List iteration

Learn to iterate over lists with for and forEach.

List iteration: for and forEach

The most natural way to walk all elements of a list is the for (x in list) loop. On each pass x is the current element. forEach { } does the same with a lambda.

val cities = listOf("Vienna", "Berlin", "Madrid")
for (city in cities) {
    println(city)
}

If you also need indexes, you can iterate cities.indices (the range 0..size-1) or walk index-value pairs with cities.withIndex().

forEach, indices and withIndex

forEach takes a lambda and runs it for every element. Inside the lambda the element is known as it. indices returns the index range and withIndex() yields (index, value) pairs.

cities.forEach { println(it) }
for (i in cities.indices) println(i)
for ((i, c) in cities.withIndex()) println("$i: $c")

When you need an index, withIndex() is the most readable choice; it gives position and value at once.

Exercise Print the cities with a for loop

Create a listOf("Vienna", "Berlin", "Madrid") and use a for (city in cities) loop to print each city on its own line.

Show example solution

fun main() {
    val cities = listOf("Vienna", "Berlin", "Madrid")
    for (city in cities) {
        println(city)
    }
}

Test yourself

Which one prints each element on its own line?
- for (x in list) println(x)
- for (x of list) println(x)
- foreach (x in list) println(x)
- for (x = 0; x < list; x++) println(x)
Show answer

for (x in list) println(x) — Kotlin's loop form is for (x in list).

Fill in the blank: for loop

for (name ____ names) println(name)

Show answer

for (name in names) println(name)

Match loop expression to use

for (x in list)

Walks each element
list.forEach { }

Walks with a lambda
list.indices

Range 0..size-1
list.withIndex()

Index+value pairs
What does list.withIndex() return?
- Only indexes
- IndexedValue stream
- Only values
- Boolean
Show answer

IndexedValue stream — withIndex() yields IndexedValue pairs; index and value together.

Next lesson: List methods

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