Swift 中 String 取下標及性能問題
取下標
String
String 用 String.Index 取下標(subscript)得到 Character，String.Index 要從 String 中獲取
let greeting = "Guten Tag!"
greeting[greeting.startIndex] // Character "G"
greeting[greeting.index(before: greeting.endIndex)] // Character "!"
greeting[greeting.index(after: greeting.startIndex)] // Character "u"
let index = greeting.index(greeting.startIndex, offsetBy: 7)
greeting[index] // Character "a"
String 用 Range<String.Index> 或 ClosedRange<String.Index> (以下 Range 和 ClosedRange 統稱為 Range) 取下標得到 String
let str = "abc"
str[str.startIndex..<str.index(after: str.startIndex)] // String "a"
str[str.startIndex...str.index(after: str.startIndex)] // String "ab"
Character
String 通過 characters 屬性獲得 String.CharacterView，表示屏幕上顯示的內容。String.CharacterView 通過 String.CharacterView.Index 取下標得到 Character，String.CharacterView.Index 要從 String.CharacterView 中獲取
let str = "abc"
let characters = str.characters // String.CharacterView
characters[characters.startIndex] // Character "a"
注意，String.CharacterView 不遵循 RandomAccessCollection 協議，用 String.CharacterView.Index 取下標不可以隨機訪問。另外，String.CharacterView.Index 與 String.Index 是相同的類型，屬於 Struct。String.Index 的文檔在 String 文檔下
typealias Index = String.CharacterView.Index
String.CharacterView 通過 Range<String.CharacterView.Index> 得到 String.CharacterView。用 Character 和 String.CharacterView 都可以生成 String
let str = "abc"
let characters = str.characters // String.CharacterView
let characters2 = characters[characters.startIndex..<characters.index(after: characters.startIndex)] // String.CharacterView
String(characters.first!) == String(characters2) // true. characters.first! is Character
用 String.CharacterView 生成 Array<Character>，可以用 Int、Range<Int> 取下標。用 Array<Character> 也可以生成 String
let str = "abc"
let arr = Array(str.characters) // Array<Character> ["a", "b", "c"]
arr[1] // Character "b"
arr[1...2] // ArraySlice<Character> ["b", "c"]
String(arr) // String "abc"
Character 可以直接與 "a" 比較
let str = "abc"
let a = str[str.startIndex] // Character "a"
let b = str[str.index(str.startIndex, offsetBy: 1)] // Character "b"
a == "a" // true
b > "a" // true
UTF-8
String 通過 utf8 屬性獲得 String.UTF8View，表示 UTF-8 編碼的內容。String.UTF8View 通過 String.UTF8View.Index 取下標得到 UTF8.CodeUnit，實際上是 UInt8；通過 Range<String.UTF8View.Index> 取下標得到 String.UTF8View。String.UTF8View.Index 要從 String.UTF8View 中獲取。String.UTF8View 不遵循 RandomAccessCollection 協議，用 String.UTF8View.Index 取下標不可以隨機訪問。用 String.UTF8View 生成 Array<UInt8>，可以用 Int、Range<Int> 取下標。用 String.UTF8View 可以生成 String。用 UInt8 或 Array<UInt8> 也可以生成 String，但內容表示數字或數字數組，不是數字的 UTF-8 編碼內容。
let str = "abc"
let utf8 = str.utf8 // String.UTF8View
let n = utf8[utf8.startIndex] // UInt8 97
let a = utf8[utf8.startIndex..<utf8.index(after: utf8.startIndex)] // String.UTF8View "a"
let ab = utf8[utf8.startIndex...utf8.index(after: utf8.startIndex)] // String.UTF8View "ab"
