R help - Oct 2011 - Foreach (doMC)

Hello,

I am trying to run a small example with foreach, but I am having some
problems. Here is the code:

*library(doMC)
registerDoMC()

zappa = list()
frank = list()

foreach (i = 1:4) %dopar% {
 zappa[[i]] = kmeans (iris[-5],4)
 frank[[i]] = warnings()
}*

The code runs without error. However the zappa and frank will be empty
lists.
If I use regular *for *instead, the list will be filled up with kmeans and
warnings results. However, it will not be processed distributed.

What am I doing wrong?
What am I not understanding?

Thanks

Rui

	[[alternative HTML version deleted]]

Rui,

I suggest you read the following tutorial to give you an introduction to
foreach:

http://cran.r-project.org/web/packages/doMC/vignettes/gettingstartedMC.pdf

Regarding your question I could would suspect that you can not assign values to
some variable inside foreach and use them "outside" in the global
environment. It is analog to using 'loacal' variables inside ordinary
dfunctions.

Try running:

zappa <- foreach (i = 1:4) %dopar% {
  kmeans(iris[-5],4)$cluster
 }

You need to define the .combine function to combine the results from your
calculation. I would suggest you try to restrict yourself to saving single
vectors from each foreach loop for the sake of simplicity. I myself tried to
save different types of R objects from foreach iterations once and it was a hard
fight against the code....


HTH
Jannis


P.S. Is there any particular reason why there are so seldom answers to posts
regarding foreach and all these doMC/doSMP packages ? Do so few people use these
packages or does this have anything to do with the commercial origin of these
packages?


--- Rui Esteves <ruimaximo at gmail.com> schrieb am Mo, 17.10.2011:
> Von: Rui Esteves <ruimaximo at gmail.com>
> Betreff: [R] Foreach (doMC)
> An: r-help at r-project.org
> Datum: Montag, 17. Oktober, 2011 19:31 Uhr
> Hello,
> 
> I am trying to run a small example with foreach, but I am
> having some
> problems. Here is the code:
> 
> *library(doMC)
> registerDoMC()
> 
> zappa = list()
> frank = list()
> 
> foreach (i = 1:4) %dopar% {
>  zappa[[i]] = kmeans (iris[-5],4)
>  frank[[i]] = warnings()
> }*
> 
> The code runs without error. However the zappa and frank
> will be empty
> lists.
> If I use regular *for *instead, the list will be filled up
> with kmeans and
> warnings results. However, it will not be processed
> distributed.
> 
> What am I doing wrong?
> What am I not understanding?
> 
> Thanks
> 
> Rui
> 
> ??? [[alternative HTML version deleted]]
> 
> ______________________________________________
> R-help at r-project.org
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> PLEASE do read the posting guide
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> and provide commented, minimal, self-contained,
> reproducible code.
>

> P.S. Is there any particular reason why there are so seldom answers to
posts regarding foreach and all these doMC/doSMP packages ?  Do so few people
use these packages or does this have anything to do with the commercial origin
of these packages?
Jannis,

An interesting question.  I'm a huge fan of foreach and the parallel
backends, and have used foreach in some of my packages.  It leaves the
choice of backend to the user, rather than forcing some environment.
If you like multicore, great -- the package doesn't care.  Someone
else may use doSNOW.  No problem.

To answer your question, foreach was originally written by (primarily,
at least) Steve Weston, previously of REvolution Computing.  It, along
with some of the parallel backends (perhaps all at this point, I'm out
of touch) are available open-source.  Hence, I'd argue that the
"commercial origin" is a moot point -- it doesn't matter, it will
always be available, and it's really useful.  Steve is no longer with
REvolution, however, and I can't speak for the responsiveness/interest
of current REvolution folks on this point.  Scanning R-help daily for
things relating to my own packages is something I try to do, but it
doesn't always happen.

I would like to think foreach is widely used -- it does have a growing
list of reverse depends/suggests.  And was updated as recently as last
May, I just noticed.
http://cran.r-project.org/web/packages/foreach/index.html

Jay

-- 
John W. Emerson (Jay)
Associate Professor of Statistics
Department of Statistics
Yale University
http://www.stat.yale.edu/~jay