[R] extremely slow recursion in R?

[MARK LEEDS]

i'm sure someone else will explain the recursion issue but , as far as your 
program running a few days, you don't have to wait this long. if you are in 
windows and do
a ctrl  alt delete and then click on processes, if the memory usage being 
used by that R process  is staying EXACTLY the same and not moving at all, 
this is a sign
( atleast i have found this to be true for my cases. i guess it may not 
always hold ) that nothing is happening and your job has gone into never 
never land or has somehow become frozen.



----- Original Message ----- 
From: "Jason Liao" <jg_liao at yahoo.com>
To: <r-help at stat.math.ethz.ch>
Sent: Thursday, August 24, 2006 4:05 PM
Subject: [R] extremely slow recursion in R?


>I recently coded a recursion algorithm in R and ir ran a few days
> without returning any result. So I decided to try a simple case of
> computing binomial coefficient using recusrive relationship
>
> choose(n,k) = choose(n-1, k)+choose(n-1,k-1)
>
> I implemented in R and Fortran 90 the same algorithm (code follows).
> The R code finishes 31 minutes and the Fortran 90 program finishes in 6
> seconds. So the Fortran program is 310 times faster. I thus wondered if
> there is room for speeding up recursion in R. Thanks.
>
> Jason
>
> R code
>
> my.choose = function(n,k)
> {
>  if(k>n) value = 0.
>  else if(k==0) value = 1.
>  else if(k==n) value = 1.
>  else value = my.choose(n-1,k) + my.choose(n-1, k-1)
>
>  value
> }
>
> print(date())
> my.choose(30,15)
> print(date())
>
>
>
> Fortran code
>
>  recursive function choose(n, k) result(value)
>  implicit none
>  integer n, k
>  double precision value
>  if(k>n) then
>    value = 0.
>  elseif(k==0) then
>    value = 1.
>  else if(k==n) then
>    value = 1.
>  else
>    value = choose(n-1, k) + choose(n-1, k-1)
>  end if
>  end function
>
>  program main
>    write(*,*) choose(30, 15)
>  end program
>
> Jason Liao, http://www.geocities.com/jg_liao
> Department of Epidemiology and Biostatistics
> Drexel University School of Public Health
> 245 N. 15th Street, Mail Stop 660
> Philadelphia, PA 19102-1192
> phone 215-762-3934
>
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