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trim and pad

Learn formatting with trim, padStart and padEnd.

trim: clean edge whitespace

trim removes leading and trailing whitespace from a string. Inner spaces are untouched. trimStart and trimEnd clean only one side.

val s = "  hello  "
println(s.trim())      // "hello"
println(s.trimStart()) // "hello  "
println(s.trimEnd())   // "  hello"

It is a good habit to trim user input before processing. Then stray spaces do not break comparisons. You can also trim specific chars: trim('*').

padStart and padEnd: pad to fixed length

padStart adds a given char to the front, padEnd to the back, until the string reaches the requested length.

val code = "5"
println(code.padStart(3, '0'))  // "005"
println(code.padEnd(3, '.'))    // "5.."

padStart is handy for fixed-width tables or sequence numbers. For example, month numbers become 01, 12. If you omit the second argument, a space is used by default.

Practice Pad the month to two digits

Pad the string "5" to 2 characters with padStart; fill the missing char with '0'. Print the result.

Starter code

Main.kt
fun main() {
    val month = "5"
    // Use padStart to make it 2 chars and print
}

Sample solution

Main.kt
val month = "5"
println(month.padStart(2, '0'))

Expected output

05

Questions and answers

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

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

  • " hi "
  • " hi"
  • "hi "
  • "hi" Correct answer

Explanation: trim removes leading and trailing spaces; not inner ones.

2 Fill in the blank: trim only the end

Question 
val s = "  hi  "
val clean = s.____()
Answer 
val s = "  hi  "
val clean = s.trimEnd()

Explanation: trimEnd removes only trailing spaces.

3 Match method to result

Term Meaning
trim() Edges cleaned
trimStart() Only front
padStart(3, '0') Pad zeros at front
padEnd(3, '.') Pad dots at end

4 Fill in the blank: pad with zeros

Question 
val code = "5"
val padded = code.____(3, '0')
Answer 
val code = "5"
val padded = code.padStart(3, '0')

Explanation: padStart(3, '0') puts two zeros in front, giving "005".

All lessons in Beginner

1 Kotlin basics

  1. What is Kotlin?
  2. Your first program
  3. The main function
  4. Comments
  5. String templates
  6. Summary - Kotlin basics

2 Variables

  1. val and var
  2. Type inference
  3. const
  4. lateinit
  5. by lazy
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. Boolean and Char
  4. String
  5. Any and Unit
  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. Elvis operator
  6. Summary - Operators

6 Strings

  1. Creating strings
  2. String templates
  3. String methods
  4. substring
  5. split and replace
  6. Summary - Strings

7 Conditionals

  1. The if statement
  2. else and else if
  3. if expression
  4. when
  5. when expression
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. Range
  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 Arrays

  1. Creating arrays
  2. Array access
  3. Array size
  4. Array loop
  5. Array methods
  6. Summary - Arrays

11 Lists

  1. Creating lists
  2. List operations
  3. List access
  4. List iteration
  5. List methods
  6. Summary - Lists

12 Maps and Sets

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

13 Functions

  1. Defining functions
  2. Return values
  3. Expression body
  4. Default parameters
  5. Named arguments
  6. Summary - Functions

14 Null safety

  1. Nullable types
  2. ?, !! and ?:
  3. let
  4. Null check
  5. lateinit
  6. Summary - Null safety

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. Regex
  2. matches and find
  3. replace
  4. StringBuilder
  5. trim and pad
  6. Summary - Advanced strings

17 Enums

  1. enum class
  2. Enum values
  3. Enum properties
  4. when with enum
  5. Enum and range
  6. Summary - Enums

18 Exceptions

  1. try/catch
  2. throw
  3. finally
  4. try expression
  5. Custom exceptions
  6. Summary - Exceptions

19 Class basics

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

20 Beginner final

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