import Foundation

var str: String = "0123456789"
let start = str.index(str.startIndex,offsetBy: 3);
let str1 = str.substring(from: start)
print(str1) //3456789

let end = str.index(str.endIndex,offsetBy: -3)
let str2 = str.substring(to: end)
print(str2) //0123456

let range = Range<String.Index>(uncheckedBounds: (lower: start,upper: end))
let str3 = str.substring(with: range)
print(str3) //3456

extension String {
    subscript(r: Range<Int>) -> String {
        get {
            let startIndex = self.index(self.startIndex,offsetBy: r.lowerBound);
            let endIndex = self.index(self.startIndex,offsetBy: r.upperBound)
            return self.substring(with: Range(startIndex..<endIndex))
        }
        set {
            let startIndex = self.index(self.startIndex,offsetBy: r.upperBound)
            self.replaceSubrange(Range(startIndex..<endIndex),with: newValue)
        }
    }
}

let i = 3
let j = 7

print(str[i..<j]) //3456
str[1..<3] = "aaa"
print(str) //0aaa3456789