Is there an nice way to generate a sequence like 0 1 2 3 2 1 0 …?

# getting-started

[blogscot]

Is there an nice way to generate a sequence like 0 1 2 3 2 1 0 …?

[antonis]

check https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.sequences/sequence.html

[Alan Evans]

fun countUpAndDownSequence(i: Int) =
        (0 until i).asSequence() + (i downTo 0).asSequence()

[blogscot]

I came up with

fun cycle(max: Int): Sequence<Int> =
  sequence {
    var step = 1
    var output = 0

    while (true) {
      step *= when {
        (output == max) and (step == 1) ||
            (output == 0) and (step == -1) -> -1
        else -> 1
      }
      yield(output)
      output += step
    }
  }

but I imagined there was something better

[commanderpepper]

Using Alan's code I came up with

[commanderpepper]

fun countUpAndDownSequence(iterations: Int, max: Int): Sequence<Int> {
    var sequence = emptySequence<Int>()
    for (i in 0..iterations) {
        sequence += (0 until max).asSequence() + (max downTo 0).asSequence()
    }
    return sequence
}

[Alan Evans]

did you run it?

[Alan Evans]

got bugs, `0`s are repeated, and an out by one error on the number iterations.

[ilya.gorbunov]

fun countUpAndDownSequence(iterations: Int, max: Int) = 
    (0 until iterations * max * 2).asSequence()
        .map { max - abs(max - it % (max * 2)) }

If you're not afraid some math 🙂
Though doesn't work with max = 0 well.

[commanderpepper]

fun countUpAndDownSequence(iterations: Int, max: Int): Sequence<Int> {
    var sequence = emptySequence<Int>()
    for (i in 0 until iterations) {
        when (i) {
            0 -> sequence += (0 until max).asSequence() + (max downTo 0).asSequence()
            else -> sequence += (1 until max).asSequence() + (max downTo 0).asSequence()
        }
    }
    return sequence
}

[commanderpepper]

I think that code works

[ursus]

cycle?