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Map operations

Learn Map management with [] and .putIfAbsent.

Map operations

Read a value from a Map with square brackets: ages['Ayşe'] returns the value for 'Ayşe', or null if it is missing. Adding a pair uses the same syntax: ages['Cem'] = 26;.

putIfAbsent places a value only when the key is absent: ages.putIfAbsent('Deniz', () => 30);. If the key exists it does nothing; otherwise it calls the given function and stores the result.

Use remove to delete and entries to see all pairs. length reports the count. Because the [] operator returns null for a missing key, null safety matters here; the next group dives into that.

putIfAbsent vs update

putIfAbsent only sets a value when the key is absent; if present, it leaves it untouched. update changes the existing value; if the key is missing it may throw or compute a default.

For example ages.putIfAbsent('Cem', () => 25); sets 25 when 'Cem' is missing. ages.update('Ayşe', (v) => v + 1); increments Ayşe's age by one.

Reach for putIfAbsent on first insert and update when you need to modify. Pick the one that matches your task.

Practice Phone book

Create an empty Map<String, String>. Use putIfAbsent to add two people: name as key, phone as value. Then print the phone book with print.

Starter code

main.dart
void main() {
  var phones = <String, String>{};
  // putIfAbsent two people

  // print
}

Sample solution

main.dart
void main() {
  var phones = <String, String>{};
  phones.putIfAbsent('Ayşe', () => '555-1000');
  phones.putIfAbsent('Mert', () => '555-2000');
  print(phones);
}

Expected output

{Ayşe: 555-1000, Mert: 555-2000}

Questions and answers

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

1 When does putIfAbsent set a value?

  • Always
  • When the key exists
  • Never
  • When the key is missing Correct answer

Explanation: putIfAbsent only sets a value when the key is absent.

2 Read a value

Question 
void main() {
  var ages = <String, int>{'Ayşe': 28};
  print(ages____);
}
Answer 
void main() {
  var ages = <String, int>{'Ayşe': 28};
  print(ages['Ayşe']);
}

Explanation: square brackets fetch the value for the key.

3 Match the operations

Term Meaning
[] Read or write a value
putIfAbsent Only set if absent
update Modify existing
remove Delete the key

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