Running foreach without returning any value in R

Question

I have a function doSomething() which runs in a foreach loop and as a result saves some calculations as .csv files. Hence I have no need for a return value of foreach, in fact I don't want a return value because it clutters my memory to the point where I cannot run as many iterations as I would want to.

How can I force foreach to not have a return value, or delete the return values of the iterations?

Here is a minimal example that illustrates my problem:

cl <- parallel::makePSOCKcluster(1)
doParallel::registerDoParallel(cl)

"%dopar%" <- foreach::"%dopar%"

doSomething <- function () {
  a <- as.numeric(1L)
}

foreach::foreach (i = 1:4) %dopar% {

  doSomething()

}

The output is:

[[1]]
[1] 1

[[2]]
[1] 1

[[3]]
[1] 1

[[4]]
[1] 1

Answer 1

From ?foreach:

The foreach and %do%/%dopar% operators provide a looping construct that can be viewed as a hybrid of the standard for loop and lapply function. It looks similar to the for loop, and it evaluates an expression, rather than a function (as in lapply), but it's purpose is to return a value (a list, by default), rather than to cause side-effects.

The line

but it's purpose is to return a value (a list, by default)

Says that this is the intended behaviour of foreach. Not sure how you want to proceed from that...

Answer 2

As noted by dario; foreach returns a list. Therefore, what you want to do is to use for loop instead. You can use write.csv function inside the loop to write the results of each iteration inside the csv file.

For parallel computing, try using parSapply function from parallel package:

library(parallel)
cl <- parallel::makePSOCKcluster(1)
doParallel::registerDoParallel(cl)
parSapply(cl, 1:4, function(doSomething) a <- as.numeric(1L))

Edit;

Combining this with Freakozoid's suggestion (set the argument of the rm funciton to a);

library(parallel)
cl <- parallel::makePSOCKcluster(1)
doParallel::registerDoParallel(cl)
parSapply(cl, 1:4, function(doSomething) {a <- as.numeric(1L); write.csv(a, "output.csv"); rm()})

will give you the resulting output as csv file, as well as a list of NAs. Since the list consists of only NAs, it may not take lots of space.

Please let me know the result.

Answer 3

As other mentioned, if you are only interested in the side-effects of the function, returning NULL at the end will not save any input, saving on RAM.

If on top of that, you want to reduce the visual clutter (avoid having a list of 100 NULL), you could use the .final argument, setting it to something like .final = function(x) NULL.

library(foreach)
doSomething <- function ()  as.numeric(1L)

foreach::foreach(i = 1:4, .final = function(x) NULL) %do% {
  
  doSomething()
}
#> NULL

^{Created on 2022-05-24 by the reprex package (v2.0.1)}

Answer 4

You can use add method like

var nodes = edges.add([returnNodes()]);

or

var nodes = new vis.DataSet([,returnNodes()]);