# 1。使用
# 2。使用
# 3。使用
# 4。使用
# 5。使用for循环和
# 6。使用
# 7。使用
使用
使用递归函数
我已经看到了一些这样的例子，但所有这些似乎都依赖于知道要计算哪个元素的出现次数。我的数组是动态生成的，所以我无法知道我想要计算哪个元素的出现次数(我想要计算所有元素的出现次数)。有人能给点建议吗?
编辑:也许我应该更清楚，数组将包含多个不同的字符串(例如:["FOO", "FOO", "BAR", "FOOBAR"]
在事先不知道foo, bar和foobar是什么的情况下，如何计算它们的出现次数?
迅速迅速3和2:
您可以使用[String: Int]类型的字典来建立[String]中每个项目的计数:
let arr = ["FOO", "FOO", "BAR", "FOOBAR"]
var counts: [String: Int] = [:]
for item in arr {
counts[item] = (counts[item] ?? 0) + 1
}
print(counts) // "[BAR: 1, FOOBAR: 1, FOO: 2]"
for (key, value) in counts {
print("(key) occurs (value) time(s)")
}
BAR occurs 1 time(s)
FOOBAR occurs 1 time(s)
FOO occurs 2 time(s)
迅速4:
Swift 4引入了(SE-0165)在字典查找中包含默认值的能力，并且结果值可以通过+=和-=等操作进行修改，因此:
counts[item] = (counts[item] ?? 0) + 1
就变成:
counts[item, default: 0] += 1
这使得使用forEach:
let arr = ["FOO", "FOO", "BAR", "FOOBAR"]
var counts: [String: Int] = [:]
arr.forEach { counts[$0, default: 0] += 1 }
print(counts) // "["FOOBAR": 1, "FOO": 2, "BAR": 1]"
Swift 4: reduce(into:_:)
Swift 4引入了reduce的新版本，它使用inout变量来累积结果。使用它，计数的创建真正变成了一行:
let arr = ["FOO", "FOO", "BAR", "FOOBAR"]
let counts = arr.reduce(into: [:]) { counts, word in counts[word, default: 0] += 1 }
print(counts) // ["BAR": 1, "FOOBAR": 1, "FOO": 2]
或者使用默认参数:
let counts = arr.reduce(into: [:]) { $0[$1, default: 0] += 1 }
最后，您可以将此扩展为Sequence，以便它可以在包含Hashable项的任何Sequence上调用，包括Array, ArraySlice, String和String.SubSequence:
extension Sequence where Element: Hashable {
var histogram: [Element: Int] {
return self.reduce(into: [:]) { counts, elem in counts[elem, default: 0] += 1 }
}
}
Sequence而不是Array以获取额外的类型。
例子:
print("abacab".histogram)
["a": 3, "b": 2, "c": 1]
print("Hello World!".suffix(6).histogram)
["l": 1, "!": 1, "d": 1, "o": 1, "W": 1, "r": 1]
print([1,2,3,2,1].histogram)
[2: 2, 3: 1, 1: 2]
print([1,2,3,2,1,2,1,3,4,5].prefix(8).histogram)
[1: 3, 2: 3, 3: 2]
print(stride(from: 1, through: 10, by: 2).histogram)
[1: 1, 3: 1, 5: 1, 7: 1, 9: 1]
array.filter{$0 == element}.count
在Swift 5中，根据你的需要，你可以选择下面的7个Playground示例代码中的一个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:)下标
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:)初始化器
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:)初始化器和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:)初始化器和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(需要基础)
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(需要基础)
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)]
学分:
- 迅速习语
- 通用的集合，使用字典
我更新了oisdk对Swift2的回答。
16/04/14我更新了这个代码到Swift2.2
16/10/11更新为Swift3
Hashable:
extension Sequence where Self.Iterator.Element: Hashable {
private typealias Element = Self.Iterator.Element
func freq() -> [Element: Int] {
return reduce([:]) { (accu: [Element: Int], element) in
var accu = accu
accu[element] = accu[element]?.advanced(by: 1) ?? 1
return accu
}
}
}
Equatable:
extension Sequence where Self.Iterator.Element: Equatable {
private typealias Element = Self.Iterator.Element
func freqTuple() -> [(element: Element, count: Int)] {
let empty: [(Element, Int)] = []
return reduce(empty) { (accu: [(Element, Int)], element) in
var accu = accu
for (index, value) in accu.enumerated() {
if value.0 == element {
accu[index].1 += 1
return accu
}
}
return accu + [(element, 1)]
}
}
}
使用
let arr = ["a", "a", "a", "a", "b", "b", "c"]
print(arr.freq()) // ["b": 2, "a": 4, "c": 1]
print(arr.freqTuple()) // [("a", 4), ("b", 2), ("c", 1)]
