08-loops.md

This material was written by Aasmund Eldhuset; it is owned by Khan Academy and is licensed for use under CC BY-NC-SA 3.0 US. Please note that this is not a part of Khan Academy's official product offering.

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for

Kotlin's loops are similar to Python's. for iterates over anything that is iterable (anything that has an iterator() function that provides an Iterator object), or anything that is itself an iterator:

val names = listOf("Anne", "Peter", "Jeff")
for (name in names) {
    println(name)
}

Note that a for loop always implicitly declares a new read-only variable (in this example, name) - if the outer scope already contains a variable with the same name, it will be shadowed by the unrelated loop variable. For the same reason, the final value of the loop variable is not accessible after the loop.

You can also create a range with the .. operator - but beware that unlike Python's range(), it includes its endpoint:

for (x in 0..10) println(x) // Prints 0 through 10 (inclusive)

If you want to exclude the last value, use until:

for (x in 0 until 10) println(x) // Prints 0 through 9

You can control the increment with step:

for (x in 0 until 10 step 2) println(x) // Prints 0, 2, 4, 6, 8

The step value must be positive. If you need to count downwards, use the inclusive downTo:

for (x in 10 downTo 0 step 2) println(x) // Prints 10, 8, 6, 4, 2, 0

Any of the expressions to the right of in in the loops above can also be used outside of loops in order to generate ranges (one type of iterables - this is similar to xrange() in Python 2 and range() in Python 3), which can be iterated over later or turned into lists:

val numbers = (0..9).toList()

If you need to know the index of the current element when you're iterating through something, you can use withIndex(), which corresponds to enumerate(). It produces a sequence of objects that have got two properties (the index and the value) and two specially-named accessor functions called component1() and component2(); Kotlin lets you destructure such an object into a declaration:

for ((index, value) in names.withIndex()) {
    println("$index: $value")
}

while

The while loop is similar to Python (but keep in mind that the condition must be an actual boolean expression, as there's no concept of truthy or falsy values).

val x = 0
while (x < 10) {
    println(x)
    x++ // Same as x += 1
}

The loop variable(s), if any, must be declared outside of the while loop, and are therefore available for inspection afterwards, at which point they will contain the value(s) that made the loop condition false.

continue and break

A plain continue or break works the same way as in Python: continue skips to the next iteration of the innermost containing loop, and break stops the loop. However, you can also label your loops and reference the label in the continue or break statement in order to indicate which loop you want to affect. A label is an identifier followed by @, e.g. outer@ (possibly followed by a space). For example, to generate primes:

outer@ for (n in 2..100) {
    for (d in 2 until n) {
        if (n % d == 0) continue@outer
    }
    println("$n is prime")
}

Note that there must be no space between continue/break and @.

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