Introduction to C++ Do…While Loop

In C++, a do…while loop is a unique control structure that executes a block of code at least once, even if the condition is initially false. Unlike other loops, the condition is checked after the loop body executes. This makes it perfect for scenarios like user input validation and menu-driven programs.

2. C++ Do…While Loop Syntax

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

Parts of the do…while loop:

  1. do → Keyword that starts the loop.
  2. Loop Body → Code executed at least once.
  3. while (condition) → Boolean expression checked after execution.
  4. Semicolon (;) → Required after the while statement.

3. How the Do…While Loop Works

  1. Execute the loop body first.
  2. Check the condition.
  3. If true, repeat the loop.
  4. If false, exit the loop.

4. Example 1 – Printing Numbers in C++

#include <iostream>
using namespace std;

int main() {
    int i = 1;
    do {
        cout << "Number: " << i << endl;
        i++;
    } while (i <= 5);
    return 0;
}

Flow Explanation:

  • Step 1: i = 1 → print → i++ → i = 2 → check (2 ≤ 5) → true.
  • Step 2: i = 2 → print → i++ → i = 3 → check → true.
  • …
  • Step 5: i = 6 → check (6 ≤ 5) → false → stop.

5. Example 2 – Password Validation in C++

#include <iostream>
using namespace std;

int main() {
    string password;
    do {
        cout << "Enter password: ";
        cin >> password;
    } while (password != "admin123");
    cout << "Access granted!" << endl;
    return 0;
}

Flow Explanation:

  • Ask for password → check → if wrong, repeat.
  • Repeat until correct input → print "Access granted!"

6. Execution Flow Diagram

 ┌─────────────┐
 │ Start Loop  │
 └──────┬──────┘
        ↓
┌─────────────────┐
│ Execute Body     │
└──────┬──────────┘
       ↓
┌─────────────────┐
│ Check Condition? │
└──────┬──────────┘
       │ Yes
       ↓
   Repeat Loop
       │
      No
       ↓
  End Program

7. Features of C++ Do…While Loop

  • Condition checked after execution.
  • Executes at least once.
  • Ideal for input-based repetition.
  • Requires semicolon after while.

8. Advantages

  • Guarantees at least one execution.
  • Best for menu-driven programs.
  • Useful for input validation in C++.

9. Disadvantages

  • May execute unwanted iteration if initial condition is false.
  • Risk of infinite loop if condition never becomes false.

10. Importance of Do…While Loop in C++

  • Ensures mandatory first execution.
  • Reduces the need for extra checks before loops.
  • Helps in interactive programs like menus and input forms.

11. Real-Life Examples

  • ATM asking for PIN until correct.
  • Game menu that reappears after a choice.
  • Login system prompting for credentials.

12. Conclusion

The C++ do…while loop is an essential looping structure that guarantees at least one execution, making it ideal for user input validation, menu-driven programs, and interactive applications. Understanding its syntax and flow helps beginners and advanced programmers write efficient and error-free C++ programs.