", even, odd);

    return 0;
}

For Loop – Example 5 (Sum of multiples of 3)

#include <stdio.h>

int main() {

    int sum = 0;

    for(int i = 3; i <= 30; i += 3){
        sum += i;
    }

    printf("Sum = %d", sum);

    return 0;
}

While Loop – Example 1 (Print digits of a number)

#include <stdio.h>

int main() {

    int n = 54321;

    while(n > 0){
        printf("%d\n", n % 10);
        n /= 10;
    }

    return 0;
}

While Loop – Example 2 (Sum until user enters 0)

#include <stdio.h>

int main() {

    int n, sum = 0;

    while(1){
        scanf("%d", &n);
        if(n == 0)
            break;
        sum += n;
    }

    printf("Total Sum = %d", sum);

    return 0;
}

While Loop – Example 3 (Find factorial)

#include <stdio.h>

int main() {

    int n = 5, fact = 1;

    while(n > 0){
        fact *= n;
        n--;
    }

    printf("Factorial = %d", fact);

    return 0;
}

While Loop – Example 4 (Print ASCII characters)

#include <stdio.h>

int main() {

    int ch = 65;

    while(ch <= 90){
        printf("%c ", ch);
        ch++;
    }

    return 0;
}

While Loop – Example 5 (Find smallest digit)

#include <stdio.h>

int main() {

    int n = 75329;
    int min = 9;

    while(n > 0){
        int d = n % 10;
        if(d < min)
            min = d;
        n /= 10;
    }

    printf("Smallest digit = %d", min);

    return 0;
}

Do-While – Example 1 (Print menu until exit)

#include <stdio.h>

int main() {

    int choice;

    do{
        printf("1.Add 2.Sub 3.Exit\n");
        scanf("%d", &choice);
    }while(choice != 3);

    return 0;
}

Do-While – Example 2 (Password retry)

#include <stdio.h>

int main() {

    int pass;

    do{
        printf("Enter password: ");
        scanf("%d", &pass);
    }while(pass != 1234);

    printf("Access Granted!");

    return 0;
}

Do-While – Example 3 (Print 1 to 10)

#include <stdio.h>

int main() {

    int i = 1;

    do{
        printf("%d ", i);
        i++;
    }while(i <= 10);

    return 0;
}

Do-While – Example 4 (Always executes once)

#include <stdio.h>

int main() {

    int x = 100;

    do{
        printf("This runs once!\n");
    }while(x < 10);

    return 0;
}

Do-While – Example 5 (Number guessing game)

#include <stdio.h>

int main() {

    int guess;

    do{
        printf("Guess number (10): ");
        scanf("%d", &guess);
    }while(guess != 10);

    printf("Correct Guess!");

    return 0;
}

Difference Between Loops

for loop:
- Used when repetition count is known

while loop:
- Used when repetition count is unknown

do-while loop:
- Executes at least once

Practice Questions

1. Write a program to print the multiplication table of any number.
2. Write a program to count digits of a number.
3. Print numbers from 100 to 1 using a loop.
4. Use a while loop to reverse a number.
5. Use a do-while loop to create a login system.

Practice Task