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

Parallel computing in R works (as far as I experienced) such that for each cluster node the memory will be allocated.

That means if you have a big data set which each node needs for calculation, this data will be allocated multiple times. This yields to high RAM consumption. Since you want to write the output in each loop and throw away the result afterwards you can try the rm function and call the garbage collection (for example with gc) in each function call.

This worked for E L M as mention above. Thx for testing!

Answer 2

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...