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length and contains

Learn list info with .length and .contains.

length and contains

You find how many elements a List has with the .length property: [1, 2, 3].length evaluates to 3. You ask whether an element exists with the .contains() method: [1, 2, 3].contains(2) evaluates to true.

.length is a property (no parentheses); .contains() is a method (parentheses required). Both are common: length for loop bounds, contains for existence checks. For example if (cart.contains("Notebook")) checks whether the cart has a product.

Querying information

.length and .contains are two ways to query a list without changing it. With length you set a loop bound; with contains you check an element's existence. Neither mutates the original list.

For example in a shopping cart cart.length tells how many products there are; cart.contains("Notebook") tells whether a notebook is in the cart. Query methods are side-effect free; they only read. Practical tip: use non-mutating methods when querying information.

Practice Print length and contains

Declare a List with var numbers = [1, 2, 3];. Print the count with print(numbers.length);. Print whether 2 is in the list with print(numbers.contains(2));. The output must be two lines: 3 and true.

Starter code

main.dart
// Declare numbers, print length and contains

Sample solution

main.dart
var numbers = [1, 2, 3];
print(numbers.length);
print(numbers.contains(2));

Expected output

3
true

Questions and answers

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

1 What does [1, 2, 3].contains(2) return?

main.dart
[1, 2, 3].contains(2)
  • 2
  • 1
  • true Correct answer
  • -1

Explanation: 2 exists in the list; contains returns true.

2 Match each method to its job

Term Meaning
.length Element count
.contains(x) Exists (bool)
.add(x) Appends to end
[i] I-th element

3 Write the length call

Question 
var numbers = [1, 2, 3];
print(numbers____);
Answer 
var numbers = [1, 2, 3];
print(numbers.length);

Explanation: .length gives the element count of the List.

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