How can I square each item of an integer array in Kotlin

You could use map:

val array = arrayOf(1, 2, 3, 4, 5)
println(array.map { n: Int -> n * n }) 

Output:

[1, 4, 9, 16, 25]

In Kotlin you use joinToString:

val array = arrayOf(1, 2, 3, 4, 5)
println(array.joinToString(separator = " ") { n -> "${n * n}" })

You can also use joinTo to join directly to a buffer (e.g. System.out) and avoid the intermediate String:

array.joinTo(System.out, separator = " ") { n -> "${n * n}" }