如何计算Swift数组中元素的出现次数？

Question

我已经看到了一些这样的示例，但是所有这些似乎都依赖于知道要计算发生次数的元素。我的数组是动态生成的，所以我无法知道要计算哪个元素的出现（我想计算所有元素的出现）。有人可以建议吗？提前致谢编辑：也许我应该更清楚一点，数组将包含多个不同的字符串（例如 ["FOO", "FOO", "BAR", "FOOBAR"]我如何在不知道它们是什么的情况下计算foo，bar和foobar的出现？

Answer 1

使用Swift 5时，您可以根据需要选择以下7个Playground示例代码之一来计算数组中可哈希项的出现次数。

＃1。使用Array的reduce(into:_:)和Dictionary的subscript(_:default:)标

let array = [4, 23, 97, 97, 97, 23]

let dictionary = array.reduce(into: [:]) { counts, number in

    counts[number, default: 0] += 1

}

print(dictionary) // [4: 1, 23: 2, 97: 3]

＃2。使用repeatElement(_:count:)函数，zip(_:_:)函数和Dictionary的init(_:uniquingKeysWith:)初始值设定项

let array = [4, 23, 97, 97, 97, 23]

let repeated = repeatElement(1, count: array.count)

//let repeated = Array(repeating: 1, count: array.count) // also works

let zipSequence = zip(array, repeated)

let dictionary = Dictionary(zipSequence, uniquingKeysWith: { (current, new) in

    return current + new

})

//let dictionary = Dictionary(zipSequence, uniquingKeysWith: +) // also works

print(dictionary) // prints [4: 1, 23: 2, 97: 3]

＃3。使用Dictionary的init(grouping:by:)初始值设定项和mapValues(_:)方法

let array = [4, 23, 97, 97, 97, 23]

let dictionary = Dictionary(grouping: array, by: { $0 })

let newDictionary = dictionary.mapValues { (value: [Int]) in

    return value.count

}

print(newDictionary) // prints: [97: 3, 23: 2, 4: 1]

＃4。使用Dictionary的init(grouping:by:)初始值设定项和map(_:)方法

let array = [4, 23, 97, 97, 97, 23]

let dictionary = Dictionary(grouping: array, by: { $0 })

let newArray = dictionary.map { (key: Int, value: [Int]) in

    return (key, value.count)

}

print(newArray) // prints: [(4, 1), (23, 2), (97, 3)]

＃5。使用for循环和Dictionary的subscript(_:)下标

extension Array where Element: Hashable {

    func countForElements() -> [Element: Int] {

        var counts = [Element: Int]()

        for element in self {

            counts[element] = (counts[element] ?? 0) + 1

        }

        return counts

    }

}

let array = [4, 23, 97, 97, 97, 23]

print(array.countForElements()) // prints [4: 1, 23: 2, 97: 3]

＃6。使用NSCountedSet和NSEnumerator的map(_:)方法（需要Foundation）

import Foundation

extension Array where Element: Hashable {

    func countForElements() -> [(Element, Int)] {

        let countedSet = NSCountedSet(array: self)

        let res = countedSet.objectEnumerator().map { (object: Any) -> (Element, Int) in

            return (object as! Element, countedSet.count(for: object))

        }

        return res

    }

}

let array = [4, 23, 97, 97, 97, 23]

print(array.countForElements()) // prints [(97, 3), (4, 1), (23, 2)]

＃7。使用NSCountedSet和AnyIterator（需要Foundation）

import Foundation

extension Array where Element: Hashable {

    func counForElements() -> Array<(Element, Int)> {

        let countedSet = NSCountedSet(array: self)

        var countedSetIterator = countedSet.objectEnumerator().makeIterator()

        let anyIterator = AnyIterator<(Element, Int)> {

            guard let element = countedSetIterator.next() as? Element else { return nil }

            return (element, countedSet.count(for: element))

        }

        return Array<(Element, Int)>(anyIterator)

    }

}

let array = [4, 23, 97, 97, 97, 23]

print(array.counForElements()) // [(97, 3), (4, 1), (23, 2)]