I know we can use subscript to cut a part of the string in Swift 4, .
let s = "aString" let subS = s[..<s.endIndex] But the problem is, how to cut the s to a subString like aStr.
I mean, What I want to do is something like s[..<(s.endIndex-3)].
But it's not right.
So, how to do it in Swift 4.
String.Index is not an integer, and you cannot simply subtract s.endIndex - 3, as "Collections move their index", see A New Model for Collections and Indices on Swift evolution.
Care must be taken not to move the index not beyond the valid bounds. Example:
let s = "aString" if let upperBound = s.index(s.endIndex, offsetBy: -3, limitedBy: s.startIndex) { let subS = String(s[..<upperBound]) } else { print("too short") } Alternatively,
let upperBound = s.index(s.endIndex, offsetBy: -3, limitedBy: s.startIndex) ?? s.startIndex let subS = String(s[..<upperBound]) which would print an empty string if s has less then 3 characters.
If you want the initial portion of a string then you can simply do
let subS = String(s.dropLast(3)) or as a mutating method:
var s = "aString" s.removeLast(min(s.count, 3)) print(s) // "aStr" As you said error is Binary operator '-' cannot be applied to operands of type 'String.Index' and 'Int'
You have to find the Swift.Index first.
let s = "aString" let index = s.index(s.endIndex, offsetBy: -3) let subS = s[..<index] print(String(subS)) 
All I wanted was a simple Substring, from the start, of x characters. Here's a function.
func SimpleSubstring(string : String, length : Int) -> String { var returnString = string if (string.count > length) { returnString = String(string[...string.index(string.startIndex, offsetBy: length - 1)]) } return returnString } usage
let myTruncatedString : String = SimpleSubstring(string: myBigString, length: 10) 
s[..<(s.endIndex-3)], the error isBinary operator '-' cannot be applied to operands of type 'String.Index' and 'Int'