R help - Feb 2014 - if(...){next} with "foreach" loop?

Dear all,

I want to do a loop with foreach. But, I would like to include a condition
with "next" to don't loop over some elements.

For example, I would like to translate this "for loop" to a
"foreach loop":

for (i in 1:10) {
  for (j in 1:2) {
  ...
  if((i==2 & j==1) | (i==4 & j==3)) { next }
  ...
  }
}

Foreach loop???

foreach (i = c(1:10)) %do%{
  foreach (j = c(1:2)) %do%{

#I don't know how to include a "next"!!
...

  }
}

Thank you!

Marc

	[[alternative HTML version deleted]]

Why?

Are you 100% sure that you need to use for loop?

What is %do%, I did not find it among standard R functions?

Regards
Petr
> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Marc Mar? Dell'Olmo
> Sent: Tuesday, February 11, 2014 11:31 AM
> To: r-help at r-project.org
> Subject: [R] if(...){next} with "foreach" loop?
>
> Dear all,
>
> I want to do a loop with foreach. But, I would like to include a
> condition with "next" to don't loop over some elements.
>
> For example, I would like to translate this "for loop" to a
"foreach
> loop":
>
> for (i in 1:10) {
>   for (j in 1:2) {
>   ...
>   if((i==2 & j==1) | (i==4 & j==3)) { next }
>   ...
>   }
> }
>
> Foreach loop???
>
> foreach (i = c(1:10)) %do%{
>   foreach (j = c(1:2)) %do%{
>
> #I don't know how to include a "next"!!
> ...
>
>   }
> }
>
> Thank you!
>
> Marc
>
>       [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-
> guide.html
> and provide commented, minimal, self-contained, reproducible code.
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On 14-02-11 5:30 AM, Marc Mar? Dell'Olmo wrote:
> Dear all,
>
> I want to do a loop with foreach. But, I would like to include a condition
> with "next" to don't loop over some elements.
>
> For example, I would like to translate this "for loop" to a
"foreach loop":
>
> for (i in 1:10) {
>    for (j in 1:2) {
>    ...
>    if((i==2 & j==1) | (i==4 & j==3)) { next }
>    ...
>    }
> }
>
> Foreach loop???
>
> foreach (i = c(1:10)) %do%{
>    foreach (j = c(1:2)) %do%{
>
> #I don't know how to include a "next"!!
> ...
>
>    }
> }
I don't know the foreach package; you might want to direct your 
questions to its maintainer.  But you can always emulate next using an 
if(), e.g. with a regular for loop,

for (i in 1:10) {
   if (cond) next
   ...
}

is the same as

for (i in 1:10) {
   if (!cond) {
     ...
   }
}

My vague understanding of the idea behind the foreach package is that 
the different runs through the loop can be run independently so they can 
be parallelized.  If that's the case, you might be able to evaluate cond 
for each i before the loop starts, and then do the equivalent of

for (i in (1:10)[!cond]) {
   ...
}

Duncan Murdoch
>
> Thank you!
>
> Marc
>
> 	[[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

Since you may eventually do this in parallel (the reason for doing
foreach), it seems inefficient to use the overhead of sending a
proccess to a core only to have it skip any computations.  I think
instead I would create the combinations with `expand.grid` and then
remove the rows corresponding to the conditions that I want  to split
and then loop over the rows of the resulting data frame.  Possibly
something like

combs <- expand.grid( i= 1:10, j=1:2 )
combs <- combs[ ! ( (combs$i==2 & combs$j==1) | (combs$i=4 &
combs$j==3)), ]
foreach( k= seq_len(nrow(combs)) %do% {

use combs$i[k] and combs$j[k] in the calculations

}





On Tue, Feb 11, 2014 at 3:30 AM, Marc Mar? Dell'Olmo
<marceivissa at gmail.com> wrote:
> Dear all,
>
> I want to do a loop with foreach. But, I would like to include a condition
> with "next" to don't loop over some elements.
>
> For example, I would like to translate this "for loop" to a
"foreach loop":
>
> for (i in 1:10) {
>   for (j in 1:2) {
>   ...
>   if((i==2 & j==1) | (i==4 & j==3)) { next }
>   ...
>   }
> }
>
> Foreach loop???
>
> foreach (i = c(1:10)) %do%{
>   foreach (j = c(1:2)) %do%{
>
> #I don't know how to include a "next"!!
> ...
>
>   }
> }
>
> Thank you!
>
> Marc
>
>         [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.


-- 
Gregory (Greg) L. Snow Ph.D.
538280 at gmail.com