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Map iteration

Learn to iterate over maps with for.

Map iteration: walking with for

To walk all pairs of a map you use the for ((k, v) in map) loop. Here k is the key and v is the value. To walk only keys or only values you use map.keys or map.values.

val ages = mapOf("Ava" to 25, "Liam" to 30)
for ((name, age) in ages) {
    println("$name: $age")
}

This destructuring loop splits each Entry into its key and value components automatically.

forEach and the Entry type

forEach { } walks each pair with a lambda. Inside the lambda the element is an Entry<K, V>; you reach its parts with .key and .value.

ages.forEach { entry ->
    println("${entry.key} = ${entry.value}")
}
// or the short form:
ages.forEach { println("${it.key} = ${it.value}") }

it is the automatic name of the single lambda parameter; that is why naming is optional for one-parameter lambdas.

Practice Print city-population pairs

Create a mapOf("Vienna" to 1, "Berlin" to 3). Use a for ((city, pop) in pops) loop to print each pair as "$city: $pop million" on its own line.

Starter code

Main.kt
fun main() {
    val pops = mapOf("Vienna" to 1, "Berlin" to 3)
    // Print each city-population pair as "city: pop million"
}

Sample solution

Main.kt
fun main() {
    val pops = mapOf("Vienna" to 1, "Berlin" to 3)
    for ((city, pop) in pops) {
        println("$city: $pop million")
    }
}

Expected output

Vienna: 1 million
Berlin: 3 million

Questions and answers

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

1 Fill in the blank: map loop

Question 
for ((k, v) ____ map) println("$k = $v")
Answer 
for ((k, v) in map) println("$k = $v")

Explanation: for ((k, v) in map) walks each pair.

2 Match loop expression to use

Term Meaning
for ((k, v) in map) Walks key-value pairs
for (k in map.keys) Walks only keys
for (v in map.values) Walks only values
map.entries All Entry pairs

3 In for (x in map), what is the type of x?

  • Exception
  • Only the key
  • Only the value
  • Entry<K, V> Correct answer

Explanation: When you iterate a map with for, each element is an Entry<K, V>.

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