Ranges and for Loops

1..10 // 1 to 10 inclusive (closed range)
1 until 10 // 1 to 9 inclusive (open end, excludes 10)
10 downTo 1 // 10 down to 1 (descending)
1..10 step 2 // 1, 3, 5, 7, 9 (with step)

// Basic for loop syntax
for (variable in rangeOrCollection) {
    // body
}

sample_range_for.kt

fun main() {
    // Basic range loop
    for (i in 1..5) {
        print("$i ") // 1 2 3 4 5
    }
    println()

    // Loop using until to exclude the end value
    val list = listOf("A", "B", "C")
    for (i in 0 until list.size) {
        print("${list[i]} ") // A B C
    }
    println()

    // Reverse loop using downTo
    for (i in 5 downTo 1) {
        print("$i ") // 5 4 3 2 1
    }
    println()

    // Loop with a custom step size
    for (i in 0..10 step 3) {
        print("$i ") // 0 3 6 9
    }
    println()

    // Iterating over a collection directly with for
    val pilots = listOf("Ayanami Rei", "Ikari Shinji", "Soryu Asuka")
    for (pilot in pilots) {
        println(pilot)
    }

    // Use withIndex() to get both index and value
    for ((index, value) in pilots.withIndex()) {
        println("$index: $value")
    }

    // Using the in operator to check if a value is within a range
    val score = 85
    if (score in 70..89) {
        println("Grade B")
    }

    // Ranges also work with characters
    for (ch in 'A'..'E') {
        print("$ch ") // A B C D E
    }
}

The command looks like this:

kotlinc sample_range_for.kt -include-runtime -d sample_range_for.jar
java -jar sample_range_for.jar
1 2 3 4 5 
A B C 
5 4 3 2 1 
0 3 6 9 
Ayanami Rei
Ikari Shinji
Soryu Asuka
0: Ayanami Rei
1: Ikari Shinji
2: Soryu Asuka
Grade B
A B C D E 

fun main() {
    val pilots = listOf("Ayanami Rei", "Ikari Shinji", "Soryu Asuka")

    // using .. includes the end, causing an out-of-bounds index
    // for (i in 0..pilots.size) { // 0..3 — pilots[3] throws IndexOutOfBoundsException
    //     println(pilots[i])
    // }

    // trying to use a negative step instead of downTo
    // for (i in 5..-1) // This produces an empty range — it does not count down

    // passing a negative step value causes a runtime error
    // for (i in 0..10 step -2) // IllegalArgumentException: Step must be positive
}

Use until or indices for index-safe loops, downTo for descending loops, and downTo step for descending loops with a step.

sample_range_for_ok.kt

fun main() {
    val pilots = listOf("Ayanami Rei", "Ikari Shinji", "Soryu Asuka")

    // Use until to avoid out-of-bounds
    for (i in 0 until pilots.size) {
        println(pilots[i])
    }

    // Use downTo for descending loops
    for (i in 3 downTo 1) {
        println("Countdown: $i")
    }

    // Combine downTo with step for descending step loops
    for (i in 10 downTo 0 step 3) {
        print("$i ")
    }
    println()
}

The command looks like this:

kotlinc sample_range_for_ok.kt -include-runtime -d sample_range_for_ok.jar
java -jar sample_range_for_ok.jar
Ayanami Rei
Ikari Shinji
Soryu Asuka
Countdown: 3
Countdown: 2
Countdown: 1
10 7 4 1

Kotlin ranges are commonly used for index-based iteration over collections. For iterating over array or list indices, list.indices is more concise than 0 until list.size. You can also control nested loops with labeled break and continue (e.g., break@outer).

Ranges can also be used inside when expressions (see when expression). For iteration using higher-order functions, see Higher-Order Functions.