How to return null for custom function used in %dopar%?

I want to return NULL with a custom function used in a parallel foreach loop. The usage is that, if some result does not fulfill certain criteria, that result won't get stored and it'll get filtered out as a postprocess step.

However, my program returns

task 1 failed - "replacement has length zero"

The problem can be reproduced with following code:

 library(doParallel)
 Input <- c(F,T,F)
 f.parallel <- function(x){
   ifelse(x,T,NULL)
 }
 no_cores <- detectCores()-1
   # for mac OS, linux
 c1 <- makeCluster(no_cores,type = "FORK")
   # for windows
   #   cl <- makeCluster(no_cores)
   #   clusterExport(cl, list("Input","f.parallel"))
 registerDoParallel(cl)
 output <- foreach(i=1:length(Input),.combine = cbind) %dopar% f.parallel(Input[i])

 stopCluster(c1)

Solution

Try f.parallel <- function(x) if(x) TRUE. ifelse() won't allow you to return NULL.

When cbind() is called to combine the results of your loop, the NULL elements will be removed. If you'd like to filter them out in a post-process step, I'd recommend using ifelse(x, TRUE, NA).