Swift - 字符串（String）用法详解

2015-05-21 发布：hangge 阅读：5868

（本文代码已升级至Swift4）

下面对 String 常用的属性和方法做个总结

1，判断是否为空：isEmpty

var str:String = ""
if str.isEmpty{
    print("is empty")
}

2，获取字符数量：string.count

let str = "hangge.com"
print("\(str.count)个字符")

3，检查字符串是否有特定前缀/后缀：hasPrefix/hasSuffix

var str = "hangge.com"
if str.hasSuffix(".com"){
}

4，还可以用“\()”在字符串里包裹变量，常量

let name = "hangge.com"
let msg = "欢迎来到 \(name)"

5，多行字符串字面量（多行文字）

过去如果有一个很长的字符串也只能写在一行。到了 Swift 4 可以把字符串写在一对 """ 中，这样字符串就可以写成多行。

let str = """
欢迎访问hanggge.com
做最好的开发者知识平台
"""
print(str)

6，大小写转换

通过字符串的 uppercased()、lowercase() 方法、capitalized 属性来访问一个字符串的大写/小写/首字母大写

let str = "Welecome to hangge.com"

let uppercase = str.uppercased()  //WELECOME TO HANGGE.COM
let lowercase = str.lowercased()  //welecome to hangge.com
let capitalized = str.capitalized //Welecome To Hangge.Com

7，字符串截取

（1）将 String 转化为 NSString 再截取

let str = "Welecome to hangge.com"

let fromStr = (str as NSString).substring(from: 5) //ome to hangge.com
let toStr = (str as NSString).substring(to: 5)  //Welec
let rangeStr =  (str as NSString).substring(with: NSMakeRange(4,1)) //c

（2）直接调用 String 的对应方法（不推荐，substring 现已被废除）

let str = "Welecome to hangge.com"

let index = str.index(str.startIndex, offsetBy: 5)
let index2 = str.index(str.endIndex, offsetBy: -5)
let range = Range(uncheckedBounds: (lower: index, upper: index2))

let fromStr = str.substring(from: index)  //ome to hangge.com
let toStr = str.substring(to: index)  //Welec
let rangeStr = str.substring(with: range) //ome to hangg

现在还是推荐使用如下方式截取：

let str = "Welecome to hangge.com"

let index = str.index(str.startIndex, offsetBy: 5)
let index2 = str.index(str.endIndex, offsetBy: -5)

let rangeStr = str[index..<index2] //ome to hangg

（3）Swift4 新增了一个语法糖 ... 可以对字符串进行单侧边界取子串。

let str = "Welecome to hangge.com"

let index = str.index(str.startIndex, offsetBy: 5)

let rangeStr0 = str[..<index] //Welec
let rangeStr1 = str[...index] //Weleco
let rangeStr2 = str[index...] //ome to hangge.com

（4）还可以通过 prefix 和 suffix 方法进行单边截取。

let str = "Welecome to hangge.com"

let index = str.index(str.startIndex, offsetBy: 5)

let rangeStr0 = str.prefix(5) //Welec
let rangeStr1 = str.prefix(upTo: index) //Welec
let rangeStr2 = str.prefix(through: index) //Weleco

let rangeStr3 = str.suffix(5) //e.com
let rangeStr4 = str.suffix(from: index) //ome to hangge.com

当然为了更方便地通过索引截取字符串，我们还可以对 String 做个扩展：
Swift - 扩展String增加字符串截取方法subString(start:, length:)

8，获取某个下标索引的前一个、后一个下标对应的字符

import UIKit

let str = "hangge.com"

//获取字符串开始索引后一个下标对应的字符
let startIndex = str.startIndex
let char1 = str[str.index(after: startIndex)]

//获取字符串结束索引前一个下标对应的字符
let endIndex = str.endIndex //endIndex在值上与字符串的长度相等
let char2 = str[str.index(before: endIndex)]

9，获取某个子串在父串中的范围

var range = str.range(of: "Hello")

10，获取某个子串在父串中的开始位置索引

参考我的另一篇文章：Swift - 在字符串中查找另一字符串首次出现的位置

11，追加字符串（拼接字符串）

str.append("hangge.com")

12，在指定位置插入字符

（1）插入一个字符

str.insert("a", at: str.index(str.startIndex, offsetBy: 5))

（2）插入一组字符

str.insert(contentsOf: ["~","~","~"], at: str.index(str.startIndex, offsetBy: 5))

13，在指定范围替换一个字符串

str.replaceSubrange(str.startIndex...str.index(str.startIndex, offsetBy: 4), with: "hangge")

14，删除指定范围的字符串

str.removeSubrange(str.startIndex...str.index(str.startIndex, offsetBy: 2))

15，删除第一个字符

str.dropFirst()

16，字符串反转

let str1 = "hangge.com"
let str2 = String(str1.reversed()) //moc.eggnah

17，遍历字符串里的字符

let str = "hangge.com"

//方法1
for c in str {
    print(c)
}

//方法2
str.forEach {
    print($0)
}

18，获取字符对应的ASCII编码

let character: Character = "a"
let unicodeScalars = character.unicodeScalars
let startIndex = unicodeScalars.startIndex
let asciiCode = unicodeScalars[startIndex].value
print(asciiCode)

19，map方法

let str = "hangge"

_ = str.map {
    print($0.description)
}

20，filter方法

let str = "hangge"

let filtered = str.filter { $0 == "g" }
print(filtered) //gg

21，reduce方法

let str = "hangge"

let result = str.reduce("1") { (result, c) -> String in
    print(result, c)
    return result + String(c)
}
print("最终结果：\(result)")