How does String substring work in Swift

I've been updating some of my old code and answers with Swift 3 but when I got to Swift Strings and Indexing with substrings things got confusing. Specifically I was trying the following: let str = "Hello, playground" let prefixRange = str.startIndex..<str.startIndex.advancedBy(5) let prefix = str.substringWithRange(prefixRange) where the second line was giving me the following error Value of type

[Nyoho]

Swift 4 & 5: extension String subscript のところ参考