Shift kotlin array

问题:

Assume that I have an array like 1 2 3 4 5, I want to shift them to left by n rotation and get a new one.

For example the 2 rotation of the above array will result in 3 4 5 1 2. I didn't found any extension function to do that.

回答1:

You can write your own extension function on Array<T>

fun <T> Array<T>.leftShift(d: Int): Array<T> {
    val newList = this.copyOf()
    var shift = d
    if (shift > size) shift %= size
    forEachIndexed { index, value ->
        val newIndex = (index + (size - shift)) % size
        newList[newIndex] = value
    }
    return newList
}

回答2:

You can use built-in java Collections.rotate method, but you need to convert your array to list firstly:

val arr = intArrayOf(1, 2, 3, 4, 5)
val list = arr.toList()
Collections.rotate(list, -2)
println(list.toIntArray().joinToString())

Outputs

3, 4, 5, 1, 2

回答3:

I interpret "get a new one" to mean that the extension function should return a new instance, like so (boundary checks omitted) :

fun <T> Array<T>.shift(n: Int) = 
    let { sliceArray(n until size) + sliceArray(0 until n) }

回答4:

Another extension function, by slicing the array in 2 parts left and right and reassembling it to right + left:

fun <T> Array<T>.leftShift(d: Int) {
    val n = d % this.size  // just in case
    if (n == 0) return  // no need to shift

    val left = this.copyOfRange(0, n)
    val right = this.copyOfRange(n, this.size)
    System.arraycopy(right, 0, this, 0, right.size)
    System.arraycopy(left, 0, this, right.size, left.size)
}

so this:

val a = arrayOf(1, 2, 3, 4, 5, 6, 7)
a.leftShift(2)
a.forEach { print(" " + it) }

will print

3 4 5 6 7 1 2