What is and how to use the WHILE and DO-WHILE loops in C++

The while and do-while loops in C++ are control structures that allow repeating the execution of a block of code as long as a specific condition is met.

These loops are useful when the exact number of iterations is not known in advance and, instead, we want to repeat code under certain conditions.

WHILE Loop

The while loop executes a block of code as long as a condition is true. The basic syntax of a while loop in C++ is as follows:

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

In this loop:

• condition: It is an expression that is evaluated in each iteration. If it is true, the block of code inside the while is executed. If it is false, the loop ends.
• Code to execute: This is where the code to be repeated is placed.

Basic Example

Let’s see a basic example that uses a while loop to print numbers from 0 to 4:

#include <iostream>

int main() {
    int counter = 0;

    while (counter < 5) {
        std::cout << counter << std::endl;
        counter++;
    }

    return 0;
}

In this example:

• The while loop prints the numbers from 0 to 4, since the condition counter < 5 is true as long as counter is less than 5.

DO-WHILE Loop

The do-while loop is similar to the while loop, but it guarantees that the block of code will be executed at least once. The basic syntax is as follows:

do
{
    // Code to execute at least once
} while (condition);

In this loop:

• Code to execute: It is executed at least once before the condition is checked.
• condition: It is evaluated after each iteration. If it is true, the block of code executes again; if it is false, the loop ends.

Basic Example

Let’s see an example that uses a do-while loop to print numbers from 0 to 4:

#include <iostream>

int main() {
    int counter = 0;

    do {
        std::cout << counter << std::endl;
        counter++;
    } while (counter < 5);

    return 0;
}

In this example, the do-while loop

• Produces the same result as the previous while loop
• But guarantees that the code executes at least once, even if counter starts at 5.

Practical Examples

Count to 10

In this example, a while loop is used to count from 1 to 10 and print each number:

#include <iostream>

int main() {
    int counter = 1;

    while (counter <= 10) {
        std::cout << counter << std::endl;
        counter++; // Increments the counter by 1
    }

    return 0;
}

This while loop executes while counter is less than or equal to 10, printing each value of counter and then incrementing it.

Sum Positive Numbers Entered by the User

In this example, a while loop is used to sum positive numbers entered by the user until a negative number is entered:

#include <iostream>

int main() {
    int sum = 0;
    int number;

    std::cout << "Enter a positive number (a negative number to end): ";
    std::cin >> number;

    while (number >= 0) {
        sum += number; // Adds the entered number to the 'sum' variable
        std::cout << "Enter another positive number (a negative number to end): ";
        std::cin >> number;
    }

    std::cout << "The total sum of the positive numbers is: " << sum << std::endl;

    return 0;
}

This while loop executes while the user enters positive numbers. Each number is added to the sum variable. The loop ends when a negative number is entered.

Search for a Number in a List

In this example, a while loop is used to search for a specific number in a list of numbers:

#include <iostream>
#include <vector>

int main() {
    std::vector<int> numbers = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
    int searched = 7;
    int index = 0;
    bool found = false;

    while (index < numbers.size() && !found) {
        if (numbers[index] == searched) {
            found = true; // Marks that the number has been found
            std::cout << "Number " << searched << " found at position " << index << std::endl;
        }
        index++; // Increments the index
    }

    if (!found) {
        std::cout << "Number " << searched << " not found in the list" << std::endl;
    }

    return 0;
}

This while loop searches for a specific number in a vector. If the number is found, its position is printed; if it is not found, it indicates that it is not in the list.

Confirm Program Continuation

In this example, a do-while loop is used to ask the user to enter “y” or “n” to confirm whether they want to continue:

#include <iostream>
#include <string>

int main() {
    std::string response;

    do {
        std::cout << "Do you want to continue? (y/n): ";
        std::cin >> response;
    } while (response != "y" && response != "n");

    std::cout << "Program finished." << std::endl;

    return 0;
}

This do-while loop asks the user to enter “y” or “n”. The block of code executes at least once and then repeats while the response is not “y” or “n”.