Ranges

A range is a shortcut for describing a whole sequence of numbers or letters, like saying 1-to-10 instead of listing every number.

In this page:

The .. operator creates a range that includes both its start and end values, commonly used directly inside for loops or in checks.

Example: Inclusive Ranges with ..

markup

fun main() {
    val range = 1..5
    for (n in range) {
        println("Value: $n")
    }
}

until builds a range that excludes its upper bound, which is the idiomatic way to loop exactly n times starting from zero.

Example: Excluding the End with until

markup

fun main() {
    for (i in 0 until 5) {
        println("Index: $i")
    }
}

downTo creates a descending range, letting a for loop count backward from a higher number to a lower one.

Example: Counting Down with downTo

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fun main() {
    for (i in 5 downTo 1) {
        println("Countdown: $i")
    }
    println("Liftoff!")
}

Chaining .step(n) onto a range changes how much the value increases (or decreases) each iteration, letting you skip over values.

Example: Custom Increments with step

markup

fun main() {
    for (i in 0..10 step 2) {
        println("Even step: $i")
    }
}

Common Mistakes

Using .. when the upper bound should be excluded, off-by-one errors are common; until (or ..<) excludes the end value.
Forgetting downTo for counting backward; a plain .. range always goes in increasing order.
Ignoring the step function when a non-1 increment is needed, and instead manually skipping values inside the loop body.

Chapter Summary

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