SWIFT: Why do we have to put exclamation point for: str.characters.index(of: " ")!

Question

Why do we have to put exclamation point when I create, in this case, a range?

Example:

var str = "joe satriani"
var range = str.startIndex..<str.characters.index(of: " ")!

If I don't put it I get an error.

Answer 1

There might not be an index of " " if there is no space in the string, therefore returning nil.

var str = "stringwithnospaces"
var range = str.startIndex..<str.characters.index(of: " ") // nil