String, substring, Range, NSRange in Swift 4

Question

I am using the following code to get a String substring from an NSRange:

func substring(with nsrange: NSRange) -> String? {
    guard let range = Range.init(nsrange)
        else { return nil }
    let start = UTF16Index(range.lowerBound)
    let end = UTF16Index(range.upperBound)
    return String(utf16[start..<end])
}

(via: https://mjtsai.com/blog/2016/12/19/nsregularexpression-and-swift/)

When I compile with Swift 4 (Xcode 9b4), I get the following errors for the two lines that declare start and end:

'init' is unavailable
'init' was obsoleted in Swift 4.0

I am confused, since I am not using an init.

How can I fix this?

Answer 1

Use Range(_, in:) to convert an NSRange to a Range in Swift 4.

extension String {
    func substring(with nsrange: NSRange) -> Substring? {
        guard let range = Range(nsrange, in: self) else { return nil }
        return self[range]
    }
}

Answer 2

With Swift 4 we can get substrings this way.

1. Substring from index

   let originStr = "Test"
   let offset = 1
   let str = String(originStr.suffix(from: String.Index.init(encodedOffset: offset)))
   

2. Substring to index

   let originStr = "Test"
   let offset = 1
   String(self.prefix(index))