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LESSON 106 OF 348 · FOR LOOPS - LOOPING OVER ARRAYS

Looping Through Arrays in Kotlin: The for...in Loop

Lesson Read the full explanation

A for loop is a special type of loop in Kotlin which allows us to loop over the items of a collection, such as an array.

Looping through an array:

For example, an array is a sequence of items, so we can use a for loop to iterate over each item and do something with it:

val fruits = arrayOf("banana", "orange", "pear", "kivi")

for (fruit in fruits) {
  println("I eat $fruit")
}

The output of this code is:

I eat banana
I eat orange
I eat pear
I eat kivi

Let's break down this code:

  • The for keyword that indicates the start of a for loop.
  • String fruit is the loop variable. It will take the value of each different item in the collection
  • The : separates the loop variable from the collection we will iterate on
  • fruits is the collection to loop over

Assignment
Now let's greet our friends using a for loop!

Hint
Look at the examples above if you get stuck.

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    KOTLIN · FOR LOOPS - LOOPING OVER ARRAYS

    Looping Through Arrays in Kotlin, explained

    The for loop

    A for loop runs a block of code once for every item of a collection, such as an array or a list. You name a variable, and on each pass it holds the next item:

    val fruits = arrayOf("banana", "orange", "pear", "kiwi")

for (fruit in fruits) {
    println("I eat $fruit")
}

    The output is:

    I eat banana
I eat orange
I eat pear
I eat kiwi

    The same four lines of code would have taken four println calls with indices. Now the loop handles any number of fruits, including none.

    The parts of the loop

    • for starts the loop.
    • fruit is the loop variable. On the first pass it is "banana", on the second "orange", and so on. You choose the name.
    • in separates the loop variable from the collection. Other languages use a colon here. Kotlin reads like English: "for each fruit in fruits".
    • fruits is the collection being looped over.
    • The block between { and } is the body. It runs once per item.

    The loop variable needs no val or var and no type. Kotlin declares it for you and works out the type from the collection, so here fruit is a String. It is read-only inside the body: you cannot assign a new value to it.

    String templates

    Inside the body, "I eat $fruit" is a string template. The $ followed by a name inserts that variable's value into the text. For anything more complex than a name, wrap it in braces: "${fruit.length} letters". To print a literal dollar sign, escape it as \$.

    The exercise

    The starting code has an array of friends:

    val friends = arrayOf("Andy", "Mary", "John")

    The task is to greet each friend with a loop, in two steps:

    1. Write the loop. Use for with a variable named friend and loop through the friends array: for (friend in friends) { }. The names are different on purpose: friends is the whole array, and friend is one item of it.
    2. Print inside the body. For every friend, print Hey, followed by the name and an exclamation mark. A template does this in one line: println("Hey, $friend!").
    val friends = arrayOf("Andy", "Mary", "John")

for (friend in friends) {
    println("Hey, $friend!")
}

    The output is:

    Hey, Andy!
Hey, Mary!
Hey, John!

    The template stops reading the name at the !, because an exclamation mark cannot be part of a variable name. A letter would not stop it. That is the reason for the braces: "$friendly" looks for a variable named friendly, while "${friend}ly" uses friend.

    Other ways to loop

    Sometimes you need the position of each item as well as the item. Kotlin gives two common ways:

    for (i in friends.indices) {
    println("$i ${friends[i]}")
}

for ((i, friend) in friends.withIndex()) {
    println("$i $friend")
}

    Both print each friend with its index, starting at 0. The property indices is the range of valid indices, 0 to size - 1, so the loop cannot run past the end. A loop also works on a number range: for (x in 1..3) visits 1, 2 and 3, since .. includes both ends. Use 0 until n when the end should be left out.

    Common mistakes

    • Using a colon. for (friend : friends) is Java syntax. Kotlin uses in.
    • Declaring the loop variable. Writing val friend or String friend inside the parentheses is an error.
    • Printing outside the body. A statement after the closing brace runs once, after the loop, and cannot see the loop variable.
    • Looping to size by index. for (i in 0..friends.size) includes the index friends.size, which does not exist, and throws an ArrayIndexOutOfBoundsException. Loop over the items directly, or use indices.
    • Changing a list while looping over it. Adding to or removing from a list inside a for loop over that list usually throws a ConcurrentModificationException.

    Key takeaways

    • for (item in collection) { ... } runs the body once per item.
    • The loop variable is declared by the loop itself and is read-only.
    • Use $name or ${expression} to put values in strings.
    • Use indices or withIndex() when you need the position as well.
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