Histogram of Array in Swift

Question

I\'m trying to write a generic histogram function that operates on an Array, but I\'m running into difficulties as Type \'Element\' does not conform to prot

Answer 1

First, you could limit the Element type to only be Hashable.

extension Array where Array.Element:Hashable {

After this, you might get another error because the swift compiler is a little "overstrained". Try to give him a hint:

typealias RT = [Array.Element: Int]

and use it everywhere. So:

extension Array where Array.Element:Hashable {
    typealias RT = [Array.Element: Int]
    func histogram() -> RT {
        return self.reduce(RT()) { (acc, key) in
            let value = (acc[key] == nil) ? 1 : (acc[key]! + 1)
            return acc.dictionaryByUpdatingKey(key: key, value: value)
        }
    }
}

finally should work.

Answer 2

Add the generic where clause: where Element: Hashable to your extension:

extension Sequence where Element: Hashable {
    func histogram() -> [Element: Int] {
        return self.reduce([Element: Int]()) { (acc, key) in
            let value = acc[key, default: 0] + 1
            return acc.dictionaryByUpdatingKey(key: key, value: value)
        }
    }
}

I also incorporated @MartinR's suggestion of using the new default value for dictionary look ups.

---

Using reduce(into:_:) you can do this much more simply and efficiently:

extension Sequence where Element: Hashable {
    func histogram() -> [Element: Int] {
        return self.reduce(into: [:]) { counts, elem in counts[elem, default: 0] += 1 }
    }
}