The general combinator you’re looking for is often called scan
, and can be defined (like all higher-order functions on lists) in terms of reduce
:
extension Array {
func scan<T>(initial: T, _ f: (T, Element) -> T) -> [T] {
return self.reduce([initial], combine: { (listSoFar: [T], next: Element) -> [T] in
// because we seeded it with a non-empty
// list, it's easy to prove inductively
// that this unwrapping can't fail
let lastElement = listSoFar.last!
return listSoFar + [f(lastElement, next)]
})
}
}
(But I would suggest that that’s not a very good implementation.)
This is a very useful general function, and it’s a shame that it’s not included in the standard library.
You can then generate your cumulative sum by specializing the starting value and operation:
let cumSum = els.scan(0, +)
And you can omit the zero-length case rather simply:
let cumSumTail = els.scan(0, +).dropFirst()