Commenting multiple objects at once

Question

Suppose, you have a list of variables a through j

for(x in 1:10) {
assign(letters[x],x)
} 

How would you go about commenting those recently created objects? I've tried something like:

for(x in 1:10) { 
comment(get(letters[x])) <- paste(x)
} 

But that seems to fail. With:

Error in comment(get(letters[x])) <- paste(x) : 
  could not find function "get<-"

What am I missing here?

Answer 1

If you are ever gong to want to loop over things, then store them in a list. It makes life easier. But if you really want to do this, you might just need a loop over an eval(parse( thingum:

> for(i in 1:10){
+  eval(parse(text=paste("comment(",letters[i],")<-'",as.character(i*2),"'",sep="")))
+ }

Answer 2

You cannot assign a value to a returned variable, see:

> x <- 'cars'
> get(x) <- 1
Error in get(x) <- 1 : could not find function "get<-"

But reading/loading the comment of a returned variable is possible with get, see:

> comment(cars) <- "test"
> comment(get(x))
[1] "test"

---

You might concatenate your variables to e.g. a list and comment the elements of the list, like:

> l <- list(a=1,b=2,c=3)
> for (x in 1:3) {
+     comment(l[[letters[x]]]) <- paste(x)
+ }
> str(l)
List of 3
 $ a: atomic [1:1] 1
  ..- attr(*, "comment")= chr "1"
 $ b: atomic [1:1] 2
  ..- attr(*, "comment")= chr "2"
 $ c: atomic [1:1] 3
  ..- attr(*, "comment")= chr "3"

And if you insist on using different variables, just attach the given list, like:

> attach(l)
The following object(s) are masked _by_ '.GlobalEnv':

    a, b, c
> a
[1] 1
> str(a)
 atomic [1:1] 1
 - attr(*, "comment")= chr "1"