Loops and ranges

Why loops?

Many jobs repeat: print a bill line for each item, send a reminder to every student, try an OTP up to three times. A loop runs a block of code again and again, so you write it once. Each run is called an iteration.

Ranges

A range is a sequence of values from a start to an end. Kotlin has several ways to write one:

for loops

fun main() {
    for (i in 1..3) {
        println("Round $i")
    }
    for (i in 10 downTo 0 step 5) {
        print("$i ")           // 10 5 0
    }
    println()

    val cities = listOf("Delhi", "Agra", "Jaipur")
    for (city in cities) {
        println("Tour stop: $city")
    }
    for ((index, city) in cities.withIndex()) {
        println("${index + 1}. $city")
    }
}

Most of the time in real apps you loop over a collection directly (for (city in cities)) rather than over numbers. Use a numeric range only when you really need the numbers, such as a table or a countdown.

while and do-while

while checks its condition before each round; if it is false at the start, the body never runs. do-while checks after, so the body always runs at least once, which suits "ask, then check" situations.

fun main() {
    var balance = 1000
    var months = 0
    while (balance > 0) {
        balance -= 300            // monthly EMI
        months++
    }
    println("Paid off in $months months")   // 4

    var attempts = 0
    do {
        attempts++
        println("Attempt $attempts")
    } while (attempts < 2)
}

repeat

repeat(3) {
    println("Reminder: pay your electricity bill")
}

break and continue

fun main() {
    for (n in 1..10) {
        if (n % 2 == 0) continue    // skip even numbers
        if (n > 7) break            // stop the loop
        print("$n ")                // 1 3 5 7
    }
}

With nested loops, break and continue affect only the innermost loop. To leave an outer loop, Kotlin lets you add a label like outer@ and write break@outer. Use this sparingly; often a function with return is clearer.

Common mistakes

• Using 0..list.size to loop over positions: the last value equals size, which is past the end and crashes. Use 0 until list.size, list.indices, or loop over the items directly.
• A while loop whose condition never becomes false: an endless loop. Make sure something inside changes the condition.
• Writing 5..1 to count down: it is an empty range. Use 5 downTo 1.
• Changing a val counter inside a loop. Counters must be var.