Why does R store the loop variable/index/dummy in memory?

Question

I\'ve noticed that R keeps the index from for loops stored in the global environment, e.g.:

for (ii in 1:5){ }

print(ii)
# [1] 5
<         


        

Answer 1

I agree with the comments above. Even if you have to use for loop (using just side effects, not functions' return values) it would be a good idea to structure your code in several functions and store your data in lists.

However, there is a way to "hide" index and all temporary variables inside the loop - by calling the for function in a separate environment:

do.call(`for`, alist(i, 1:3, {
  # ...
  print(i)
  # ... 
}), envir = new.env())

But ... if you could put your code in a function, the solution is more elegant:

for_each <- function(x, FUN) {
  for(i in x) {
    FUN(i)
  }
}

for_each(1:3, print)

Note that with using "for_each"-like construct you don't even see the index variable.