Does Swift have a function similar to numpy.diff that calculates the difference between adjacent elements of an array

Question

I'm trying to convert some Python code to Swift and wondering if there is an existing function to calculate the difference between successive elements in a Swift array. For example:

diff([1,3,5,6,10]) would return [2,2,1,4]

Answer 1

No, but it could be very easily implemented:

let a = [1, 3, 5, 6, 10]
zip(a.dropFirst(), a).map(-) // => [2, 2, 1, 4]

It's simple enough that it's probably not worth wrapping into a function, but if you insist:

extension Collection where Element: Numeric {
    func diff() -> [Element] {
        return zip(self.dropFirst(), self).map(-)
    }
}

[1, 3, 5, 6, 10].diff() // => [2, 2, 1, 4]

If you need the result to be lazily evaluated, you can do this:

extension Collection where Element: Numeric {
    func diff() -> AnyCollection<Element> {
        return AnyCollection(zip(self.dropFirst(), self).lazy.map(-))
    }
}

Answer 2

You can use reduce(into:) combined with dropfirst to achieve what you want:

extension Collection where Element: SignedNumeric {
    func diff() -> [Element] {
        guard var last = first else { return [] }
        return dropFirst().reduce(into: []) {
            $0.append($1 - last)
            last = $1
        }
    }
}

Another option is to use map and defer:

extension Collection where Element: SignedNumeric {
    func diff() -> [Element] {
        guard var last = first else { return [] }
        return dropFirst().map { element in
            defer { last = element }
            return element - last
        }
    }
}

---

let arr = [1,3,5,6,10]
print(arr.diff())  // "[2, 2, 1, 4]\n"