C++ do‑while Loop: Syntax, Practical Examples, and Nested Loops

What Is a do‑while Loop?

The do‑while loop is a control‑flow construct that executes a block of code at least once and repeats it as long as a specified condition remains true. Unlike a standard while loop, the condition is evaluated after the loop body, guaranteeing one execution regardless of the initial value of the condition.

When to Use a do‑while Loop?

Use a do‑while loop when you need:

• At least one execution of the loop body.
• The number of iterations to be determined at runtime.
• A prompt‑first approach, such as reading user input until a sentinel value is entered.

Syntax

do {
    // code
} while (condition);

The loop body precedes the while condition, so the compiler executes the body first and then checks the condition to decide whether to iterate again.

How It Works

Flow‑chart steps:

1. Start the loop.
2. Execute the loop body.
3. Evaluate the condition.
4. If true, repeat from step 2.
5. If false, exit the loop and continue with subsequent statements.

Example 1: Counting with a do‑while Loop

#include <iostream>
using namespace std;
int main() {
    int x = 1;
    do {
        cout << "X is: " << x << endl;
        ++x;
    } while (x < 5);
    return 0;
}

Output:

The loop prints values 1 through 4 because the condition x < 5 becomes false after x reaches 5.

Example 2: Summing Numbers Until Zero Is Entered

#include <iostream>
using namespace std;
int main() {
    int num, sum = 0;
    do {
        cout << "Enter a number: ";
        cin >> num;
        sum += num;
    } while (num != 0);
    cout << "Sum is " << sum;
    return 0;
}

Output:

In this example, the loop continues until the user enters 0, ensuring that at least one number is processed.

Nested do‑while Loops

Placing a do‑while loop inside another creates a nested structure, useful for multi‑dimensional iterations such as printing a grid or performing compound calculations.

Syntax

do {
    // outer body
    do {
        // inner body
    } while (inner_condition);
} while (outer_condition);

How It Works

Execution proceeds as follows:

1. Initialize outer variables.
2. Enter the outer loop body.
3. Enter the inner loop body.
4. After each inner iteration, evaluate inner_condition to decide whether to continue the inner loop.
5. Once the inner loop ends, update outer variables and evaluate outer_condition to decide whether to repeat the outer loop.

Example

#include <iostream>
using namespace std;
int main() {
    int a = 1;
    do {
        int b = 1;
        do {
            cout << a << "\n";
            ++b;
        } while (b <= 3);
        ++a;
    } while (a <= 3);
    return 0;
}

Output:

Key Takeaways

• The do‑while loop guarantees at least one execution of the loop body.
• Use it when the iteration count is unknown until runtime.
• The loop’s condition is checked after the body, not before.
• Nested loops enable multi‑level iteration, but beware of complexity and readability.