[R] dopar parallel assignments

[Vivek Ayer]

Hi David,

Thanks for the help. It's working! I still find it to be a new
concept. I haven't encountered storing loops in objects in any other
languages. Can it even be done in other languages? Very novel, quite
intriguing.

Thanks again,
Vivek

On Thu, Apr 29, 2010 at 3:44 PM, David M Smith
<david at revolution-computing.com> wrote:
> [This relates to the foreach function in library(foreach)]
>
> Vivek,
>
> In the %dopar% example, the assignments are being made into "child" R
> sessions in parallel. foreach is generally pretty good about detecting
> variables you reference in the parallel loops and making sure the
> objects are copied over to the parent, but in this case where you're
> using "assign" directly, it doesn't detect that test_1.25 and
> tets_1.50 are variables you're modifying in parallel. (In the %do%
> case everything is run in the same R session, so the assignments are
> naturally preserved.)
>
> Also, in general, think of foreach more of an analogue to "lapply"
> than to "for", in the sense that you're running it to collect the
> values of the body of the loop, not for their side effects.
>
> So what you probably really want is code like this:
>
> test <- foreach(i = c(1.25, 1.50)) %dopar% some_timeconsuming_operation(i)
>
> and then (if you really need the variables named as specified)
>
> v <- c(1,25,1.50)
> for (i in seq(along=v)) assign(paste("test_",v[i],sep=""),test[[i]])
>
> In your specific example,
>
>  some_timeconsuming_function <- function(i) i
>
> but that's not timeconsuming, and so you're not going to get any
> benefit from parallelization.
>
> # David Smith
>
>
> On Thu, Apr 29, 2010 at 2:07 PM, Vivek Ayer <vivek.ayer at gmail.com> wrote:
>> Hi guys,
>>
>> I was wondering why this piece of code doesn't work:
>>
>> foreach (i = c(1.25,1.50)) %dopar% {
>> assign(paste("test_",i,sep=""),i)
>> }
>>
>> but, this does:
>>
>> foreach (i = c(1.25,1.50)) %do% {
>> assign(paste("test_",i,sep=""),i)
>> }
>>
>> Obviously, the difference is %dopar% vs. %do%. If I use %do%, I get
>> objects test_1.25 and test_1.50, but I don't get these objects if I
>> use %dopar% even though it seemed to run through the loop in parallel.
>>
>> Thanks in advance,
>> Vivek
>>
>> ______________________________________________
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>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>
>
>
> --
> David M Smith <david at revolution-computing.com>
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