Count the occurences a particular integer has in an array [duplicate]

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Xcode 9 or later • Swift 4 or later

In Swift 4 you can use the new Dictionary method reduce(into:) as follow:

extension Sequence where Element: Hashable {
    var frequency: [Element: Int] {
        return reduce(into: [:]) { $0[$1, default: 0] += 1 }
    }
    func frequency(of element: Element) -> Int {
        return frequency[element] ?? 0
    }
}

Playground testing:

let numbers = [0, 0, 1, 1, 1, 2, 3, 4, 5, 5]
print(numbers.frequency) // [2: 1, 4: 1, 5: 2, 3: 1, 1: 3, 0: 2]

print(numbers.frequency(of: 0))   // 2  
print(numbers.frequency(of: 1))   // 3
print(numbers.frequency(of: 2))   // 1
print(numbers.frequency(of: 3))   // 1
print(numbers.frequency(of: 4))   // 1
print(numbers.frequency(of: 5))   // 2

By extending Collection it supports Strings as well

"aab".frequency   // ["a": 2, "b": 1]

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