How do you iterate a for loop with multiplication or division? [Kotlin]

Question

How do you write an iteration step to multiply, divide, or have some custom iteration function in Kotlin? I know I can use while loops instead, but for loops seem more restrictive in scope than in other languages. Is this by design?

What is available in most other languages:

for(let i = 1; i <= 10; i *= 2) // [JavaScript]
for i := 1; i <= 10; i*=2 // [Golang]

What I want to do in Kotlin (or something close to it):

for(i in 1..10 step 2 * i) // [Kotlin]

It would be easy if we could reference the iterator in the for loop body itself, but the above will cause an 'Unresolved reference' error since i cannot be referenced in the iteration step.

Answer 1

Yes, it's a pity that Kotlin designers decided not to support C-style for loops, but you can do some tricks with extension functions.

Declare infix function

// for increasing rangeTo (..)
infix fun IntRange.step(next: (Int) -> Int) =
        generateSequence(first, next).takeWhile { if (first < last) it <= last else it >= last }

// for decreasing downTo
infix fun IntProgression.step(next: (Int) -> Int) = (first..last).step(next)

and use it like following

for (i in 1..10 step { it * 2 }) {
    println(i)
}

Output

1
2
4
8

And

for (i in 16 downTo 1 step { it / 2 }) {
    println(i)
}

Output

16
8
4
2
1