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contains and indexOf

Learn search with contains and indexOf.

contains and indexOf

To ask whether a substring exists inside a String you use the .contains() method: "welcome".contains("come") evaluates to true. The return type is bool; either true or false.

If you want the position of the substring you use .indexOf(): "welcome".indexOf("come") returns 3. Indexing is zero-based. If the substring is not found .indexOf() returns -1.

-1 when not found

When the substring is not found .indexOf() returns -1. This value is often used to check whether a substring exists: if (email.indexOf("@") != -1) roughly tests whether a text might be an email.

A more readable way is to use .contains(): if (email.contains("@")). Both forms give the same result; .contains() is clearer. Practical rule: use .contains() for existence, .indexOf() for position.

Practice Find the @ index

Declare a String variable email with the value "alex@dart.com". Declare a new variable at and assign the result of email.indexOf("@") to it. Then print at with print(at);. The output on a single line must be 4.

Starter code

main.dart
// Declare email, assign the @ index to at via indexOf, print it

Sample solution

main.dart
String email = "alex@dart.com";
var at = email.indexOf("@");
print(at);

Expected output

4

Questions and answers

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

1 What does "welcome".contains("come") return?

main.dart
"welcome".contains("come")
  • true Correct answer
  • false
  • 3
  • -1

Explanation: "come" exists inside "welcome"; the result is true.

2 Match each method to its job

Term Meaning
contains(x) Exists (bool)
indexOf(x) First index (-1 if absent)
substring(0, 3) Takes a part
.length Character count

3 Write the indexOf call

Question 
String email = "a@b.com";
print(email____);
Answer 
String email = "a@b.com";
print(email.indexOf("@"));

Explanation: .indexOf("@") gives the index of the @ sign.

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