Tag: loops

Suppose you wanted to print the string “Hello there!” three times. In any language that borrows its syntax from C, you’d probably use the classic for loop, as shown below:

// C99
for (int i = 0; i < 3; i++) {
    printf("Hello there!\n");
}

// JavaScript
for (let i = 0; i < 3; i++) {
    console.log("Hello there!");
}

With Kotlin’s for loop, you have a couple of options:

// Kotlin

// Here’s one way you can do it:
for (index in 0 until 3) {
    println("Hello there!")
}

// Here’s another way:
for (i in 1..3) {
    println("Hello there!")
}

If you’re used to programming in a C-style language, you’ll probably reach for a for loop when you need to perform a task a given number of times. The Kotlin language designers noticed this and came up with something more concise: the repeat() function:

repeat(3) {
    println("Hello there!")
}

As you can see, using repeat() give you a shorter, easier to read line than for. And in most cases, shorter and easier to read is better.

Do you need to know the current iteration index? Here’s how you’d get that value:

fun main() {
    repeat(3) { iteration ->
    	println("Hello there from iteration $iteration!")
    }
}

Remember, you can use whatever variable name you want for that index, as long as it’s a valid name:

fun main() {
    repeat(3) { thingy ->
    	println("Hello there from iteration $thingy!")
    }
}

Note that repeat() is for performing a task a known number of times, without exceptions. It’s a function that takes a closure, not a loop structure. This means that the break and continue statements won’t work:

// This WILL NOT compile!

fun main() {
    repeat(3) { iteration ->

        // 🚨🚨🚨 You can’t do this 🚨🚨🚨
        if (iteration == 1) {
            break
        }

        // 🚨🚨🚨 You can’t do this, either 🚨🚨🚨
        if (iteration == 2) {
            continue
        }

        // “break” and “continue” work only
        // inside a loop, and repeat()
        // *isn’t* a loop...
        // ...it’s a *function*!

    	println("Hello there from iteration $iteration!")
    }
}

For the curious, here’s the source code for repeat():

@kotlin.internal.InlineOnly
public inline fun repeat(times: Int, action: (Int) -> Unit) {
    contract { callsInPlace(action) }

    for (index in 0 until times) {
        action(index)
    }
}

To summarize: If you need to perform a task n times in Kotlin — and only n times — use repeat instead() of for!