Is there any way to break out of a foreach loop?

Question

I am using the R package foreach() with %dopar% to do long (~days) calculations in parallel. I would like the ability to stop the entire set of cal

Answer 1

Instead of trying to break out of a loop, I write a small file to the disk when I've reached my terminal loop, then have all remaining iterations simply skip based on the existence of that file.

Checking if a file exists costs us less than a milisecond of computing time.

# 1.4 seconds to check if a file exists a million times
system.time(lapply(1:1e6, function(x) file.exists("checker.txt")))
   user  system elapsed 
  1.204   0.233   1.437 

This is great when you don't have a fixed number of iterations or your process can finish before all of the iterations are complete (like a convergence, for example)

library(foreach)

alist <- foreach(i = 1:5000) %dopar% { 
  if(file.exists("checker.txt")) {
    return(NULL)
  } else {
    if(i = 20) {
      write("", "checker.txt") # write an empty file
    }
    return(i)
  }
}

file.remove("checker.txt")

The great thing about this is that even if your list is extremely long, if you just unlist() you only get the values.

> length(alist)
[1] 5000

> unlist(res)
 [1]  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20

Don't bother trying to break, instead, just "skip the rest"!