for (k, v) in arr.freq() {
print("(k) -> (v) time(s)")
}
// b -> 2 time(s)
// a -> 4 time(s)
// c -> 1 time(s)
for (element, count) in arr.freqTuple() {
print("(element) -> (count) time(s)")
}
// a -> 4 time(s)
// b -> 2 time(s)
// c -> 1 time(s)
使用NSCountedSet。在objective - c中:
NSCountedSet* countedSet = [[NSCountedSet alloc] initWithArray:array];
for (NSString* string in countedSet)
NSLog (@"String %@ occurs %zd times", string, [countedSet countForObject:string]);
我想你可以自己把它翻译成Swift。
如何:
func freq<S: SequenceType where S.Generator.Element: Hashable>(seq: S) -> [S.Generator.Element:Int] {
return reduce(seq, [:]) {
(var accu: [S.Generator.Element:Int], element) in
accu[element] = accu[element]?.successor() ?? 1
return accu
}
}
freq(["FOO", "FOO", "BAR", "FOOBAR"]) // ["BAR": 1, "FOOBAR": 1, "FOO": 2]
它是通用的，所以无论你的元素是什么，它都可以工作，只要它是可哈希的:
freq([1, 1, 1, 2, 3, 3]) // [2: 1, 3: 2, 1: 3]
freq([true, true, true, false, true]) // [false: 1, true: 4]
如果你不能让你的元素可哈希，你可以用元组:
func freq<S: SequenceType where S.Generator.Element: Equatable>(seq: S) -> [(S.Generator.Element, Int)] {
let empty: [(S.Generator.Element, Int)] = []
return reduce(seq, empty) {
(var accu: [(S.Generator.Element,Int)], element) in
for (index, value) in enumerate(accu) {
if value.0 == element {
accu[index].1++
return accu
}
}
return accu + [(element, 1)]
}
}
freq(["a", "a", "a", "b", "b"]) // [("a", 3), ("b", 2)]
我喜欢避免内部循环，并尽可能使用。map。因此，如果我们有一个字符串数组，我们可以执行以下操作来计算
的出现次数var occurances = ["tuples", "are", "awesome", "tuples", "are", "cool", "tuples", "tuples", "tuples", "shades"]
var dict:[String:Int] = [:]
occurances.map{
if let val: Int = dict[$0] {
dict[$0] = val+1
} else {
dict[$0] = 1
}
}
打印
["tuples": 5, "awesome": 1, "are": 2, "cool": 1, "shades": 1]
Swift 4
let array = ["FOO", "FOO", "BAR", "FOOBAR"]
// Merging keys with closure for conflicts
let mergedKeysAndValues = Dictionary(zip(array, repeatElement(1, count: array.count)), uniquingKeysWith: +)
// mergedKeysAndValues is ["FOO": 2, "BAR": 1, "FOOBAR": 1]
另一种方法是使用filter方法。我发现最优雅的
var numberOfOccurenses = countedItems.filter(
{
if $0 == "FOO" || $0 == "BAR" || $0 == "FOOBAR" {
return true
}else{
return false
}
}).count
您可以使用这个函数来计算数组
中条目的出现次数func checkItemCount(arr: [String]) {
var dict = [String: Any]()
for x in arr {
var count = 0
for y in arr {
if y == x {
count += 1
}
}
dict[x] = count
}
print(dict)
}
你可以这样实现它-
let arr = ["FOO", "FOO", "BAR", "FOOBAR"]
checkItemCount(arr: arr)
public extension Sequence {
public func countBy<U : Hashable>(_ keyFunc: (Iterator.Element) -> U) -> [U: Int] {
var dict: [U: Int] = [:]
for el in self {
let key = keyFunc(el)
if dict[key] == nil {
dict[key] = 1
} else {
dict[key] = dict[key]! + 1
}
//if case nil = dict[key]?.append(el) { dict[key] = [el] }
}
return dict
}
let count = ["a","b","c","a"].countBy{ $0 }
// ["b": 1, "a": 2, "c": 1]
struct Objc {
var id: String = ""
}
let count = [Objc(id: "1"), Objc(id: "1"), Objc(id: "2"),Objc(id: "3")].countBy{ $0.id }
// ["2": 1, "1": 2, "3": 1]
extension Collection where Iterator.Element: Comparable & Hashable {
func occurrencesOfElements() -> [Element: Int] {
var counts: [Element: Int] = [:]
let sortedArr = self.sorted(by: { $0 > $1 })
let uniqueArr = Set(sortedArr)
if uniqueArr.count < sortedArr.count {
sortedArr.forEach {
counts[$0, default: 0] += 1
}
}
return counts
}
}
// Testing with...
