Swift 2 count duplicate in array

Question

Could you please help me to simplify the flow below? the scope is to count how many time each element is duplicate within the given array.

var test = [1,2,3,4,5,6,7,8,5,9,3,3,9,9,9]

var testCount = [Int:Int]()

for curr in test {
    if let x = testCount[curr] {
        testCount[curr] = x  + 1
        continue;
    }
    testCount[curr] = 1
}

print(testCount)

I'm working on it in order to have something like:

test.map_duplicate() or map_duplicate(test)

Answer 1

extension Array where Element: Hashable {

    func mergeDuplicates() -> [Element: Int] {
        var result = [Element: Int]()

        self.forEach({ result[$0] = result[$0] ?? 0 + 1 })

        return result
    }

}

var test = [1,2,3,4,5,6,7,8,5,9,3,3,9,9,9]

let testCount = test.mergeDuplicates()

print(testCount)

The code above provides an extension to any Array type that holds Hashable elements (anything else is not eligible to be the key of a dictionary). Method mergeDuplicates produces the dictionary you want. In its implementation, forEach() executes a closure over every element in the array (this is typically more straightforward and transparent alternative to old-fashioned for-loop). Then on each iteration we check whether there is already existing value in the dictionary with a running count (and use nil-coalescing operator ?? to handle cases when a particular value was not yet met before, hence it's zero). Then increment, and store (back) into the dictionary.

Answer 2

You could extend SequenceType to have a function freq which does exactly what you need here.

extension SequenceType where Self.Generator.Element: Hashable {
    func freq() -> [Self.Generator.Element: Int] {
        return reduce([:]) { (var accu: [Self.Generator.Element: Int], element) in
            accu[element] = accu[element]?.successor() ?? 1
            return accu
        }
    }
}