fold

Accumulates value starting with initial value and applying operation from left to right to current accumulator value and each element.

inline fun <T, R> Array<out T>.fold(initial: R, operation: (acc: R, T) -> R): R(source)

val numbers = arrayOf(1, 2, 3, 4, 5)
val sum = numbers.fold(0) { acc, value -> acc + value }
println("Sum: $sum")  // Output: Sum: 15

Source