[R] "Safe" use of iterator (package iterators)

[Doran, Harold]

I believe I now see the light vis-à-vis iterators when combined with foreach() calls in R. I have now been able to reduce computational workload to minutes instead of hours. I want to verify that the way I am using them is "safe". By safe I mean does the iterator traverse elements in the same way as I have below in my toy example to illustrate what I mean.

In the first "traditional" example, I have only one index variable for the loop and so I know that the same list in r1 and r2 are always being grabbed. That is, in iteration 1 it is guaranteed to use r1[[1]] + r2[[1]].

In the example that uses the iterators, is this also guaranteed even though I now have two iterator objects? That is, will the index for element i always be the same as the index for element j when using this across many different cores?

It seems to be true and in all my test cases so far I am seeing it to be true. But, that could be just luck, so I wonder if there is a condition under which that would NOT be true.

Thank you
Harold


library(foreach)
library(doParallel)
cl <- makeCluster(2) 
registerDoParallel(cl)

### Create random data
r1 <- vector("list", 20)
for(i in 1:20){
	r1[[i]] <- rnorm(10)
}

### Create random data
r2 <- vector("list", 20)
for(i in 1:20){
	r2[[i]] <- rnorm(10)
}

### Use a for loop traditionally 
result1 <- vector("list", 20)
for(i in 1:20){
	result1[[i]] <- r1[[i]] + r2[[i]]
}
	
### Use iterators
itx1 <- iter(r1)
itx2 <- iter(r2)

result2 <- foreach(i = itx1, j = itx2) %dopar% {
	i + j
	}	

all.equal(result1, result2)