String(n) // "97", NOT "a"
String(a) // "a"
String(ab) // "ab"
let arr = Array(utf8) // Array<UInt8> [97, 98, 99]
let n2 = arr[0] // UInt8 97
let arr2 = arr[0...1] // // ArraySlice<UInt8> [97, 98]
String 通過 utf8CString 屬性獲得 ContiguousArray<CChar>，實際上是 ContiguousArray<Int8>，表示 UTF-8 編碼的內容並且末尾增加一個 0，所以長度比 utf8 屬性的長度大 1。ContiguousArray<Int8> 可以用 Int、Range<Int> 取下標，分別得到 Int8 和 ArraySlice<Int8>。ContiguousArray 遵循 RandomAccessCollection 協議，用 Int 取下標可以隨機訪問。
let str = "abc"
let utf8 = str.utf8CString // ContiguousArray<Int8> [97, 98, 99, 0]
let a = utf8[0] // Int8 97
let ab = utf8[0...1] // ArraySlice<Int8> [97, 98]
UTF-16
String 通過 utf16 屬性獲得 String.UTF16View，表示 UTF-16 編碼的內容。String.UTF16View 通過 String.UTF16View.Index 取下標得到 UTF16.CodeUnit，實際上是 UInt16；通過 Range<String.UTF16View.Index> 取下標得到 String.UTF16View。String.UTF16View.Index 要從 String.UTF16View 中獲取。String.UTF16View 遵循 RandomAccessCollection 協議，用 String.UTF16View.Index 取下標可以隨機訪問。用 String.UTF16View 生成 Array<UInt16>，可以用 Int、Range<Int> 取下標。用 String.UTF16View 可以生成 String。用 UInt16 或 Array<UInt16> 也可以生成 String，但內容表示數字或數字數組，不是數字的 UTF-16 編碼內容。
let str = "abc"
let utf16 = str.utf16 // String.UTF16View
let n = utf16[utf16.startIndex] // UInt16 97
let a = utf16[utf16.startIndex..<utf16.index(after: utf16.startIndex)] // String.UTF16View "a"
let ab = utf16[utf16.startIndex...utf16.index(after: utf16.startIndex)] // String.UTF16View "ab"
String(n) // "97", NOT "a"
String(a) // "a"
String(ab) // "ab"
let arr = Array(utf16) // Array<UInt16> [97, 98, 99]
let n2 = arr[0] // UInt16 97
let arr2 = arr[0...1] // // ArraySlice<UInt8> [97, 98]
性能對比
對 String、String.CharacterView、Array<Character>、String.UTF8View、Array<UInt8>、ContiguousArray<Int8>、String.UTF16View、Array<UInt16> 進行判空(isEmpty)、獲取長度(count)、一個位置的取下標([index])、一段距離的取下標([range])測試，統計執行時間。
定義測試類型、打印和更新時間的方法、要測試的 String
import Foundation
enum TestType {
case isEmpty
case count
case index
case range
}
func printAndUpdateTime(_ date: inout Date) {
let now = Date()
print(now.timeIntervalSince(date))
date = now
}
let s = "aasdfsdfsdfgfdsg vrutj7edbj7 ergcwhmkl5lknjklqawkrcqjljkljqjlqjhbrlqwfcbhafcpiluioufnlkqjvjakjn fnvjalgkhlkdkjlkasdfsdfsdfgfdsg vrutj7edbj7 ergcwhmkl5lknjklqawkrcqjliopjktyuljqjlqjhbrlqwfcbhafciluioufnlkjvjakjn fnvjalgkhlkdkjlkasdfsdfsdfgfdsg vrutj7edbj7 ergcwhmkl5lknjklqawkrcqjljkljqjlqjhbrlqwfcbhafciluioufnlkjvjakjn fnvjalgkhlkdkjlkasderwytwghfsdfsdfgfdsg vrutj7edbj7 fdgotuyoergcwhmkl5lknjklqawkyrcqjljkljqjlqjhbrlqwfcbhafci luioufnlkjvjakjn fnvjalgkhlkdkjlkasdfsdfsdfgfdsg vrutj7edbj7 ergcvcnvbwhmkl5lknjklqawkrcqjljkljqjlqjhbrlqwfcbhafciluioufnlkjvjakjn fnvjalgkhlkdkjlkasdfsdfsdfgfdsg vrutj7edbj7 ergcwhmkl5lknjklqawkrcqjljkljqjlqjhbrlqwfcbhafciluioufnlkjvjakjkn fnvjalgkhlkdkjlkasdfsdfsdfgfdsg vrutj7edbj7 ergcwhmkl5lknjklqawkrcqjljkljqjlqjhbrlqwfcbhafciluioufnlkjvjakjn fnvjalgkhlkdkjlkasdfsdfsdfgfdsg