[Kotlin] Enhanced Loops

In Kotlin, you can write much simpler and more convenient loops compared to Java. Let's see how you can use them.

1. .. operator

val fruits = listOf("Apple", "Banana", "Cherry", "Durian")

fun main() {
    for (index in 0..fruits.size - 1) {
        val fruit = fruits[index]
        println("$index: $fruit")
    }
}

Using .. creates a traditional loop that increments by 1.

2. downTo

val fruits = listOf("Apple", "Banana", "Cherry", "Durian")

fun main() {
    for (index in fruits.size - 1 downTo 0) {
        val fruit = fruits[index]
        println("$index: $fruit")
    }
}

Using downTo creates a loop that decrements as expected.

3. step

val fruits = listOf("Apple", "Banana", "Cherry", "Durian")

fun main() {
    for (index in 0..fruits.size - 1 step 2) {
        val fruit = fruits[index]
        println("$index: $fruit")
    }
}

With the step keyword, you can implement a loop that skips a specific number of elements. This also applies to downTo.

4. until

val fruits = listOf("Apple", "Banana", "Cherry", "Durian")

fun main() {
    for (index in 0 until fruits.size) {
        val fruit = fruits[index]
        println("$index: $fruit")
    }
}

Using until creates a loop that does not include the last number, eliminating the need for -1.

5. lastIndex

val fruits = listOf("Apple", "Banana", "Cherry", "Durian")

fun main() {
    for (index in 0 .. fruits.lastIndex) {
        val fruit = fruits[index]
        println("$index: $fruit")
    }
}

Now, using the lastIndex property, loops start to become easier to read. But of course, there's more to explore.

6. indices

val fruits = listOf("Apple", "Banana", "Cherry", "Durian")

fun main() {
    for (index in fruits.indices) {
        val fruit = fruits[index]
        println("$index: $fruit")
    }
}

indices returns the index range of the collection.

7. withIndex()

val fruits = listOf("Apple", "Banana", "Cherry", "Durian")

fun main() {
    for ((index, fruit) in fruits.withIndex()) {
        println("$index: $fruit")
    }
}

By using withIndex(), extracting both index and value simultaneously simplifies the code, resembling Python's simplicity. This should be sufficient for most loop scenarios, but there's one more method left.

8. forEachIndexed

val fruits = listOf("Apple", "Banana", "Cherry", "Durian")

fun main() {
    fruits.forEachIndexed { index, fruit ->
        println("$index: $fruit")
    }
}

Using a lambda function with forEachIndexed can make the code more concise, intuitive, and straightforward. Choose the appropriate method that suits your needs.

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Reference

Kotlin Tips: Loops