Swift — String的索引佑吝、遍歷
一、基于EnumeratedSequence的遍歷
//字符串的遍歷
let randomString = "hello everybody"
//基于EnumeratedSequence的遍歷
for (i, char) in randomString.enumerated() {
print("The results of the ergodic\(i): \(char)")
/*打印輸出：
The results of the ergodic0: h
The results of the ergodic1: e
The results of the ergodic2: l
The results of the ergodic3: l
The results of the ergodic4: o
The results of the ergodic5:
The results of the ergodic6: e
The results of the ergodic7: v
The results of the ergodic8: e
The results of the ergodic9: r
The results of the ergodic10: y
The results of the ergodic11: b
The results of the ergodic12: o
The results of the ergodic13: d
The results of the ergodic14: y
*/
二绳匀、for in 正序遍歷
//正序
let randomString = "hello everybody"
for char in randomString{
print("The results of the ergodic:\(char)")
/*打印輸出：
The results of the ergodic:h
The results of the ergodic:e
The results of the ergodic:l
The results of the ergodic:l
The results of the ergodic:o
The results of the ergodic:
The results of the ergodic:e
The results of the ergodic:v
The results of the ergodic:e
The results of the ergodic:r
The results of the ergodic:y
The results of the ergodic:b
The results of the ergodic:o
The results of the ergodic:d
The results of the ergodic:y*/
}
三芋忿、for in 逆序遍歷
//逆序
let randomString = "hello everybody"
var index = randomString.count
for char in randomString.reversed() {
index -= 1
print("The results of the ergodic:\(index)=\(char)")
/*打印輸出：
The results of the ergodic:14=y
The results of the ergodic:13=d
The results of the ergodic:12=o
The results of the ergodic:11=b
The results of the ergodic:10=y
The results of the ergodic:9=r
The results of the ergodic:8=e
The results of the ergodic:7=v
The results of the ergodic:6=e
The results of the ergodic:5=
The results of the ergodic:4=o
The results of the ergodic:3=l
The results of the ergodic:2=l
The results of the ergodic:1=e
The results of the ergodic:0=h
*/
}
字符串索引
在講解String基于索引的遍歷前先擴(kuò)展一下 字符串的索引，那么在提到字符串的索引前就要先認(rèn)識(shí)一下
字形群集
1.可擴(kuò)展的字符群集可以組成一個(gè)或者多個(gè) Unicode 標(biāo)量疾棵。這意味著
不同的字符以及相同字符的不同表示方式可能需要不同數(shù)量的內(nèi)存空間來(lái)存儲(chǔ)戈钢。所以 Swift 中的字符在一個(gè)字符串中并不一定占用相同的內(nèi)存空間數(shù)量。因此在沒有獲取字符串的可擴(kuò)展的字符群的范圍時(shí)候是尔，就不能計(jì)算出字符串的字符數(shù)量殉了。如果您正在處理一個(gè)長(zhǎng)字符串，需要注意characters屬性必須遍歷全部的 Unicode 標(biāo)量拟枚，來(lái)確定字符串的字符數(shù)量薪铜。2.另外需要注意的是通過characters屬性返回的字符數(shù)量并不總是與包含相同字符的NSString的length屬性相同。
NSString的length屬性是利用 UTF-16 表示的十六位代碼單元數(shù)字恩溅，而不是 Unicode 可擴(kuò)展的字符群集隔箍。3.前面提到，不同的字符可能會(huì)占用不同數(shù)量的內(nèi)存空間暴匠，所以
要知道Character的確定位置鞍恢，就必須從String開頭遍歷每一個(gè) Unicode 標(biāo)量直到結(jié)尾傻粘。因此每窖，Swift 的字符串不能用整數(shù)(integer)做索引。
如下的字符：
//字符串字面量的特殊字符
let dollarSign = "\u{24}" // $, Unicode 標(biāo)量 U+0024
let blackHeart = "\u{2665}" // ?, Unicode 標(biāo)量 U+2665
let sparklingHeart = "\u{1F496}" // ??, Unicode 標(biāo)量 U+1F496
print(dollarSign,blackHeart,sparklingHeart)//打印 $ ? ??
//可擴(kuò)展的字形群集
let eAcute: Character = "\u{E9}" // é
let combinedEAcute: Character = "\u{65}\u{301}" // e 后面加上 ?
// eAcute 是 é, combinedEAcute 是 é
print("eAcuteValue:\(eAcute),combinedEAcuteValue:\(combinedEAcute)")//打印 eAcuteValue:é,combinedEAcuteValue:é
索引
1.使用
startIndex屬性可以獲取一個(gè)String的第一個(gè)Character的索引弦悉。使用endIndex屬性可以獲取最后一個(gè)Character的后一個(gè)位置的索引窒典。因此，endIndex屬性不能作為一個(gè)字符串的有效下標(biāo)稽莉。如果String是空串瀑志，startIndex和endIndex是相等的。2.通過調(diào)用 String 的
index(before:)或index(after:)方法污秆，可以立即得到前面或后面的一個(gè)索引劈猪。您還可以通過調(diào)用index(_:offsetBy:) 方法來(lái)獲取對(duì)應(yīng)偏移量的索引，這種方式可以避免多次調(diào)用 index(before:) 或 index(after:) 方法良拼。
let stringIndexStr = "hello word!"
