how use down to in kotlin code example

Example: for loop kotlin

val array = arrayOf(1, 3, 9)
for (item in array) {
    //loops items
}
for (index in 0..array.size - 1) {
	//loops all indices
}
for (index in 0 untill array.size) {
    //loops all indices
}
for (index in array.indices) {
    //loops all indices (performs just as well as two examples above)
}