[6, 7, 4, 5, 6, 0, 6].occurrencesOfElements()
// Expected result (see number 6 occurs three times) :
// [7: 1, 4: 1, 5: 1, 6: 3, 0: 1]
计数排序的第一步。
var inputList = [9,8,5,6,4,2,2,1,1]
var countList : [Int] = []
var max = inputList.maxElement()!
// Iniate an array with specific Size and with intial value.
// We made the Size to max+1 to integrate the Zero. We intiated the array with Zeros because it's Counting.
var countArray = [Int](count: Int(max + 1), repeatedValue: 0)
for num in inputList{
countArray[num] += 1
}
print(countArray)
两个解决方案:
- 使用
forEach循环
let array = [10,20,10,40,10,20,30]
var processedElements = [Int]()
array.forEach({
let element = $0
// Check wether element is processed or not
guard processedElements.contains(element) == false else {
return
}
let elementCount = array.filter({ $0 == element}).count
print("Element: (element): Count (elementCount)")
// Add Elements to already Processed Elements
processedElements.append(element)
})
let array = [10,20,10,40,10,20,30]
self.printElementsCount(array: array)
func printElementsCount(array: [Int]) {
guard array.count > 0 else {
return
}
let firstElement = array[0]
let filteredArray = array.filter({ $0 != firstElement })
print("Element: (firstElement): Count (array.count - filteredArray.count )")
printElementsCount(array: filteredArray)
}
import Foundation
var myArray:[Int] = []
for _ in stride(from: 0, to: 10, by: 1) {
myArray.append(Int.random(in: 1..<6))
}
// Method 1:
var myUniqueElements = Set(myArray)
print("Array: (myArray)")
print("Unique Elements: (myUniqueElements)")
for uniqueElement in myUniqueElements {
var quantity = 0
for element in myArray {
if element == uniqueElement {
quantity += 1
}
}
print("Element: (uniqueElement), Quantity: (quantity)")
}
// Method 2:
var myDict:[Int:Int] = [:]
for element in myArray {
myDict[element] = (myDict[element] ?? 0) + 1
}
print(myArray)
for keyValue in myDict {
print("Element: (keyValue.key), Quantity: (keyValue.value)")
}
执行计数的结构
struct OccureCounter<Item: Hashable> {
var dictionary = [Item: Int]()
mutating func countHere(_ c: [Item]) {
c.forEach { add(item: $0) }
printCounts()
}
mutating func add(item: Item) {
if let value = dictionary[item] {
dictionary[item] = value + 1
} else {
dictionary[item] = 1
}
}
func printCounts() {
print("::: START")
dictionary
.sorted { $0.value > $1.value }
.forEach { print("::: ($0.value) — ($0.key)") }
let all = dictionary.reduce(into: 0) { $0 += $1.value }
print("::: ALL: (all)")
print("::: END")
}
}
struct OccureTest {
func test() {
let z: [Character] = ["a", "a", "b", "a", "b", "c", "d", "e", "f"]
var counter = OccureCounter<Character>()
counter.countHere(z)
}
}
它打印:
::: START
::: 3 — a
::: 2 — b
::: 1 — c
::: 1 — f
::: 1 — e
::: 1 — d
::: ALL: 9
::: END