Ranges and for Loops

// Creating ranges.
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
}

fun main() {
    // Basic range loop.
    for (i in 1..5) {
        print("$i ")  // Prints "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]} ") // Prints "A B C ".
    }
    println()

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

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

    // Iterating over a collection directly with for.
    val fruits = listOf("apple", "orange", "grape")
    for (fruit in fruits) {
        println(fruit) // Prints each fruit name.
    }

    // Use withIndex() to get both index and value.
    for ((index, value) in fruits.withIndex()) {
        println("$index: $value") // Prints "0: apple", etc.
    }

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

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

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.