Summary

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

Returns the specified [initial](kotlinlang.org/api/latest/jvm/stdlib/kotlin..) value if the array is empty.

Parameters

operation - function that takes current accumulator value and an element, and calculates the next accumulator value.

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

Tip

1. Similar to reduce() but fold allows you to set an initial value.

2. Like reduce(), it accumulates the return values of the operation. (In the case of a lambda, it is the last line.)

3. The return type is the same as the data type of the initial value T.

In Practice

Numbers with intervals of x up to n

class Solution {
    fun solution(x: Int, n: Int): LongArray {
        // return LongArray(n){x.toLong() * (it+1)}
        var answer = mutableListOf<Long>()
        (0 until n).fold(x.toLong()){ acc, _ ->
            answer.add(acc)
            acc + x
        }
        return answer.toLongArray() 
    }
}

Supported Data Types

• Array

• ByteArray

• ShortArray

• IntArray

• LongArray

• FloatArray

• DoubleArray

• BooleanArray

• CharArray

References

https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/fold.html