CODEWITHSUNDEEP
iosarraysswifthistogram
ajrlewis
ajrlewis

Reputation: 3058

Histogram of Array in Swift

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 protocol 'Hashable'.

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

where dictionaryByUpdatingKey(...) mutates an existing dictionary as follows:

extension Dictionary {
    func dictionaryByUpdatingKey(key: Dictionary.Key, value: Dictionary.Value) -> Dictionary {
        var mutableSelf = self
        let _ = mutableSelf.updateValue(value, forKey: key)
        return mutableSelf
    }
}

I have tried replacing Array.Element with AnyHashable and then forcing the key as! AnyHashable, but this seems messy and the return type should preferably be of the same type as the Array.Element and not of AnyHashable.

I wish to use the Array extension as follows:

let names = ["Alex", "Alex", "James"]
print(names.histogram()) // ["James": 1, "Alex": 2]

or

let numbers = [2.0, 2.0, 3.0]
print(numbers.histogram()) // [3.0: 1, 2.0: 2]

Upvotes: 5

Views: 1971

Answers (2)

vacawama
vacawama

Reputation: 154691

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 }
    }
}

Upvotes: 12

Andreas Oetjen
Andreas Oetjen

Reputation: 10209

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.

Upvotes: 1

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