iopiouvrutj7edbj7 ergcwhmkl5lknjklqawkrcqjljkfghngdljqjlqjhbrlqwfcbhafciluioufnlkjvjakjn fnvjalgkhlkdkjlkasdfsdfsdfgfdsg vrutj7edbj7 ergcwhmbkl5lknjklqawkrcqjljkljqjlqjhbrlqwfcbhafciluioufnlkjvjakjn fnvjalgkhlkdkjlkasdfsdfsdfgfdsg vrutj7edbj7 ergcwhmkl5lknjklqawkrcqjljkljqjlqjhbrlqwfcbhafciluioufnlkjvjakjn fnvjalgkhlkdkjlkasdfsdfsdfgfdsg vrutj7edbj7 ergcwhmkl5lknjklqawkrcqjljkljqjlqjhbrlqwfcbhafciluioufnlkjvjakjn fnvjalgkhlkdkjlkasdfsdfsdfgfdsg vrutj7edbj7 ergcwhmkl5lknjklqawkrcqjljkljqjlqjhbrlqwfcbhafciluioufnlkjvjakjn fnvjalgkhlkdkjlkasdfsdfsdfgfdsg vrutj7edbj7 ergcwhmkl5lknjklqasdfsdwkrcqjljkljqjlqjhbrlqwfcbhafciluioufnlkjvjakjn fnvjalgkhlkdkjlkasdfsdfsdfgfdsg vrutj7edbj7 ergcwhmkl5lknjklqawkrcqjljkljqjlqjhbrlqwfcbhafciluioufnlkjvjakjn fnvjalgkhlkdkjlkasdfsdfsdfgfdsg vrutj7edbj7 ergcwhmkl5lknjklqawkrcqjljkljdqjlqjhbrlqwfcbhafciluioufnlkjvjakjn fnvjalgkhlkdkjlkasddfsdfsdfgfdsg vrutj7edbj7 ergcwhmkl5lknjklqawkrcqjljkljqjlqjhbrlqwfcbhafciluioufnlkjvjakjn fnvjalgkhlkdkjlkasdfsdfsdfgfdsg vrutj7edbj7 ergcwhmkl5lknjklqawkrcqjljkljqjlqjhbsdfdsrlqwfcbhafciluioufnlkjvjakjn fnvjalgkhlkdkjlkasdfsdfsdfgfdsg vrutj7edbj7 ergcwhmkl5lknjklqawkrcqjljkljqjlqjhbrlqwfcbhafciluioufnlkjvjakjn fnvjalgkhlkdkjlkasdfsdfsdfgfdsg vrutj7edbj7 ergcwhmkl5lknjklqawkrcqjljkljqjlqjhbrlqwfcbhafciluioufnlkjvjakjn fnvjalgkhlkdkjlkasdfsdfsdfgfdsg vrutj7edbj7 ergcwhmkl5lknjklqawkrcqjljkljqjlqjhbrlqwfcbhafciluioufnlkjvjakjn fnvjalgkhlkdkjlkasdfsdfsdfsadfsdgfdsg vrutj7edbj7 ergcwhmkl5lknjklqawkrcqjljkljqsdfasjlqjhbrlqwfcbhafciluioufnlkjvjakjn fnvjalgkhlkdkjlkasdfsdfsdfgfdsg vrutj7edbj7 ergcwhmkl5lknjklqawkrcqjljkljqjlqjhbrlqwfcbhafciluioufnlkjvjakjn fnvjalgkhlkdkjlkasdfsdfsdfgfdsg vrutj7edbj7 ergcwhmkl5lknjklqawkrcqjljkljqjlqjhbrlqwfcbhafciluioufnlkjvjakjn fnvjalgkhlkdkjlkasdfsdfsdafgfdsg vrutj7edbj7 ergcwhmkl5lknjklqawkrcqjljkljqjlqjhbrlqwfcbhafciluioufnlkjvjakjn fnvjalgkhlkdkjlkasdfsdfsdfgfdsg vrutj7edbj7 ergcwhmkl5lknjklqawkrcqjljkljqjlqjhbrlqwfcbhafciluioufnlkjvjakjn fnvjalgkhlkdkjlkasdfsdfsdfgfdsg vrutj7edbj7 ergcwhmkl5lknjklqawkrcqjljkljqjlqjhbrlqwfcbhafciluioufnlkjvjakjn fnvjalgkhlkdkjlkasdfsdfsdfgfdsg vrutj7edbj7 ergcwhmkl5lknjklqawkrcqjljkljqjlqjhbrlqwfcbhafciluioufnlkjvjakjn fnvjalgkhlkdkjlk"
測試代碼
let loopCount = 10000
let index = s.characters.count / 2
let testType: TestType = .range
print(testType)
var date = Date()
forLoop: for _ in 0..<loopCount {
switch testType {
case .isEmpty:
_ = s.isEmpty
case .count:
break forLoop
case .index:
_ = s[s.index(s.startIndex, offsetBy: index)]
case .range:
let endIndex = s.index(s.startIndex, offsetBy: index)
_ = s[s.startIndex..<endIndex]
}
}
if testType == .count {
date = Date()
} else {
print("String")
printAndUpdateTime(&date)
}
let characters = s.characters
for _ in 0..<loopCount {
switch testType {
case .isEmpty:
_ = characters.isEmpty
case .count:
_ = characters.count
