[R] string/character to number

[Jake Michaelson]

I did a very quick search of the archive and couldn't find a readily 
available answer to this one:

I'd like to convert, for example:

c("a", "b", "a", "b")

to

c(1, -1, 1, -1)

In the case of the first vector, it may be any length, but will always 
only have two unique values.  It must always be replaced by 
corresponding values of 1 and -1.

Any thoughts?

Thanks in advance,

Jake