Swift running sum

Question

I\'d like a function runningSum on an array of numbers a (or any ordered collection of addable things) that returns an array of the same length where each eleme

Answer 1

I thought I'd be cool to extend Sequence with a generic scan function as is suggested in the great first answer.

Given this extension, you can get the running sum of an array like this: [1,2,3].scan(0, +)

But you can also get other interesting things…

• Running product: array.scan(1, *)
• Running max: array.scan(Int.min, max)
• Running min: array.scan(Int.max, min)

Because the implementation is a function on Sequence and returns a Sequence, you can chain it together with other sequence functions. It is efficient, having linear running time.

Here's the extension…

extension Sequence {

    func scan(_ initialResult: Result, _ nextPartialResult: @escaping (Result, Self.Element) -> Result) -> ScanSequence {
        return ScanSequence(initialResult: initialResult, underlying: self, combine: nextPartialResult)
    }
}

struct ScanSequence: Sequence {

    let initialResult: Result
    let underlying: Underlying
    let combine: (Result, Underlying.Element) -> Result

    typealias Iterator = ScanIterator

    func makeIterator() -> Iterator {
        return ScanIterator(previousResult: initialResult, underlying: underlying.makeIterator(), combine: combine)
    }

    var underestimatedCount: Int {
        return underlying.underestimatedCount
    }
}

struct ScanIterator: IteratorProtocol {

    var previousResult: Result
    var underlying: Underlying
    let combine: (Result, Underlying.Element) -> Result

    mutating func next() -> Result? {
        guard let nextUnderlying = underlying.next() else {
            return nil
        }

        previousResult = combine(previousResult, nextUnderlying)
        return previousResult
    }
}