case .index:
_ = characters[characters.index(characters.startIndex, offsetBy: index)]
case .range:
let endIndex = characters.index(characters.startIndex, offsetBy: index)
_ = characters[characters.startIndex..<endIndex]
}
}
print("Characters")
printAndUpdateTime(&date)
let characterArr = Array(characters)
for _ in 0..<loopCount {
switch testType {
case .isEmpty:
_ = characterArr.isEmpty
case .count:
_ = characterArr.count
case .index:
_ = characterArr[index]
case .range:
_ = characterArr[0..<index]
}
}
print("Characters array")
printAndUpdateTime(&date)
let utf8 = s.utf8
for _ in 0..<loopCount {
switch testType {
case .isEmpty:
_ = utf8.isEmpty
case .count:
_ = utf8.count
case .index:
_ = utf8[utf8.index(utf8.startIndex, offsetBy: index)]
case .range:
let endIndex = utf8.index(utf8.startIndex, offsetBy: index)
_ = utf8[utf8.startIndex..<endIndex]
}
}
print("UTF-8")
printAndUpdateTime(&date)
let utf8Arr = Array(utf8)
for _ in 0..<loopCount {
switch testType {
case .isEmpty:
_ = utf8Arr.isEmpty
case .count:
_ = utf8Arr.count
case .index:
_ = utf8Arr[index]
case .range:
_ = utf8Arr[0..<index]
}
}
print("UTF-8 array")
printAndUpdateTime(&date)
let utf8CString = s.utf8CString
for _ in 0..<loopCount {
switch testType {
case .isEmpty:
_ = utf8CString.isEmpty
case .count:
_ = utf8CString.count
case .index:
_ = utf8CString[index]
case .range:
_ = utf8CString[0..<index]
}
}
print("UTF-8 C string")
printAndUpdateTime(&date)
let utf16 = s.utf16
for _ in 0..<loopCount {
switch testType {
case .isEmpty:
_ = utf16.isEmpty
case .count:
_ = utf16.count
case .index:
_ = utf16[utf16.index(utf16.startIndex, offsetBy: index)]
case .range:
let endIndex = utf16.index(utf16.startIndex, offsetBy: index)
_ = utf16[utf16.startIndex..<endIndex]
}
}
print("UTF-16")
printAndUpdateTime(&date)
let utf16Arr = Array(utf16)
for _ in 0..<loopCount {
switch testType {
case .isEmpty:
_ = utf16Arr.isEmpty
case .count:
_ = utf16Arr.count
case .index:
_ = utf16Arr[index]
case .range:
_ = utf16Arr[0..<index]
}
}
print("UTF-16 array")
printAndUpdateTime(&date)
測試結果
判空
獲取長度
一個位置的取下標
一段距離的取下標
以上比較中，判斷 String 是否為空，訪問 String 的 isEmpty 速度最快。對於其他操作，遵循 RandomAccessCollection 協議(ContiguousArray<Int8>、String.UTF16View 以及其他 Array)的類型效率較高。
進一步比較判空操作
let loopCount = 10000
var date = Date()
for _ in 0..<loopCount {
_ = s.isEmpty
}
print("isEmpty")
printAndUpdateTime(&date)
for _ in 0..<loopCount {
_ = s == ""
}
print("== \"\"")
printAndUpdateTime(&date)
與訪問 String 的 isEmpty 相比，判斷 String 是否等於空 String 速度更快！
注意到文檔中，對 String.UTF8View 和 String.UTF16View 的 Range 取下標方法的說明
subscript(bounds: Range<String.UTF8View.Index>) -> String.UTF8View { get }
subscript(bounds: Range<String.UTF16View.Index>) -> String.UTF16View { get }
Complexity: O(n) if the underlying string is bridged from Objective-C, where n is the length of the string; otherwise, O(1).
