Kotlin - Convert while loop to functional style

Question

I have the following Kotlin function:

fun func(n: Int): Int {

    var count = 1

    var m = n
    while(m != 1) {

        m = if(m.isOdd()) 3 * m + 1 else m / 2

        count++
    }

    return count
}

I would like to write that simple algorithm in a "functional" style, using Kotlin's operators like map(), count(), etc. The closest thing I could come up with was this:

fun func(n: Int): Int {

    return n.toList()
            .map{ if(it.isOdd()) 3*it+1 else it/2 }
            .takeWhile { it != 1 }
            .count()

}

Obviously, the above code does not work because map is executed only once, but you get the idea of what I am trying to achieve.

PS: toList() is just an extension function that converts an int to a list containing that int:

fun Int.toList() = listOf(this)

While @hotkey probably provided the right answer, I think we all should agree that your original code is actually more memory efficient and will not result in creating a sequence with potentially thousands elements just to find out how many elements were generated. — Strelok
– Strelok, Commented Aug 17, 2017 at 0:37
@Strelok That is a good point. What I was looking for is a solution where all variables were immutable and hotkey's answer delivers that. However, if someone knows a way to do the same thing but 'discarding elements on-the-fly' (so memory is not an issue) please let us know. — Bitcoin Cash - ADA enthusiast
– Bitcoin Cash - ADA enthusiast, Commented Aug 17, 2017 at 0:43

hotkey · Accepted Answer · 2017-08-17 00:39:01Z

10

Since you do not know how many items there will be, you can construct a (possibly infinite) sequence where each item is calculated based on the previous one, then limit it with your condition it != 1 and count how many items there are:

return generateSequence(n) { if (it.isOdd()) 3 * it + 1 else it / 2 }
        .takeWhile { it != 1 }
        .count()

Here, generateSequence(n) { ... }, constructs a Sequence<Int> that has n for its first element, and each of the following elements is calculated by the code passed as a lambda (it is called on the previous element, and only when another item is queried, i.e. lazily).

edited Aug 17, 2017 at 0:39

answered Aug 17, 2017 at 0:22

hotkey

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