How to stop emacs from replacing underbar with <- in ess-mode

From http://www.r-bloggers.com/a-small-customization-of-ess/ and How to change smart assign key ("_" to "<-") binding in ESS

To assign ":" to "<-" and to stop the assignment of underscore (underbar) "_" to "<-" put the following in .emacs (yes, the repeated line is correct)

(setq ess-smart-S-assign-key ":")
(ess-toggle-S-assign nil)
(ess-toggle-S-assign nil)
(ess-toggle-underscore nil) ; leave underscore key alone!

A more recent version which seemed to work for me, and is a lot less verbose (you essentially keep normal underscores, but can set your own key for this smart behaviour!):

(global-set-key (kbd "C-;")  (lambda () (interactive) (insert " <- ")))
(ess-toggle-underscore nil)

Insert your shortkey choice instead of C-;.

Since the feature is useful. You can assign it to other key which is less used by you in R it will automatically unassign it from underscore. I personally assign it to ";" by adding following line in .emacs file.

(setq ess-smart-S-assign-key ";")

My version of emacs is 24.3 All-in-one installation file by Vincent Goulet.(Installed on windows 7)

hope this helps

Edit In emacs 25.2 above do not work instead add following in the .emacs file

(setq ess-smart-S-assign-key ";")
(ess-toggle-S-assign nil)
(ess-toggle-S-assign nil)

From ESS's manual (look under "Changes/New Features in 5.2.0"):

ESS[S]: Pressing underscore ("_") once inserts " <- " (as before); pressing underscore twice inserts a literal underscore. To stop this smart behaviour, add "(ess-toggle-underscore nil)" to your .emacs after ess-site has been loaded