C++ do-while Loop: How It Works and When to Use It

The do-while loop is the third kind of loop in C++ (after for and while). It has one key difference from the others: it always runs at least once, because it checks the condition after executing the body.

Syntax

do {
    // body — runs at least once
} while (condition);
//         ^--- note the semicolon — required!

Don’t forget the semicolon after the closing while (condition). Forgetting it is the #1 syntax mistake with do-while.

How It Works

int i = 0;
do {
    cout << i << " ";
    i++;
} while (i < 5);
// Output: 0 1 2 3 4

Execution order:

1. Run the body: print 0, increment to 1
2. Check condition: 1 < 5 → true → repeat
3. Run body: print 1, increment to 2
4. Check condition: 2 < 5 → true → repeat
5. …
6. Run body: print 4, increment to 5
7. Check condition: 5 < 5 → false → stop

The body ran 5 times.

The Key Difference: Runs at Least Once

With a while loop, if the condition is false from the start, the body never runs:

int x = 10;

while (x < 5) {
    cout << "This never prints" << endl;
}

do {
    cout << "This prints once" << endl;
} while (x < 5);

The while loop body never executes. The do-while body executes once, then checks the condition (which is false), so it stops — but it did run once.

The Classic Use Case: Input Validation

The most common reason to reach for do-while is asking for user input. You always need to prompt at least once, then repeat if the input is invalid:

#include <iostream>
using namespace std;

int main() {
    int age;

    do {
        cout << "Enter your age (must be positive): ";
        cin >> age;

        if (age <= 0) {
            cout << "Invalid age. Try again." << endl;
        }
    } while (age <= 0);

    cout << "Your age is: " << age << endl;
    return 0;
}

Sample session:

Enter your age (must be positive): -5
Invalid age. Try again.
Enter your age (must be positive): 0
Invalid age. Try again.
Enter your age (must be positive): 25
Your age is: 25

Without do-while, you’d need to write the prompt twice — once before the loop and once inside it. do-while eliminates that duplication.

Menu-Driven Programs

Another great fit: programs with menus where you always show the menu at least once, then repeat until the user chooses to quit:

#include <iostream>
using namespace std;

int main() {
    int choice;

    do {
        cout << "\n--- Menu ---\n";
        cout << "1. Say hello\n";
        cout << "2. Say goodbye\n";
        cout << "3. Quit\n";
        cout << "Choice: ";
        cin >> choice;

        switch (choice) {
            case 1: cout << "Hello!\n"; break;
            case 2: cout << "Goodbye!\n"; break;
            case 3: cout << "Exiting...\n"; break;
            default: cout << "Invalid choice.\n";
        }
    } while (choice != 3);

    return 0;
}

The menu always displays at least once. If the user picks 3, the loop exits. Otherwise, it loops back to show the menu again.

Comparing while vs do-while

// while loop: checks first
int n = 0;
while (n > 0) {
    cout << n;
    n--;
}
// Nothing prints — condition was false from the start

// do-while: runs first, then checks
do {
    cout << n;
    n--;
} while (n > 0);
// Prints: 0 (ran once before checking)

Rule of thumb:

• Use while when you might not want to execute the body at all
• Use do-while when the body must execute at least once (usually for prompts and menus)

Nested do-while

You can nest do-while inside other loops, or vice versa:

int i = 1;
do {
    int j = 1;
    do {
        cout << i * j << "\t";
        j++;
    } while (j <= 5);
    cout << "\n";
    i++;
} while (i <= 3);

Output (multiplication table):

1  2  3  4  5
2  4  6  8  10
3  6  9  12 15

Common Mistakes

Forgetting the semicolon:

do {
    cout << "hi\n";
} while (true)   // Error: missing semicolon

Infinite loop:

do {
    cout << "infinite\n";
} while (true);  // Runs forever — add a break or condition change

Counter not updating:

int i = 0;
do {
    cout << i;
    // Forgot i++; — infinite loop!
} while (i < 5);

Summary

The do-while loop is simple but has one important property: the body always executes at least once. Use it when you need this guarantee — especially for input validation and menu loops. For everything else, for and while are more common choices.

Related Articles

• C++ Loops Tutorial — for, while, and do-while compared in full
• C++ User Input with cin — reading keyboard input (pairs with do-while for validation)
• C++ Calculator Program — a project using do-while for looping

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