Swift String下标扩展

对swift的String进行扩展，方便进行下标访问元素:

extension String{
    subscript(range: Range) -> String {
        get{
            let startIndex = self.startIndex.advancedBy(range.endIndex)
            let endIndex = self.startIndex.advancedBy(range.endIndex)
             
            return self[Range(start: startIndex, end: endIndex)]
        }
    }
     
    subscript(index: Int) -> String{
        get{
            return self[index...index]
        }
    }
}
 
let string = "string"
// string[1]  "t"
// string[0...2]  "str"

// string[-1...6]  error
// string[-2...-1]  error
// string[6...7]  error

在这种情况下如果访问的下标有一边不在String的范围之内就会报错

可以定义当访问越界时同意返回空字符串,修改方式如下:

extension String{
    subscript(range: Range) -> String {
        get{
            let rStart = range.startIndex < 0 ? 0 : range.startIndex
            let rEnd = range.endIndex < 0 ? 0 : range.endIndex
             
            let startIndex = self.startIndex.advancedBy(rStart, limit: self.endIndex)
            let endIndex = self.startIndex.advancedBy(rEnd, limit: self.endIndex)
             
            return self[Range(start: startIndex, end: endIndex)]
        }
    }
     
    subscript(index: Int) -> String{
        get{
            return self[index...index]
        }
    }
}
 
let string = "string"
// string[1]  "t"
// string[0...2]  "str"
// string[-1...6]  "string"
// string[-2...-1]  ""
// string[6...7]  ""

如果对越界返回有其他处理,可以参照上述代码进行相应修改。