[R] Loops and memory

Spencer Graves spencer.graves at pdf.com
Tue May 6 22:44:41 CEST 2003

Hi, Duncan:

I agree totally with your priorities:  First get the right answer.  Then 
worry about speed if necessary.  Regarding priority number 1, I often 
program simple special cases in Excel first.  Then if I can get the same 
answer in S-Plus or R, it gives me confidence in the algorithm.

Spencer Graves

Duncan Murdoch wrote:
> On Tue, 06 May 2003 19:35:01 +0000, you wrote in message
> <Law11-F110twoPhPVKP0002a2b8 at hotmail.com>:
>>The other day I was surprised by how much longer a for loop takes to
>>add two vectors a and b compared to a + b.  (I think that I made a and
>>b have a million entries.)
>>I guess my problem is that I don't really what the issues are, I guess,
>>so it's not clear to me when and where loops should be avoided.  I
>>guess I should try to get a copy of this new book to find out.
> I think the main issue nowadays is that your code will go much slower
> if it is interpreted R code (as a for loop would be) than if it is
> compiled C or Fortran code (the way "a + b" is implemented
> internally).  In the old days, there was an additional penalty for
> doing a loop (S tried to allocate memory each step through the loop,
> but wouldn't clean up until the end), but that isn't normally a
> problem in R.  
> But as a general principle, I think you should always write your code
> to be readable first; if it turns out to be too slow that way, then
> worry about optimizing it.  As you gain experience in R you'll find
> the vectorized versions of formulas more readable than the loops and
> you'll need to redo things less often, but at first, loops may be the
> best way to get the right answer fast.
> Duncan Murdoch
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