Ranges and progressions

Kotlin lets you easily create ranges of values using the .rangeTo() and .rangeUntil() functions from the kotlin.ranges package.

To create:

a closed-ended range, call the .rangeTo() function with the .. operator.
an open-ended range, call the .rangeUntil() function with the ..< operator.

For example:

fun main() {
//sampleStart
    // Closed-ended range
    println(4 in 1..4)
    // true
    
    // Open-ended range
    println(4 in 1..<4)
    // false
//sampleEnd
}

Ranges are particularly useful for iterating over for loops:

fun main() {
//sampleStart
    for (i in 1..4) print(i)
    // 1234
//sampleEnd
}

To iterate numbers in reverse order, use the downTo function instead of ...

fun main() {
//sampleStart
    for (i in 4 downTo 1) print(i)
    // 4321
//sampleEnd
}

It is also possible to iterate over numbers with an arbitrary step (not necessarily 1). This is done via the step function.

fun main() {
//sampleStart
    for (i in 0..8 step 2) print(i)
    println()
    // 02468
    for (i in 0..<8 step 2) print(i)
    println()
    // 0246
    for (i in 8 downTo 0 step 2) print(i)
    // 86420
//sampleEnd
}

Progression

The ranges of integral types, such as Int, Long, and Char, can be treated as arithmetic progressions. In Kotlin, these progressions are defined by special types: IntProgression, LongProgression, and CharProgression.

Progressions have three essential properties: the first element, the last element, and a non-zero step. The first element is first, subsequent elements are the previous element plus a step. Iteration over a progression with a positive step is equivalent to an indexed for loop in Java/JavaScript.

for (int i = first; i <= last; i += step) {
  // ...
}

When you create a progression implicitly by iterating a range, this progression's first and last elements are the range's endpoints, and the step is 1.

fun main() {
//sampleStart
    for (i in 1..10) print(i)
    // 12345678910
//sampleEnd
}

To define a custom progression step, use the step function on a range.

fun main() {
//sampleStart
    for (i in 1..8 step 2) print(i)
    // 1357
//sampleEnd
}

The last element of the progression is calculated this way:

For a positive step: the maximum value not greater than the end value such that (last - first) % step == 0.
For a negative step: the minimum value not less than the end value such that (last - first) % step == 0.

Thus, the last element is not always the same as the specified end value.

fun main() {
//sampleStart
    for (i in 1..9 step 3) print(i) // the last element is 7
    // 147
//sampleEnd
}

Progressions implement Iterable<N>, where N is Int, Long, or Char respectively, so you can use them in various collection functions like map, filter, and other.

fun main() {
//sampleStart
    println((1..10).filter { it % 2 == 0 })
    // [2, 4, 6, 8, 10]
//sampleEnd
}

Last modified: 06 July 2023