[R] Recursive algorithm

Barry Rowlingson b.rowlingson at lancaster.ac.uk
Mon Nov 1 13:23:19 CET 2010

On Mon, Nov 1, 2010 at 10:12 AM, Megh Dal <megh700004 at yahoo.com> wrote:
> Dear friend, I have to construct some recursive algorithm for which I used some for loop like:
> res <- vector(length=10000)
> res[1] = 0
> for (i in 2:(10000+1)) res[i] <- res[i-1]*........some function
> I have noticed that this is taking too much time. Is there any way to speed up things?

 You can't speed ANYTHING up until you know where the time is going.
In your case, is it in "some function" or in R's loop. Try replacing
'some function' with a constant and see what happens. I suspect it
will be very quick.

 > system.time(for (i in 2:(10000+1)) res[i] <- res[i-1]*1.12)
   user  system elapsed
  0.076   0.000   0.077

 yup. So it's all in 'some function'. Next question?


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