Printing from mclapply in R Studio

I am using mclapply from within RStudio and would like to have an output to the console from each process but this seems to be suppressed somehow (as mentioned for example here: Is mclapply guaranteed to return its results in order?). How could I get R Studio to print something like

x <- mclapply(1:20, function(i) cat(i, "\n"))

to the console?

I've tried print(), cat(), write() but they all seem not to work. I also tried to set mc.silent = FALSE explicitly without an effect.

Parallel processing with GUI's is problematic. I write a lot of parallel code and it's constantly crashing my colleague's computer because he insists on using Rstudio instead of console R.

From what I read, RStudio "does not propagate the output of forked processes to the RStudio console. If you are doing this, it is best to start R via a shell."

This makes sense as a workaround for the RStudio people because parallel processing typically breaks GUI's when people try to output to the GUI from a bunch of different processes. It works in the console (albeit often not in order) but parallel processing gurus will pinch their noses when they hear about any I/O from a forked thread.

If you must have output from forked threads, save them in a string and return it. Then collect and output from the main process. Or just use a console for your parallel runs. What I tell my colleague is to do all his debugging and development in RStudio using lapply(), then switch to a console for the real run.

Just expanding a little on the solution used by the asker, i.e. writing to a file to check progress:

write.file = '/temp_output/R_progress'

time1 = proc.time()[3]
outstuff = unlist(mclapply(1:1000000, function(i){
  if (i %% 1000 == 0 ){
    file.create(write.file)
    fileConn<-file(write.file)
    writeLines(paste0(i,'/',nrow(loc),' ',(i/nrow(loc)*100)), fileConn)
    close(fileConn)
  }
  #do your stuff here
}, mc.cores=6))
print(proc.time()[3] - time1)

And then you can monitor from a console with

tail -c +0 -f '/temp_output/R_progress'

来源：https://stackoverflow.com/questions/17345837/printing-from-mclapply-in-r-studio

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rstudio

mclapply