C++ do Keyword

The do keyword in C++ is used to create a do-while loop. This type of loop ensures that the body of the loop is executed at least once before checking the condition. The condition is evaluated at the end of the loop, making it different from other loop constructs like for and while, where the condition is evaluated before the loop body is executed.

Syntax

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do {
    // Loop body
} while (condition);
do
The keyword that starts the do-while loop and ensures the loop body is executed at least once.
condition
The boolean expression evaluated after the loop body. If true, the loop continues; otherwise, it terminates.

Examples

Example 1: Basic do-while Loop

This example demonstrates a simple do-while loop that prints numbers from 1 to 5.

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#include <iostream>
using namespace std;

int main() {
    int i = 1;

    do {
        cout << i << " ";
        i++;
    } while (i <= 5);

    return 0;
}

Output:

1 2 3 4 5

Explanation:

  1. The variable i is initialized to 1.
  2. The do block executes the loop body, printing the value of i.
  3. The value of i is incremented after each iteration.
  4. The loop continues until the condition i <= 5 becomes false.

Example 2: Input Validation with do-while Loop

This example demonstrates using a do-while loop to repeatedly prompt the user for input until a valid number is entered.

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#include <iostream>
using namespace std;

int main() {
    int number;

    do {
        cout << "Enter a number between 1 and 10: ";
        cin >> number;
    } while (number < 1 || number > 10);

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

    return 0;
}

Output:

Enter a number between 1 and 10: 15
Enter a number between 1 and 10: 5
You entered: 5

Explanation:

  1. The program uses a do-while loop to prompt the user for input.
  2. The condition number < 1 || number > 10 ensures the loop continues until a valid number is entered.
  3. Once the user enters a valid number, the loop terminates, and the program prints the number.

Example 3: Summing Numbers with do-while

This example demonstrates using a do-while loop to sum numbers until the user enters 0.

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#include <iostream>
using namespace std;

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

    do {
        cout << "Enter a number (0 to stop): ";
        cin >> number;
        sum += number;
    } while (number != 0);

    cout << "Total sum: " << sum << endl;

    return 0;
}

Output:

Enter a number (0 to stop): 5
Enter a number (0 to stop): 10
Enter a number (0 to stop): 0
Total sum: 15

Explanation:

  1. The loop executes at least once, prompting the user to enter a number.
  2. The entered number is added to the variable sum.
  3. The loop continues until the user enters 0, at which point the total sum is printed.

Key Points about do Keyword

  1. The do keyword ensures the loop body executes at least once, regardless of the condition.
  2. The condition is evaluated at the end of the loop, making do-while loops useful for scenarios like input validation and repeated tasks.
  3. The loop terminates when the condition evaluates to false.
  4. It is essential to ensure the loop condition eventually becomes false to avoid infinite loops.