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

[Peter Waltman]

Hi Gabor -

Thanks for the 2 suggestions (and to Charilaos Skiadas as well, who also 
suggested looking at proto).

I think I'm leaning towards using the new environment idea you 
suggested, however, I don't quite get what a promise is (beyond what the 
help page says and I didn't really follow it).

Is there a single document describing the relationship between 
environments, assignments, bindings, etc?  Unfortunately, I haven't been 
able to find anything which gives a good overview of what these are and 
how they interact. 

Clearly, it's beyond the introduction to R document that you can find on 
the website, and any books I've looked at focus on how to use R to 
generate statistical results, rather than on the actual language 
aspects.  Finally, while the help pages may explain the individual 
functions, I find these difficult to use without understanding the 
general framework in which they work.

Thanks,

Peter

Gabor Grothendieck wrote:
> 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:
> https://stat.ethz.ch/pipermail/r-devel/2008-January/047914.html
>
> Also you might want to look at the proto package which reframes the
> use of environments in terms of object oriented programming.
> http://r-proto.googlecode.com
>
>
> 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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>>
>>