r - Assigning and removing objects in a loop: eval(parse(paste(

Question

I am looking to assign objects in a loop. I've read that some form of eval(parse( is what I need to perform this, but I'm running into errors listing invalid text or no such file or directory. Below is sample code of generally what I'm attempting to do:

x <- array(seq(1,18,by=1),dim=c(3,2,3))
for (i in 1:length(x[1,1,])) {
  eval(parse(paste(letters[i],"<-mean(x[,,",i,"])",sep="")
}

And when I'm finished using these objects, I would like to remove them (the actual objects are very large and cause memory problems later on...)

for (i in 1:length(x[1,1,])) eval(parse(paste("rm(",letters[i],")",sep="")))

Both eval(parse(paste( portions of this script return errors for invalid text or no such file or directory. Am I missing something in using eval(parse(? Is there a easier/better way to assign objects in a loop?

Answer 1

That's a pretty disgusting and frustrating way to go about it. Use assign to assign and rm's list argument to remove objects.

> for (i in 1:length(x[1,1,])) {
+   assign(letters[i],mean(x[,,i]))
+ }
> ls()
[1] "a" "b" "c" "i" "x"
> a
[1] 3.5
> b
[1] 9.5
> c
[1] 15.5
> for (i in 1:length(x[1,1,])) {
+   rm(list=letters[i])
+ }
> ls()
[1] "i" "x"
> 

Whenever you feel the need to use parse, remember fortune(106):

If the answer is parse() you should usually rethink the question.
-- Thomas Lumley, R-help (February 2005)