Ranges and progressions

Ranges and progressions define sequences of values in Kotlin, supporting range operators, iteration, custom step values, and arithmetic progressions.

Ranges

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

A range represents an ordered set of values with a defined start and end. By default, it increments by 1 at each step. For example, 1..4 represents the numbers 1, 2, 3, and 4.

To create:

a closed-ended range, call the .rangeTo() function with the .. operator. This includes both the start and end values.
an open-ended range, call the .rangeUntil() function with the ..< operator. This includes the start value but excludes the end value.

For example:

fun main() {
//sampleStart
    // Closed-ended range: includes both 1 and 4
    println(4 in 1..4)
    // true
    
    // Open-ended range: includes 1, excludes 4
    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
}

You can also iterate over numbers with a custom step using the step() function, instead of the default increment of 1:

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 February 2025