//獲取字符串第一個(gè)索引的 字符
let starIndexC = stringIndexStr[stringIndexStr.startIndex]//獲取字符串第一個(gè)索引的 字符
print("startIndexValue:\(stringIndexStr.startIndex)")//打诱降谩： startIndexValue:Index(_base: Swift.String.UnicodeScalarView.Index(_position: 0), _countUTF16: 1)
print("starIndexCharacter:\(starIndexC)")//打印：starIndexCharacter:h
//獲取字符串最后一個(gè)索引的 字符
//錯(cuò)誤方法
//endIndex屬性不能作為一個(gè)字符串的有效下標(biāo),運(yùn)行時(shí)會(huì)崩潰
//let endIndexC = stringIndexStr[stringIndexStr.endIndex]//試圖獲取越界索引對(duì)應(yīng)的 Character庸推，將引發(fā)一個(gè)運(yùn)行時(shí)錯(cuò)誤常侦。
//print("endIndexCharacter:\(endIndexC)")
//正確方法
let endIndexC = stringIndexStr[stringIndexStr.index(before: stringIndexStr.endIndex)]//試圖獲取越界索引對(duì)應(yīng)的 Character浇冰，將引發(fā)一個(gè)運(yùn)行時(shí)錯(cuò)誤。
print("endIndexCharacter:\(endIndexC)")//打恿觥：endIndexCharacter:!
//獲取字符串 第二個(gè)索引的字符（第一個(gè)索引后面的一個(gè)索引值）
let afterIndexC = stringIndexStr[stringIndexStr.index(after: stringIndexStr.startIndex)]//獲取字符串 第二個(gè)索引的字符（第一個(gè)索引后面的一個(gè)索引值）
print(afterIndexC)//打又庀啊：e
//獲取字符串末尾的一個(gè)字符
let beforeIndexC = stringIndexStr[stringIndexStr.index(before: stringIndexStr.endIndex)]//獲取字符串末尾的一個(gè)字符(最后一個(gè)索引前面的一個(gè)索引，使用endIndex屬性可以獲取最后一個(gè)Character的后一個(gè)位置的索引坡倔，endIndex屬性不能作為一個(gè)字符串的有效下標(biāo))
print("獲取字符串末尾的一個(gè)字符：\(beforeIndexC)")//打悠濉：獲取字符串末尾的一個(gè)字符： ！
//第一個(gè)索引4個(gè)偏移量之后的一個(gè)索引
let offSetByIndex = stringIndexStr.index(stringIndexStr.startIndex, offsetBy: 4)//第一個(gè)索引4個(gè)偏移量之后的一個(gè)索引
print(offSetByIndex,stringIndexStr[offSetByIndex])//打幼锼：Index(_base: Swift.String.UnicodeScalarView.Index(_position: 4), _countUTF16: 1) o
基于索引的遍歷
四仅仆、基于索引的正序遍歷
//基于索引的正序遍歷
let randomString = "hello everybody"
for i in 0..<randomString.count {
let char: Character = randomString[randomString.index(randomString.startIndex, offsetBy: i)]
print("swift-string ergodic\(i): \(char)")
/*打印輸出：
swift-string ergodic0: h
swift-string ergodic1: e
swift-string ergodic2: l
swift-string ergodic3: l
swift-string ergodic4: o
swift-string ergodic5:
swift-string ergodic6: e
swift-string ergodic7: v
swift-string ergodic8: e
swift-string ergodic9: r
swift-string ergodic10: y
swift-string ergodic11: b
swift-string ergodic12: o
swift-string ergodic13: d
swift-string ergodic14: y
*/
}
五、基于索引的逆序遍歷
//基于索引的逆序遍歷
let randomString = "hello everybody"
for i in stride(from: randomString.count - 1, through: 0, by: -1) {
let char: Character = randomString[randomString.index(randomString.startIndex, offsetBy: i)]
print("swift-string ergodic\(i): \(char)")
/*打印輸出：
swift-string ergodic14: y
swift-string ergodic13: d
swift-string ergodic12: o
swift-string ergodic11: b
swift-string ergodic10: y
swift-string ergodic9: r
swift-string ergodic8: e
swift-string ergodic7: v
swift-string ergodic6: e
swift-string ergodic5:
swift-string ergodic4: o
swift-string ergodic3: l
swift-string ergodic2: l
swift-string ergodic1: e
swift-string ergodic0: h
*/
}
以上五種Swift中String的遍歷方法總有一款用得到
