Array/List iteration without extra object allocations

As far as I know the only allocation-less way to define a for loop is

for (i in 0..count - 1)

All other forms lead to either a Range allocation or an Iterator allocation. Unfortunately, you cannot even define an effective reverse for loop.

To iterate an array without allocating extra objects you can use one of the following ways.

1. for-loop

    for (e in arr) {
        println(e)
    }

2. forEach extension

    arr.forEach {
        println(it)
    }

3. forEachIndexed extension, if you need to know index of each element

    arr.forEachIndexed { index, e ->
        println("$e at $index")
    }

Here is an example of preparing a list and iterate with index and value.

val list = arrayListOf("1", "11", "111")
for ((index, value) in list.withIndex()) {
   println("$index: $value")
}

Output:

0:1

1:11

2:111

Also, following code works similar,

val simplearray = arrayOf(1, 2, 3, 4, 5)
for ((index, value) in simplearray.withIndex()) {
    println("$index: $value")
}