Programming in C++ – Looping Structures

Comprehensive guide to C++ looping structures — while, do-while, and for loops — with clear syntax, annotated examples, menu-driven program patterns, and practical tips for beginners.

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While Loop

do..while Loop

What is a do-while loop?

A do-while loop executes the code at least once, then checks the condition.

Syntax

do {
    // code to execute
} while (condition);

👉 Key point: The condition is checked after the loop body.

Example 1: Print Numbers from 1 to 5

#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:

• i starts at 1
• The loop prints the number
• i increases by 1
• The loop runs until i becomes greater than 5

Example 2: Print “Hello” 3 Times

#include <iostream>
using namespace std;

int main() {
    int count = 1;

    do {
        cout << "Hello\n";
        count++;
    } while (count <= 3);

    return 0;
}

Explanation:

• The loop runs exactly 3 times
• Useful when you know how many times to repeat something

Example 3: Sum of First 5 Natural Numbers

#include <iostream>
using namespace std;

int main() {
    int i = 1, sum = 0;

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

    cout << "Sum = " << sum;
    return 0;
}

Output

Sum = 15

Example 4: Menu-Driven Program (Real-Life Use)

#include <iostream>
using namespace std;

int main() {
    int choice;

    do {
        cout << "\n1. Say Hello";
        cout << "\n2. Say Bye";
        cout << "\n3. Exit";
        cout << "\nEnter your choice: ";
        cin >> choice;

        if (choice == 1)
            cout << "Hello!\n";
        else if (choice == 2)
            cout << "Bye!\n";

    } while (choice != 3);

    return 0;
}

Why do-while here?

• The menu must appear at least once
• Perfect for menus and user input programs

Example 5: Loop Runs At Least Once (Important Concept)

#include <iostream>
using namespace std;

int main() {
    int i = 10;

    do {
        cout << "This will print once\n";
    } while (i < 5);

    return 0;
}

Explanation:

• Condition is false
• Still, the message prints once
• This is the main difference from while loop

Comparison: while vs do-while

When to Use do-while

✅ When the loop must execute at least once ✅ For menus, password checks, user input