Kotlin Fold Reduce

Kotlin Fold Reduce

### Kotlin Reduce

剛開始寫 Kotlin 的時候

Array, ArrayList, List, MutableList

就被這四個搞死了，因為 Swift 只有一種叫做 Array

高階函數

swift -> map, filter, reduce

kotlin -> map, filter, reduce, fold

## Kotlin 的 reduce 不等於 Swift 的 reduce

inline fun <S, T : S> Array<out T>.reduce(
    operation: (acc: S, T) -> S
): S

### Kotlin Fold

Kotlin 的 fold 等於 Swift 的 reduce

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

### 莫名其妙就跳坑了 囧
### 需要注意的是 Reduce will throw error in empty collection ```kotlin public inline fun Iterable.reduce(operation: (acc: S, T) -> S): S { val iterator = this.iterator() if (!iterator.hasNext()) throw UnsupportedOperationException("Empty collection can't be reduced.") var accumulator: S = iterator.next() while (iterator.hasNext()) { accumulator = operation(accumulator, iterator.next()) } return accumulator } ```