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The do-while loop

Learn loops that run at least once with do-while.

The do-while loop

The do-while loop runs the body first, then checks the condition. The form is do { ... } while (condition);. Even if the condition is false the body runs at least once; this is the key difference from while.

For example var i = 1; do { print(i); i++; } while (i <= 3); prints 1, 2, 3. First print(i) and i++ run, then i <= 3 is checked. If true the loop repeats.

At least once

The key feature of do-while is that the body runs at least once. Because the condition is checked after the body, even if the condition is false from the start the body runs once. This behavior is useful especially when reading user input.

For example, to show a menu at least once and repeat until the user picks 'exit', do-while fits well. The same behavior with while needs an extra flag variable; do-while is cleaner. Practical tip: pick do-when when 'at least once' is needed.

Practice Count 1 to 5 with do-while

Start with var i = 1;. Build a do { ... } while (i <= 5); loop; in the body print with print(i); and increase with i++;. The output must be 5 lines: 1, 2, 3, 4, 5 (one number per line).

Starter code

main.dart
// Count 1 to 5 with do-while and print each

Sample solution

main.dart
var i = 1;
do {
  print(i);
  i++;
} while (i <= 5);

Expected output

1
2
3
4
5

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Match each part to its job

Term Meaning
do { Body start
} while (cond); Condition and end
i++ Increase
print(i) Print

2 Write the do-while condition

Question 
var i = 1;
do {
  print(i);
  i++;
} while (____);
Answer 
var i = 1;
do {
  print(i);
  i++;
} while (i <= 3);

Explanation: The condition i <= 3 is true while i is 3 or below.

3 How many times does do { print(i); } while (false); run?

main.dart
do {
  print("hello");
} while (false);
  • 0 times
  • 2 times
  • 1 time Correct answer
  • Infinitely

Explanation: The do-while body runs before the condition; even false it runs once.

All lessons in Beginner

1 Dart basics

  1. What is Dart?
  2. Your first program
  3. dart run
  4. Comments
  5. String interpolation
  6. Summary - Dart basics

2 Variables

  1. var and type
  2. final
  3. const
  4. late
  5. Null safety
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. bool and String
  4. Type inference
  5. dynamic
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. Error handling
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Null-aware operators
  6. Summary - Operators

6 Strings

  1. Creating strings
  2. String interpolation
  3. length and methods
  4. substring
  5. contains and indexOf
  6. Summary - Strings

7 Conditionals

  1. The if statement
  2. else and else if
  3. switch
  4. Pattern matching
  5. Ternary
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. for-in
  5. break and continue
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding items
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Lists

  1. Creating lists
  2. List access
  3. add and remove
  4. length and contains
  5. List methods
  6. Summary - Lists

11 Sets and Maps

  1. Set
  2. Set operations
  3. Map
  4. Map operations
  5. Map iteration
  6. Summary - Sets and Maps

12 Functions

  1. Defining functions
  2. Return values
  3. Arrow functions
  4. Optional parameters
  5. First-class functions
  6. Summary - Functions

13 Null safety in depth

  1. Nullable types
  2. Null-aware operators
  3. The ! operator
  4. late
  5. Null check
  6. Summary - Null safety

14 Class basics

  1. class
  2. Constructor
  3. Instance variables
  4. Methods
  5. this
  6. Summary - Class basics

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 Advanced string operations

  1. split and join
  2. replace
  3. trim and pad
  4. RegExp
  5. StringBuffer
  6. Summary - Advanced strings

17 Enums

  1. Defining enums
  2. Enum values
  3. Enhanced enums
  4. switch with enum
  5. Enum and patterns
  6. Summary - Enums

18 Exceptions

  1. try/catch
  2. throw
  3. finally
  4. The Exception class
  5. on clause
  6. Summary - Exceptions

19 Async basics

  1. Future
  2. async/await
  3. then
  4. Future.error
  5. Future.wait
  6. Summary - Async basics

20 Beginner final

  1. Project design
  2. Classes
  3. Collections
  4. Error handling
  5. Putting it together