I'm not understanding well why should indicate that is a reference or dereference x in the code of the photo.
The function both works with the map with and without &/* . The original vector is not mutable.
The valid answer in the exercise is to use &/* but after modifying and using the debugger, the output and the changes (or lack of) to v are the same.
The filter really changes it's behaviour if I don't use them (unless I do first the map instead).