In Kotlin can I create a range that counts backwards?

Question

I looked at the documentation for the Ranges and I see no mention of backwards ranges.

Is it possible to do something like:

for (n in 100..1) {
    println(n)
}

And get results:

100
99
98
...

Answer 1

Use downTo as in:

for (n in 100 downTo 1) {
//
}

Answer 2

Reversed ranges are supported using the minus - unary operator as in -(1..100).

To invoke a method on that range, you will then need to surround it with parentheses as in (-(1..100)).foreach { println(it) }.

Answer 3

As pointed by others, the correct answer is

for (n in 100 downTo 1) {
    println(n)
}

But why did Kotlin team chose 100 downTo 1 vs 100..1?

I think that the syntax 100..1 would be bad when we try to use variables instead of literals. If we typed

for (n in b..a)

then it wouldn't be clear what loop we wanted to use.

We may have intended to count backwards but if b turns out to be smaller than a, then our program would actually count upwards! That would be a source of bugs.

Answer 4

Just as an example of an universal range function for "for":

private infix fun Int.toward(to: Int): IntProgression {
    val step = if (this > to) -1 else 1
    return IntProgression.fromClosedRange(this, to, step)
}

Usage:

// 0 to 100
for (i in 0 toward 100) {
    // Do things
}

// 100 downTo 0
for (i in 100 toward 0) {
    // Do things
}

Answer 5

If you look at the exact page you linked to, there's a suggestion for a reversed function that would let you do for (n in (1..100).reversed()) but it doesn't appear to be implemented yet. The .. operator always counts up.

Answer 6

(100 downto 1).map{ println(it) }