List — forEach() / forEachIndexed()

val fruits = listOf("Apple", "Banana", "Orange")

// forEach — executes a block for each element.
fruits.forEach { println(it) }

// forEachIndexed — also provides the index.
fruits.forEachIndexed { index, fruit ->
    println("$index: $fruit")
}

// Equivalent using a for loop.
for ((index, fruit) in fruits.withIndex()) {
    println("$index: $fruit")
}

data class Student(val name: String, val score: Int)

fun main() {
    val students = listOf(
        Student("Alice", 85),
        Student("Bob", 72),
        Student("Carol", 91),
        Student("Dave", 68)
    )

    // Basic forEach
    println("=== Score List ===")
    students.forEach { student ->
        println("${student.name}: ${student.score} pts")
    }

    println()

    // Numbered output with forEachIndexed
    println("=== Numbered ===")
    students.forEachIndexed { index, student ->
        val rank = index + 1
        println("$rank. ${student.name}: ${student.score} pts")
    }

    println()

    // forEach on a map (destructuring key and value)
    val capitals = mapOf("Japan" to "Tokyo", "France" to "Paris", "USA" to "Washington D.C.")
    println("=== Capitals ===")
    capitals.forEach { (country, capital) ->
        println("Capital of $country: $capital")
    }

    println()

    // onEach — same as forEach but chainable.
    val names = students
        .onEach { println("Processing: ${it.name}") }
        .map { it.name }
    println("Name list: $names")

    println()

    // for loop with withIndex
    println("=== withIndex ===")
    for ((i, s) in students.withIndex()) {
        println("[$i] ${s.name}")
    }
}

Use forEach() for operations with side effects, such as logging or writing to a database. When you need the transformed results, use map(); when you need to filter elements by a condition, use filter() — these make your intent clearer.

To exit a forEach loop early, use return@forEach to return from the lambda (a plain return would return from the enclosing function). To break out of the loop entirely, use a for loop with break.

For basic list operations, see List — Creating / listOf() / mutableListOf(). For filtering and transformation, see List — filter() / map().