[R] foreach(): how to do calculations "between" two foreach loops?

[Steve Lianoglou]

Hi,

On Wed, May 11, 2011 at 5:44 PM, Marius Hofert <m_hofert at web.de> wrote:
> Dear expeRts,
>
> is it possible to carry out calculations between different foreach() calls?
> As for nested loops, you want to carry out calcuations not depending on the inner
> loop only once and not for each iteration of the innermost loop.
>
> Cheers,
>
> Marius
>
>
> library(foreach)
>
> foreach(i=1:3) %:%
>    foreach(j=1:2) %do% {
>            i <- i+1
>        print(paste(i,j))
>    }
>
> foreach(i=1:3) %:%
>    i <- i+1 # lengthy calculation which has to be done only once, not for each j
>    foreach(j=1:2) %do% {
>        print(paste(i,j))
>    }

If I understand your question well, one solution might be to break it
into two parallized parts, eg:

R> lengthy.calculation.i <- foreach(i=1:3) %dopar% someIntenseFunction(i)
R> foreach(i=1:3) %:% foreach(j=1:3) %dopar% {
  anotherIntenseFunction(length.calculation.i[i], j)
}

I believe the left and right hand side of the %:% "operator" need a
"foreach" object, so your original incantation wouldn't work.

-steve
-- 
Steve Lianoglou
Graduate Student: Computational Systems Biology
 | Memorial Sloan-Kettering Cancer Center
 | Weill Medical College of Cornell University
Contact Info: http://cbio.mskcc.org/~lianos/contact