CLooping StatementsWhile Loop

Mastering the while Loop in C: A Beginner’s Guide

The while loop in C is a control structure used for executing a block of code repeatedly as long as a specified condition remains true. Unlike the for loop, the while loop is best suited for scenarios where the number of iterations is not predetermined.

In this blog, we’ll explore the while loop with examples ranging from simple to complex, providing detailed explanations to help beginners understand its functionality.

1. What is a while Loop?

The while loop is a pre-tested loop that evaluates the condition before each iteration. If the condition is true, the loop body executes; otherwise, the loop terminates.

Syntax:

while (condition) {
    // Code to execute as long as condition is true
}

2. Simple Example

Let’s start with a basic example: printing numbers from 1 to 10.

main.c
#include <stdio.h>
 
int main() {
    int i = 1;  // Initialization
 
    while (i <= 10) {  // Condition
        printf("%d ", i);
        i++;  // Update
    }
 
    return 0;
}

Output:

1 2 3 4 5 6 7 8 9 10

Explanation:

  1. Initialization: Variable i is initialized to 1.
  2. Condition: i <= 10 ensures the loop runs as long as i is less than or equal to 10.
  3. Update: i++ increments i by 1 after each iteration to prevent infinite looping.

3. Practical Applications of while Loops

1. Sum of Numbers Using a while Loop

This program calculates the sum of natural numbers up to a given number n.

main.c
#include <stdio.h>
 
int main() {
    int n, sum = 0, i = 1;
 
    printf("Enter the value of n: ");
    scanf("%d", &n);
 
    while (i <= n) {
        sum += i;  // Add the current value of i to sum
        i++;       // Increment i
    }
 
    printf("The sum of the first %d natural numbers is %d.\n", n, sum);
    return 0;
}

Output (Example):

  1. Input: 5 
    The sum of the first 5 natural numbers is 15.

2. Reverse a Number Using a while Loop

This program reverses the digits of a given number.

main.c
#include <stdio.h>
 
int main() {
    int num, reversed = 0;
 
    printf("Enter a number: ");
    scanf("%d", &num);
 
    while (num != 0) {
        int digit = num % 10;  // Extract the last digit
        reversed = reversed * 10 + digit;  // Build the reversed number
        num /= 10;  // Remove the last digit
    }
 
    printf("Reversed number: %d\n", reversed);
    return 0;
}

Output (Examples):

  1. Input: 1234 
    Reversed number: 4321

3. Find the Factorial of a Number

This program calculates the factorial of a given number using a while loop.

main.c
#include <stdio.h>
 
int main() {
    int num;
    unsigned long long factorial = 1;
 
    printf("Enter a positive integer: ");
    scanf("%d", &num);
 
    if (num < 0) {
        printf("Factorial is not defined for negative numbers.\n");
    } else {
        int i = 1;  // Initialization
        while (i <= num) {
            factorial *= i;  // Multiply by current i
            i++;             // Increment i
        }
        printf("Factorial of %d = %llu\n", num, factorial);
    }
 
    return 0;
}

Output (Example):

  1. Input: 5 
    Factorial of 5 = 120

4. Generate Fibonacci Numbers

This program generates the Fibonacci sequence up to a given number of terms.

main.c
#include <stdio.h>
 
int main() {
    int n, t1 = 0, t2 = 1;
 
    printf("Enter the number of terms: ");
    scanf("%d", &n);
 
    printf("Fibonacci Series: ");
 
    int count = 1;  // Start counting from the first term
    while (count <= n) {
        printf("%d ", t1);
        int nextTerm = t1 + t2;  // Calculate the next term
        t1 = t2;  // Update t1
        t2 = nextTerm;  // Update t2
        count++;
    }
 
    return 0;
}

Output (Example):

  1. Input: 7 
    Fibonacci Series: 0 1 1 2 3 5 8

5. Infinite Loops

A while loop can create infinite loops if the condition never becomes false.

Example: Displaying a Menu Until Exit

main.c
#include <stdio.h>
 
int main() {
    int choice;
 
    while (1) {  // Infinite loop
        printf("\nMenu:\n");
        printf("1. Say Hello\n");
        printf("2. Exit\n");
        printf("Enter your choice: ");
        scanf("%d", &choice);
 
        if (choice == 1) {
            printf("Hello, World!\n");
        } else if (choice == 2) {
            printf("Exiting...\n");
            break;  // Exit the loop
        } else {
            printf("Invalid choice! Try again.\n");
        }
    }
 
    return 0;
}

Output (Example):

Menu:
1. Say Hello
2. Exit
Enter your choice: 1
Hello, World!

Menu:
1. Say Hello
2. Exit
Enter your choice: 2
Exiting...

4. Nested while Loops

Nested while loops are used to handle more complex structures, such as multi-dimensional data.

Example: Printing a Right-Angled Triangle Pattern

main.c
#include <stdio.h>
 
int main() {
    int rows, i = 1;
 
    printf("Enter the number of rows: ");
    scanf("%d", &rows);
 
    while (i <= rows) {
        int j = 1;  // Initialize the inner loop variable
        while (j <= i) {
            printf("* ");
            j++;
        }
        printf("\n");  // Move to the next line
        i++;
    }
 
    return 0;
}

Output (Example):

  1. Input: 4 
    *
* *
* * *
* * * *

5. Key Points to Remember

  1. The while loop is suitable for tasks where the number of iterations is unknown.
  2. Ensure the loop condition eventually becomes false to avoid infinite loops.
  3. Use break to exit a loop prematurely and continue to skip the current iteration.
  4. Nested while loops can solve multi-dimensional problems but may increase complexity.

Summary

The while loop is a versatile tool for solving problems requiring repeated execution of a block of code. From simple tasks like summing numbers to more complex ones like generating Fibonacci sequences, it provides flexibility and control.

In the next blog, we’ll explore the do-while loop, which guarantees at least one execution of the loop body. Stay tuned!

For LoopDo-While Loop