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Conditional Expressions and Loops  ›  Lesson

for Loops

for 12 minKotlin's for loop walks through anything that provides an iterator, such as a range, a collection or a map, rather than counting with an index and a condition.

for Loops

Printing the café menu means doing the same thing for every drink. Writing one println per drink breaks the day the menu changes. A loop repeats the work once per item, however many items there are.

The rule

for walks through things; it does not count by itself.

Kotlin's for takes each element of something iterable, one at a time:

val drinks = listOf("latte", "mocha", "chai")
for (drink in drinks) {
    println(drink)
}

There is no C-style loop like for (i = 0; i < 3; i++). When you need numbers, walk through a range:

for (i in 1..3) print(i)            // 123
for (i in 0 until drinks.size) println(drinks[i])
for (i in drinks.indices) println(drinks[i])   // same, safer
for (i in 10 downTo 0 step 5) print("$i ") // 10 5 0

drinks.indices is the range 0..size - 1, so it can never step off the end.

When you want both the position and the item, use withIndex() and unpack the pair:

for ((i, drink) in drinks.withIndex()) {
    println("${i + 1}. $drink")      // 1. latte, 2. mocha, 3. chai
}

Maps unpack into key and value:

val prices = mapOf("latte" to 3.5, "mocha" to 4.0)
for ((drink, price) in prices) {
    println("$drink costs $price")
}

The loop variable (drink, i) is read-only and exists only inside the loop. For "just do this n times" without needing the number, repeat(3) { println("Brewing…") } is a tidy alternative.

The shortcut

Ask "What do I actually need each time round: the item, the position, or both?"

  • Item only → for (x in list).
  • Position only → for (i in list.indices).
  • Both → for ((i, x) in list.withIndex()).
  • Key and value → for ((k, v) in map).
Numbering the menuStep 1 of 4

  1. What do we need?

    The drink name and its position, so both: withIndex().

The off-by-one trap for (i in 0..drinks.size) reaches index 3 and crashes. until or indices avoids it.

Drills

Answer fast — trust the rule.

quick drill

What does for (i in 1..4) print(i) print?

quick drill

val d = listOf("a", "b", "c"). Which loop crashes with an index error?

quick drill

Which loop gives both position and item?

quick drill

How do you loop over a map's keys and values together?

Remember

  • for walks through a range, collection or map; there is no counting form.
  • Pick the shape: item, indices, withIndex(), or (k, v) for maps.
  • 0..size goes one too far; use until or indices.

Practice — level up

0/1 solved

Write your answer, then compare it with a rubric and a model answer.

ExplainPrinting a numbered menu

Given val drinks = listOf("latte", "mocha", "chai") and val prices = mapOf("latte" to 3.5, "mocha" to 4.0), write for loops that (a) print "1. latte", "2. mocha", "3. chai" and (b) print each drink with its price from the map. Explain why Kotlin has no C-style for (i = 0; i < n; i++) loop and what you use instead.