[R-sig-hpc] foreach: using either %do% or %dopar% depending on condition

[Brian G. Peterson]

Matthieu,

I think I understand what you're saying, but you still don't need it, in my 
opinion.

registerDoSEQ() does what you're describing, it instructs foreach to run 
without a parallel backend.  If no parallel backend is available, this is the 
default.

As for standardizations, just put foreach in 'Depends:' in your package 
DESCRIPTION file, as you would for any other package.  'foreach' will be 
required to install your package, and it will be loaded automatically when your 
package loads, as with any other R package.

I'll note again, that when foreach is loaded, if the user takes no explicit 
action to register a parallel backend, foreach will run sequentially, 
automatically, without any of the complications you describe, or the workaround 
suggested by Steve.

Regards,

    - Brian

mat wrote:
> Thanks a lot Brian and Steve for your prompt answer!!
> 
> You noth pointed out what I'm doing is a little bit complicated... let 
> me precise why I do this, and maybe you will agree?
> 
> The function for end-user is not on parallel staff, it is a test for a 
> time series model, whose computations can be alleviated thanks to a 
> parallel computing. Say the function is (using the names as in package 
> strucchange, which is to my knowledge the only one to make an explicit 
> call to foreach):
> 
> mytest<-function(x, y, optionA=c("A", "B"), hpc=c("none", "foreach")
> 
> So the rationale is that one assumes that by default the user does not 
> use the parallel option, hence the %do% call (using by default %dopar% 
> would be adding an unexpected warning, no?). But if the user wants, then 
> we will call %dopar%
> 
> Do you think it then makes sense? Are there any recomandations / 
> standardization efforts on how one should provide link to foreach or 
> snow from other packages?
> 
> Thanks a lot!!!
> 
> Matthieu
> 
> 
> 
> 
> 
> Stephen Weston a écrit :
>> You could try something like this:
>>
>>     > '%my%' <- if (hpc == "none") get('%do%') else get('%dopar%')
>>
>> However, why not just let the user decide by either registering a 
>> parallel
>> backend or not?  The main purpose of the "registerDoSEQ" function is
>> to allow the user to say that parallel operations should be done 
>> sequentially.
>> The '%dopar%' function is the programmer's way to declare that the
>> foreach loop can be executed in parallel.  It's the user's job to declare
>> how the '%dopar%' should be executed.  That also simplifies the code
>> by not having to define and pass around yet another option to your
>> functions.
>>
>> - Steve
>>
>>
>>
>>
>> On Mon, May 3, 2010 at 6:45 AM, mat <matthieu.stigler at gmail.com> wrote:
>>  
>>> Hi
>>>
>>> I wish to be able to substitute %do% or %dopar% depending on whether the
>>> user wants or not to use the parallel feature. For this, I would do
>>> something like:
>>>
>>> if(hpc) foreach(icount(5), .combine = "rbind")  %dopar%
>>> else foreach(icount(5), .combine = "rbind")  %do%
>>>   statement of function...
>>>
>>> I did not succeed in doing it, I tried:
>>>
>>> foreach(icount(5), .combine = "rbind")  %do% else  %dopar% {
>>> a<-runif(1)
>>> b<-runif(1)
>>> c(a,b)
>>> }
>>>
>>> Or tried to assign outside the operator:
>>>
>>> "%my%" <-function(obj,ex) if(hpc=="none") "%dopar%" else "%dopar%"
>>>
>>> foreach(icount(5), .combine = "rbind")  %my% {
>>> a<-runif(1)
>>> b<-runif(1)
>>> c(a,b)
>>> }
>>>
>>> But it fails... any idea?
>>>
>>> Thanks a lot!!
>>>
>>> Matthieu
>>>
>>>     


-- 
Brian G. Peterson
http://braverock.com/brian/
Ph: 773-459-4973
IM: bgpbraverock