Sequence+categorise.swift

public extension Sequence {
    func categorise<U : Hashable>(_ key: (Iterator.Element) -> U) -> [U:Int] {
        var dict: [U:Int] = [:]

        self.forEach { el in
            let key = key(el)
            dict[key] = (dict[key] ?? 0) + 1
        }

        return dict
    }
}

print([1, 4, 7 ,4, 2, 2, 4, 7, 1, 2].categorise{ $0 })
// [7: 2, 2: 3, 4: 3, 1: 2]

print(["a", "a", "b", "b", "b", "b", "c"].categorise{ $0 })
// ["b": 4, "a": 2, "c": 1]