Do-While Loop

The do-while loop is a special type of loop that guarantees the code block will execute at least once, even if the condition is false from the beginning. Think of it like trying a new restaurant - you'll definitely go there once, and then decide if you want to go back based on your experience!

Syntax

do {
  // Code to be executed
} while (condition);

The key difference from a regular while loop is that the condition is checked after the code block executes, not before.

How It Works

1. Execute the code block first
2. Check the condition
3. If the condition is true, repeat from step 1
4. If the condition is false, exit the loop

Basic Example

void main() {
  int count = 1;
  
  do {
    print('Count: $count');
    count++;
  } while (count <= 3);
}

Output:

Count: 1
Count: 2
Count: 3

Key Difference: Do-While vs While

Let's see what happens when the condition is false from the start:

Do-While Loop

void main() {
  int number = 10;
  
  do {
    print('This will print at least once: $number');
  } while (number < 5);  // false condition
}

Output:

This will print at least once: 10

Regular While Loop

void main() {
  int number = 10;
  
  while (number < 5) {  // false condition
    print('This will never print: $number');
  }
}

Output: (nothing prints)

Practical Example: User Input Validation

import 'dart:io';

void main() {
  String? userInput;
  
  do {
    print('Enter "yes" to continue or "no" to exit:');
    userInput = stdin.readLineSync();
    
    if (userInput != 'yes' && userInput != 'no') {
      print('Invalid input! Please try again.');
    }
  } while (userInput != 'yes' && userInput != 'no');
  
  print('You entered: $userInput');
}

This ensures the user is prompted at least once and keeps asking until they provide valid input.

Common Use Cases

Use Case	Description
Menu Systems	Show menu at least once, repeat based on user choice
Input Validation	Ask for input, validate, and re-ask if invalid
Game Loops	Play game at least once, ask if player wants to continue
Data Processing	Process data once, check if more data exists

Important Notes

• Always ensure your condition can eventually become false to avoid infinite loops
• The semicolon (;) after the while condition is required
• Use do-while when you need to execute code at least once, regardless of the initial condition

The do-while loop is perfect when you want to guarantee that something happens at least once - like giving everyone a chance to try something before deciding if they want to continue!