swift3.0 废弃 swift 4.0 以后字符串截取

截取前 n 个字符：

let str = "Hello, Swift!" let prefix = str.prefix(5) print(prefix) // "Hello" 截取后 n 个字符：

let str = "Hello, Swift!" let suffix = str.suffix(6) print(suffix) // "Swift!" 截取指定范围内的字符：

let str = "Hello, Swift!" let range = str.index(str.startIndex, offsetBy: 7)..

let str = "Hello, Swift!" let index = str.index(str.startIndex, offsetBy: 7) let subString = str[index...] print(subString) // "Swift!" 从最后一个指定“.”字符开始截取，到字符串结束：

let str = "http://www.free.com/346345647457.png" if let dotIndex = str.lastIndex(of: ".") {     let fileExtension = str[dotIndex+1..

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