根据元素的出现获取数组的子集

Question

我有一个像这样的数组：

var arr = [4,1,5,5,3]

我想根据数组中元素的出现从数组中获取子集。

例如：

Elements with frequency 1 is {4,1,3}
Elements with frequency 2 is {5,5}

我遵循了这个但无法弄清楚如何做上面的事情。

有什么办法可以做到这一点吗？

Answer 1

你可以使用一个NSCountedSet来获取arr中所有元素的计数，然后你可以构建一个Dictionary，其中的键将是元素和值将是具有键出现次数的元素数组。通过迭代 Set(arr) 而不是简单地 arr 来构建字典，您可以确保重复元素只添加一次到字典中（因此例如对于您的原始示例，不会添加 5频率为 2 的两倍）。

对于打印，您只需要遍历 Dictionary 的键并打印键及其对应的值。我只是对键进行排序，使打印按出现次数的升序排列。

let arr = [4,1,5,5,3,2,3,6,2,7,8,2,7,2,8,8,8,7]
let counts = NSCountedSet(array: arr)
var countDict = [Int:[Int]]()
for element in Set(arr) {
    countDict[counts.count(for: element), default: []].append(element)
}
countDict

for freq in countDict.keys.sorted() {
    print("Elements with frequency \(freq) are {\(countDict[freq]!)}")
}

输出：

Elements with frequency 1 are {[4, 6, 1]}
Elements with frequency 2 are {[5, 3]}
Elements with frequency 3 are {[7]}
Elements with frequency 4 are {[2, 8]}

Swift 3 版本:

let arr = [4,1,5,5,3,2,3,6,2,7,8,2,7,2,8,8,8,7]
let counts = NSCountedSet(array: arr)
var countDict = [Int:[Int]]()
for element in Set(arr) {
    if countDict[counts.count(for: element)] != nil {
        countDict[counts.count(for: element)]!.append(element)  
    } else {
        countDict[counts.count(for: element)] = [element]
    }
}

for freq in countDict.keys.sorted() {
    print("Elements with frequency \(freq) are {\(countDict[freq]!)}")
}

Answer 2

就是这样：

func getSubset(of array: [Int], withFrequency frequency: Int) -> [Int]
{
    var counts: [Int: Int] = [:]

    for item in array
    {
        counts[item] = (counts[item] ?? 0) + 1
    }

    let filtered  = counts.filter{ [=10=].value == frequency}

    return Array(filtered.keys)
}

这是纯粹的 Swift（没有使用旧的 Next Step 类）并且使用了您提供的 SO link 中的想法.

counts 字典包含数组中每个整数值（键）的频率（值）：[int-value : frequency]。

Answer 3

您只需要获取元素的出现次数并过滤只出现一次或多次的元素，如answer:

extension Array where Element: Hashable {
    // Swift 4 or later
    var occurrences: [Element: Int] {
        return reduce(into: [:]) { [=10=][, default: 0] += 1 }
    }
    // // for Swift 3 or earlier
    // var occurrences: [Element: Int] {
    //     var result: [Element: Int] = [:]
    //     forEach{ result[[=10=]] = (result[[=10=]] ?? 0) + 1}
    //     return result
    // }
    func frequencies(where isIncluded: (Int) -> Bool) -> Array {
        return filter{ isIncluded(occurrences[[=10=]] ?? 0) }
    }
}

游乐场测试：

let arr = [5, 4, 1, 5, 5, 3, 5, 3]

let frequency1 = arr.frequencies {[=11=] == 1}       // [4, 1]
let frequency2 = arr.frequencies {[=11=] == 2}       // [3, 3]
let frequency3orMore = arr.frequencies {[=11=] >= 3} // [5, 5, 5, 5]

根据元素的出现获取数组的子集

Get Subset of array based on the occurrence of elements

arrays

ios

swift3