[R] += assignment operator

[David Brahm]

The thread "Finding the name ob an object" gave me an idea about how to write
an assignment operator like C's "+=":

"%+=%" <- function(a, b) {
  as <- deparse(substitute(a))
  bs <- deparse(substitute(b))
  st <- paste(as, "<-", as, "+", bs)
  eval.parent(parse(text=st), 2)
}

R> xx <- matrix(1:9, 3,3)
R> xx[2, which(xx[2, ] < 6)] %+=% 100
R> xx
        [,1] [,2] [,3]
   [1,]    1    4    7
   [2,]  102  105    8
   [3,]    3    6    9

Anyone have any better ideas on building this kind of operator?  It would be
better if the condition [which(...)] were only evaluated once.  Bonus points if
you can make %*=%, %-=%, etc. all in one fell swoop.
-- 
                              -- David Brahm (brahm at alum.mit.edu)