How do you use String.substringWithRange? (or, how do Ranges work in Swift?)

Question

I have not yet been able to figure out how to get a substring of a String in Swift:

var str = “Hello, playground”
func test(str: String) -> String {
 return str.substringWithRange( /* What goes here? */ )
}
test (str)

I'm not able to create a Range in Swift. Autocomplete in the Playground isn’t super helpful - this is what it suggests:

return str.substringWithRange(aRange: Range<String.Index>)

I haven't found anything in the Swift Standard Reference Library that helps. Here was another wild guess:

return str.substringWithRange(Range(0, 1))

And this:

let r:Range<String.Index> = Range<String.Index>(start: 0, end: 2)
return str.substringWithRange(r)

I've seen other answers (Finding index of character in Swift String) that seem to suggest that since String is a bridge type for NSString, the "old" methods should work, but it's not clear how - e.g., this doesn't work either (doesn't appear to be valid syntax):

let x = str.substringWithRange(NSMakeRange(0, 3))

Thoughts?

Answer 1

You can use the substringWithRange method. It takes a start and end String.Index.

var str = "Hello, playground"
str.substringWithRange(Range<String.Index>(start: str.startIndex, end: str.endIndex)) //"Hello, playground"

To change the start and end index, use advancedBy(n).

var str = "Hello, playground"
str.substringWithRange(Range<String.Index>(start: str.startIndex.advancedBy(2), end: str.endIndex.advancedBy(-1))) //"llo, playgroun"

You can also still use the NSString method with NSRange, but you have to make sure you are using an NSString like this:

let myNSString = str as NSString
myNSString.substringWithRange(NSRange(location: 0, length: 3))

Note: as JanX2 mentioned, this second method is not safe with unicode strings.