Cannot increment beyond endIndex - swift

Question

I'm looking for the "Swift 3" way of handling an error where I try to increment the position of a string to an out of bounds index. I have an extension that looks like the following:
extension String {
func substring(from: Int) -> String {
let fromIndex = index(from: from)
return substring(from: fromIndex)
}
}
In implementation code, I have a loop which periodically takes chunks of a string and moves the index further in the string. My problem is I'm not sure what the Swift 3 way is of handling "End of String, do not proceed if we've reached the end"
Implementation code is something as trivial as this:
myStr = myStr.substring(from: pos + 1)
if pos + 1 is the end of the string, it shouldn't error out, but should instead just exit/return from my loop. What's the best way of doing that?

Answer 1

You can write something like this
extension String {
func substring(from offset: Int) -> String {
let fromIndex = index(self.startIndex, offsetBy: offset)
return substring(from: fromIndex)
}
}
Examples
"Hello world".substring(from: 0) // "Hello world"
"Hello world".substring(from: 1) // "ello world"
"Hello world".substring(from: 2) // "llo world"
What does happen if you pass the wrong param?
Something like this will generate a fatal error.
"Hello world".substring(from: 12)
fatal error: cannot increment beyond endIndex
You can make you code safer adding a guard statement like this
extension String {
func substring(from: Int) -> String? {
guard from < self.characters.count else { return nil }
let fromIndex = index(self.startIndex, offsetBy: from)
return substring(from: fromIndex)
}
}

Answer 2

You can use the index(_, offsetBy:, limitedBy:) method
to ensure that the index is not advanced beyond the end index:
extension String {
func substring(from: Int) -> String? {
guard let fromIndex = index(startIndex, offsetBy: from, limitedBy: endIndex) else {
return nil
}
return substring(from: fromIndex)
}
}

Answer 3

extension String {
func substring(from index: Int) -> String {
guard index < characters.count else { return "" }
return substring(from: characters.index(startIndex, offsetBy: index))
}
}
"12345".substring(from: 3) // "45"
"12345".substring(from: 9) // ""
Alternatively, you might want to return nil if index is out of bounds when you change the function's return type to String?