[R] Suggestion on how to make permanent changes to a single object in a list?

Gabor Grothendieck ggrothendieck at gmail.com
Thu Jan 3 23:25:02 CET 2008

You can do it with environments.  The first line sets up fooStack with
a list of environments instead of a list of lists and the remaining lines
are the same as in your post squished to one line each to make it
easier to see the entire code at once:

fooStack <- lapply(1:5, new.env)
fooModifier <- function( foo ) foo$bar <- "bar"
fooModifier( fooStack[[ 1 ]] )
fooStack[[1]]$bar # "bar"

You may need to be a bit careful if you pursue this line of reasoning as there
is a long standing bug in R relating to lists of promises so take care that you
don't get promises in the list.  See point #2 in:

Also you might want to look at the proto package which reframes the
use of environments in terms of object oriented programming.

On Jan 3, 2008 4:35 PM, Peter Waltman <waltman at cs.nyu.edu> wrote:
>   specifically, imagine we have:
>     fooStack <- list()
>     for ( i in 1:5 )
>         fooStack[[i]] <- list()
>   and we have a function:
>     fooModifier <- function( foo ) {
>     foo$bar <- "bar"
>     }
>   then, if we invoke fooModifier, i.e.:
>     fooModifier( fooStack[[ 1 ]] )
>   the $bar elt is only set in the scope of the function, and if we use the
>   "<<-" modifier in fooModifier, R will throw an error b/c it can't find the
>   "foo" object.  I have to say that for someone coming from languages that
>   have pointers and/or references, it's really frustrating that R fails to
>   allow one to have direct access to the objects' memory space.
>   Onyway, one workaround would be to pass in the whole fooStack object and the
>   index of the elt that you want to modify to the fooModifier fn, but I'd
>   rather not have to pass the whole thing in.
>   Any suggestions?
>   Thanks!
>   Peter Waltman
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