如果 String 是從 Objective-C 的 NSString 橋接來的，時間復雜度為 O(n)，否則為 O(1)。這句話怎么理解呢？前面說了，String.UTF8View 不遵循 RandomAccessCollection 協議，而 String.UTF16View 遵循 RandomAccessCollection 協議，兩者的時間復雜度應該不同。這里怎么說時間復雜度與 String 是否橋接自 NSString 有關？以下進一步探究。
let s2 = NSString(string: s) as String
let loopCount = 10000
let index = s.characters.count / 2
let index2 = s.characters.count - 1
func test(_ s: String) {
var date = Date()
let utf8 = s.utf8
for _ in 0..<loopCount {
_ = utf8[utf8.startIndex..<utf8.index(utf8.startIndex, offsetBy: index)]
}
print("UTF-8 index")
printAndUpdateTime(&date)
for _ in 0..<loopCount {
_ = utf8[utf8.startIndex..<utf8.index(utf8.startIndex, offsetBy: index2)]
}
print("UTF-8 index2")
printAndUpdateTime(&date)
let utf16 = s.utf16
for _ in 0..<loopCount {
_ = utf16[utf16.startIndex..<utf16.index(utf16.startIndex, offsetBy: index)]
}
print("UTF-16 index")
printAndUpdateTime(&date)
for _ in 0..<loopCount {
_ = utf16[utf16.startIndex..<utf16.index(utf16.startIndex, offsetBy: index2)]
}
print("UTF-16 index2")
printAndUpdateTime(&date)
}
print("String")
test(s)
print("\nString bridged from NSString")
test(s2)
測試結果
對比 index 與 index2 的差異。測試參數 index2 約為 index 的 2 倍。UTF-8 index2 的耗時也約為 index 的 2 倍。UTF-16 的 index 和 index2 耗時相近。這與是否遵循 RandomAccessCollection 協議一致。
對比 String 與 NSString 的差異。橋接自 NSString 的 String 耗時比 String 要長，UTF-8 尤其明顯。這應該就是文檔說明的情況。用 Range 取下標，橋接自 NSString 的 String，比 String 多一些操作，多出 O(n) 級別的時間，而不是取下標的時間復雜度是 O(n)。
應用
具體應用時，選取哪種編碼方式、取下標方式？首先，編碼方式要看具體應用場景。編碼方法不同，字符串的長度可能不同。如果字符串只含英文，比較好辦。如果字符串含有中文或 Emoji，選擇編碼方式就要慎重。注意，NSString 的 length 屬性獲得的長度對應 UTF-16 編碼。
let str = "abc"
str.characters.count // 3
str.unicodeScalars.count // 3
str.utf16.count // 3
(str as NSString).length // 3
str.utf8.count // 3
str.utf8CString.count - 1 // 3
strlen(str) // 3
let emojiStr = "🇵🇷"
emojiStr.characters.count // 1
emojiStr.unicodeScalars.count // 2
emojiStr.utf16.count // 4
(emojiStr as NSString).length // 4
emojiStr.utf8.count // 8
emojiStr.utf8CString.count - 1 // 8
strlen(emojiStr) // 8
let ChineseStr = "中文"
ChineseStr.characters.count // 2
ChineseStr.unicodeScalars.count // 2
ChineseStr.utf16.count // 2
(ChineseStr as NSString).length // 2
ChineseStr.utf8.count // 6
ChineseStr.utf8CString.count - 1 // 6
strlen(ChineseStr) // 6
一般情況下，字符串要顯示出來，就用 String.CharacterView。如果要取下標，考慮性能，就用 String.CharacterView 生成 Array<Character>。如果要用其他編碼方式，也可以生成相應的 Array，以 Int 或 Range<Int> 取下標，效率高而且代碼簡潔。
現在 LeetCode 支持 Swift 了。如果做 LeetCode 的需要多次進行字符串取下標的題目，用不遵循 RandomAccessCollection 協議的類型(例如 String.CharacterView、String.UTF8View)，可能思路對了結果卻是 “Time Limit Exceeded”。參見：http://www.cnblogs.com/silence-cnblogs/p/6878223.html
為了避免 NSString 橋接帶來的性能問題，在 Swift 里盡量用 String；盡量減少 Objective-C 的代碼，盡量選擇 Swift 編寫的第三方庫。
轉載請注明出處：http://www.cnblogs.com/silence-cnblogs/p/6877463.html