R help - Nov 2003 - += assignment operator

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)

David Brahm <brahm at alum.mit.edu> writes:
> 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.
I don't think either of those are realistically possible at the R
level. Even at the C level, it would probably be a hack, but so is the
way we handle complex assignments (like foo$i[j][[k]] <- fum) already.

However, paste() always looks wrong to me:

"%+=%" <- function(a, b) eval.parent(substitute(a <- a + b))


-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907