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trim and pad

Learn formatting with trim, padLeft and padRight.

trim

To remove the whitespace characters (space, tab, newline) at the start and end of a string use trim. It is especially useful when taking user input; if the user accidentally left a space you clean it up. " apple ".trim() gives "apple".

If you only want to clean one side, there are trimLeft and trimRight as well. trim does not modify the original string; it returns a new one. So do not forget to assign the result to a variable.

padLeft and padRight

To pad a string to a given length use padLeft and padRight. They fill the missing characters with a character you choose. "7".padLeft(3, "0") gives "007". Ideal for showing numbers at a fixed width.

If no padding character is given, space is used by default. padRight does the same from the right: "ad".padRight(5, "_") gives "ad___". Neither adds anything if the string already meets or exceeds the target length; they never shorten it.

Practice Your turn: trim and padLeft

First trim the string " 7 ", then pad it to three characters on the left with padLeft(3, "0"). Print the result with print.

Starter code

main.dart
void main() {
  // First trim the spaces
  // Then padLeft(3, "0") and print
}

Sample solution

main.dart
void main() {
  final result = "  7  ".trim().padLeft(3, "0");
  print(result);
}

Expected output

007

Questions and answers

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

1 What does " apple ".trim() return?

main.dart
final x = "  apple  ".trim();
  • "apple "
  • "apple"
  • " apple" Correct answer
  • " apple "

Explanation: trim cuts spaces from both sides.

2 The trim call

Question 
final clean = "  apple  ".____();
Answer 
final clean = "  apple  ".trim();

Explanation: Write trim; it cleans spaces.

3 Match trim and pad

Term Meaning
trim cuts spaces on both sides
trimLeft cuts spaces on the left
padLeft pads on the left
padRight pads on the right

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