CLooping StatementsDo-While Loop

Understanding the do-while Loop in C: A Beginner’s Guide

The do-while loop is a control structure in C that guarantees the loop body executes at least once, regardless of the condition’s initial value. This makes it distinct from the while loop, which checks the condition before execution.

In this blog, we will dive into the do-while loop with examples ranging from simple to complex, with detailed explanations for absolute beginners.

1. What is a do-while Loop?

A do-while loop executes the block of code first, then evaluates the condition. If the condition is true, the loop continues; otherwise, it terminates.

Syntax:

do {
    // Code to execute
} while (condition);
  • The loop body is guaranteed to execute at least once, even if the condition is false initially.
  • The semicolon (;) after the while condition is mandatory.

2. Simple Example

Let’s start with a basic example: printing numbers from 1 to 5.

main.c
#include <stdio.h>
 
int main() {
    int i = 1;  // Initialization
 
    do {
        printf("%d ", i);
        i++;  // Update
    } while (i <= 5);  // Condition
 
    return 0;
}

Output:

1 2 3 4 5

Explanation:

  1. Initialization: Variable i is set to 1.
  2. Execution: The loop body runs, printing the value of i and incrementing it.
  3. Condition: After each iteration, the condition i <= 5 is checked. If true, the loop repeats.

3. Practical Applications of do-while Loops

1. User Input Validation

This program ensures the user enters a number within a specified range.

main.c
#include <stdio.h>
 
int main() {
    int number;
 
    do {
        printf("Enter a number between 1 and 10: ");
        scanf("%d", &number);
 
        if (number < 1 || number > 10) {
            printf("Invalid input! Please try again.\n");
        }
    } while (number < 1 || number > 10);  // Condition to repeat on invalid input
 
    printf("You entered: %d\n", number);
    return 0;
}

Output (Examples):

  1. Input: 15, 0, 7 
    Enter a number between 1 and 10: 15
Invalid input! Please try again.
Enter a number between 1 and 10: 0
Invalid input! Please try again.
Enter a number between 1 and 10: 7
You entered: 7

2. Sum of Positive Numbers Until a Negative Number is Entered

This program calculates the sum of positive numbers entered by the user until they enter a negative number.

main.c
#include <stdio.h>
 
int main() {
    int num, sum = 0;
 
    do {
        printf("Enter a positive number (negative to stop): ");
        scanf("%d", &num);
 
        if (num >= 0) {
            sum += num;  // Add positive numbers to the sum
        }
    } while (num >= 0);  // Stop when a negative number is entered
 
    printf("The sum of positive numbers is %d.\n", sum);
    return 0;
}

Output (Example):

  1. Input: 10, 20, -1 
    Enter a positive number (negative to stop): 10
Enter a positive number (negative to stop): 20
Enter a positive number (negative to stop): -1
The sum of positive numbers is 30.

3. Repeatedly Display a Menu Until the User Exits

This program displays a menu and processes the user’s choice until they decide to exit.

main.c
#include <stdio.h>
 
int main() {
    int choice;
 
    do {
        printf("\nMenu:\n");
        printf("1. Print Hello\n");
        printf("2. Print Goodbye\n");
        printf("3. Exit\n");
        printf("Enter your choice: ");
        scanf("%d", &choice);
 
        switch (choice) {
            case 1:
                printf("Hello!\n");
                break;
            case 2:
                printf("Goodbye!\n");
                break;
            case 3:
                printf("Exiting...\n");
                break;
            default:
                printf("Invalid choice! Try again.\n");
        }
    } while (choice != 3);  // Repeat until the user selects Exit (3)
 
    return 0;
}

Output (Example):

Menu:
1. Print Hello
2. Print Goodbye
3. Exit
Enter your choice: 1
Hello!

Menu:
1. Print Hello
2. Print Goodbye
3. Exit
Enter your choice: 3
Exiting...

4. Reverse a Number

This program reverses the digits of a given number using a do-while loop.

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

Output (Examples):

  1. Input: 1234 
    Reversed number: 4321

5. Infinite Loop Example

Sometimes, do-while loops can be used to create intentional infinite loops.

Example: Continuously Print Numbers

main.c
#include <stdio.h>
 
int main() {
    int i = 1;
 
    do {
        printf("%d ", i);
        i++;
    } while (1);  // This condition is always true, creating an infinite loop
 
    return 0;
}

Note: This program will continue indefinitely and must be terminated manually.

4. Nested do-while Loops

Nested do-while loops are useful for multi-level tasks, such as printing patterns.

Example: Printing a Square Pattern

main.c
#include <stdio.h>
 
int main() {
    int rows, cols, i = 1;
 
    printf("Enter the number of rows and columns: ");
    scanf("%d %d", &rows, &cols);
 
    do {
        int j = 1;
        do {
            printf("* ");
            j++;
        } while (j <= cols);  // Inner loop for columns
        printf("\n");
        i++;
    } while (i <= rows);  // Outer loop for rows
 
    return 0;
}

Output (Example):

  1. Input: 3 4 
    * * * *
* * * *
* * * *

5. Key Points to Remember

  1. The do-while loop ensures the loop body executes at least once.
  2. Use the semicolon (;) after the while condition.
  3. Avoid infinite loops by ensuring the condition will eventually become false.
  4. Use nested do-while loops for multi-dimensional tasks.

Summary

The do-while loop is a powerful construct when you need to guarantee execution at least once, such as for user input validation or menu-driven programs. Understanding its behavior and syntax is crucial for mastering control flow in C programming.

In the next blog, we’ll explore jump statements (break, continue, and goto) to control the flow within loops. Stay tuned!

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