Kotlin Loops and Ranges

for (i in 1..5) {
    println(i)          // 1, 2, 3, 4, 5
}

• 1..5 is a range that includes both ends.
• for (x in something) visits each item, giving it the name x.
• The loop variable is created fresh each time; you don't declare or increase it yourself.

for works with any collection, not just numbers:

val fruits = listOf("apple", "pear", "plum")
for (fruit in fruits) println(fruit)
for (i in fruits.indices) println("$i: ${fruits[i]}")
for ((i, fruit) in fruits.withIndex()) println("${i + 1}. $fruit")

indices is the range of valid positions, 0 until size. withIndex() gives the position and the item together, unpacked into two names with brackets. This is safer than counting positions by hand, which risks going one past the end.

A while loop repeats as long as its condition is true, which suits situations where you don't know in advance how many times it will run:

var balance = 100.0
var years = 0
while (balance < 200) {
    balance *= 1.05
    years++
}

do { } while (condition) always runs its body at least once before checking. repeat(3) { println("Hip hip, hooray!") } runs a block a fixed number of times, with the count available as it, starting at 0.

• break stops the loop immediately.
• continue skips the rest of this pass and moves on to the next item.

In nested loops, break only leaves the innermost loop. To leave an outer loop, give it a label with name@ and use break@name:

outer@ for (row in 1..3) {
    for (col in 1..3) {
        if (row * col == 4) break@outer
        print("${row * col} ")
    }
}

A range isn't only something to loop over. It's an ordinary value, so you can keep it in a variable, pass it to a function and ask it questions:

val teen = 13..19
println(15 in teen)        // true
println(20 !in teen)       // true
println(teen.first)        // 13
println(teen.last)         // 19
println(teen.count())      // 7
println(teen.sum())        // 112

in checks membership without looping, so age in 13..19 is a tidy replacement for age >= 13 && age <= 19. Ranges of Double work for this kind of check too, as in 0.5 in 0.0..1.0, but you can't loop over them, because there's no obvious "next" decimal number.

When you add step, the last value is the last one the steps actually reach, not necessarily the end you wrote. (1..10 step 4).toList() is [1, 5, 9], so its last is 9. A range written backwards, such as 5..1, is simply empty: (5..1).isEmpty() is true, which is why the loop over it never runs. Use reversed() or downTo when you want to go the other way.

A for loop works over anything that can hand out its items one at a time. That includes strings, which give you each Char, and maps, which give you entries you can unpack into a key and a value:

for (ch in "abc") print(ch.uppercaseChar())     // ABC
for ((name, age) in mapOf("Ana" to 30, "Ben" to 25)) {
    println("$name is $age")
}

Finally, the loop variable is read-only. Writing i = 5 inside for (i in 1..3) is a compile error, because each pass gets a fresh val. If you need a counter you can change, use a while loop with a var.