Emacs: How to use a major mode for non-standard file extension

Question

I'd like to use R major mode for another file extension in emacs (for an unsupported language with syntax similar to R).

How do I force emacs to change the major mode for a buffer I'm editing? How do I change my .emacs to permanently associate a major mode with a particular file extension?

Answer 1

This should work:

(add-to-list 'auto-mode-alist '("\\.rr" . R-mode))