Xcode 9 或更高版本 • Swift 4 或更高版本

在 Swift 4 中你可以使用新的 Dictionary 方法 reduce(into:) 如下:

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
    }
}

游乐场测试:

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

通过扩展集合，它也支持字符串

By extending Collection it supports Strings as well

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