The do-while loop in C++ is a control flow statement that executes a block of code at least once and then repeatedly executes the code block as long as a specified condition is true. This is particularly useful in scenarios where you want to ensure that the code block runs at least once, such as when prompting a user for input.

Syntax of do-while Loop

do {
    // Code to be executed
} while (condition);

• condition: This is evaluated after the loop body has executed. If it is true, the loop will execute again; if it is false, the loop will terminate.

Key Characteristics

1. Guaranteed Execution: The code inside the do block is executed at least once, even if the condition is false on the first check.
2. Condition Check After Execution: Unlike the while loop, where the condition is checked before any execution, the do-while loop checks the condition after executing the loop body.

Example: Basic do-while Loop

Here’s a simple example that uses a do-while loop to print numbers from 1 to 5:

#include <iostream>

int main() {
    int count = 1; // Initialization

    do {
        std::cout << "Count is: " << count << std::endl;
        count++; // Update
    } while (count <= 5); // Condition

    return 0;
}

Output:

Count is: 1
Count is: 2
Count is: 3
Count is: 4
Count is: 5

Explanation

1. Initialization (int count = 1): A variable is declared and initialized to control the number of loop iterations.
2. Loop Body: The block of code inside the do is executed first. In this case, it prints the current value of count and increments it.
3. Condition Check (count <= 5): After the loop body is executed, the condition is checked. If it is true, the loop runs again; if it is false, the loop terminates.

Use Case: User Input Example

do-while loops are often used for user input scenarios where you want to prompt the user until they provide valid input.

Example:

#include <iostream>

int main() {
    int number;

    do {
        std::cout << "Enter a number between 1 and 10: ";
        std::cin >> number;
    } while (number < 1 || number > 10); // Condition for valid input

    std::cout << "You entered: " << number << std::endl;

    return 0;
}

• In this example, the program prompts the user to enter a number until a valid number (between 1 and 10) is provided.
• The input is requested at least once, ensuring that the user has the opportunity to provide input.

Infinite do-while Loop

Similar to a while loop, a do-while loop can also create an infinite loop if the condition is always true.

Example:

#include <iostream>

int main() {
    int count = 0;

    do {
        std::cout << "This loop will run forever unless you break it!" << std::endl;
    } while (true); // Always true

    return 0;
}

To exit an infinite loop, you can use a control statement like break.

Comparison with while Loop

The primary difference between do-while and while loops is when the condition is evaluated:

• while Loop: The condition is checked before the loop body is executed. If the condition is false, the loop may not execute at all.
• do-while Loop: The loop body is executed first, and then the condition is checked. The loop will always execute at least once.

Example of while vs do-while:

#include <iostream>

int main() {
    int count = 6;

    // While Loop
    while (count <= 5) {
        std::cout << "This will not run." << std::endl; // This will not execute
        count++;
    }

    // Do-While Loop
    do {
        std::cout << "This will run once." << std::endl; // This will execute once
        count++;
    } while (count <= 5);

    return 0;
}

Output:

This will run once.

Summary

• The do-while loop is a control structure that guarantees at least one execution of the loop body, making it ideal for situations where input is required or when you need to perform an operation at least once.
• The condition is checked after the loop body has executed, allowing for different behaviors compared to the while loop.
• It is useful in user interaction scenarios, validating inputs, or running operations that need to happen once before checking for further conditions.

Using do-while loops effectively allows you to create robust programs that handle input and conditions with ease, ensuring a user-friendly experience.