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.

Exercise 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.

Show example solution

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

Test yourself

Fill in the blank: map loop

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

Show answer

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

Match loop expression to use

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
In for (x in map), what is the type of x?
- Exception
- Only the key
- Only the value
- Entry<K, V>
Show answer

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

Next lesson: Summary - Maps and Sets

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