[R] Help understanding environments

[William Dunlap]

Since you set
   combined <- NULL
at the top of the function
   exists("combined")
will always return TRUE.  If you initialize 'combined' to NULL,
use is.null(combined) to see if it is still in its virgin state.  If you
don't initialize it to anything then use exists("combined").
In this example you are combining the results with c(), which
essentially ignores NULL arguments so you don't need the
if-else at all if you initialize 'combined' to NULL. 

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com


> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf
> Of Rui Barradas
> Sent: Tuesday, June 18, 2013 7:57 AM
> To: Davis, Brian
> Cc: r-help at r-project.org
> Subject: Re: [R] Help understanding environments
> 
> Hello,
> 
> The first argument to exists() must be the name of an object given as a
> character string, as stated in the help page. See ?exists. You have also
> forgot to initialize combined to NULL, this time.
> Your function revised would then become
> 
> 
> c_objects <- function(FILES) {
>    combined <- NULL
>    for (FILE in FILES) {
>      load(FILE)
>      if (exists("combined")) {
>        combined <- c(combined, res)
>      } else {
>        combined <- res
>      }
>    }
>    return(combined)
> }
> 
> 
> And all works as expected.
> 
> Hope this helps,
> 
> Rui Barradas
> 
> 
> Em 18-06-2013 14:57, Davis, Brian escreveu:
> > In my haste to make a smaller example than my actual code I used 'is.null' instead of
> 'exists' as is in my code.  Here's a small reproducible example
> >
> >
> > res <- list(abc=letters, ABC=LETTERS)
> > save(res, file="results.RData")
> > res <- list(zyx=rev(letters), ZYX=rev(LETTERS))
> > save(res, file="results2.RData")
> >
> > rm(res)
> > FILES <- c("results.RData", "results2.RData")
> >
> >
> > c_objects <- function(FILES) {
> >    for (FILE in FILES) {
> >      load(FILE)
> >      if (exists(combined)) {
> >        combined <- c(combined, res)
> >      } else {
> >        combined <- res
> >      }
> >    }
> >    return(combined)
> > }
> >
> > combined_results <- c_objects(FILES)
> >
> >
> > -----Original Message-----
> > From: Duncan Murdoch [mailto:murdoch.duncan at gmail.com]
> > Sent: Monday, June 17, 2013 5:40 PM
> > To: Davis, Brian
> > Cc: r-help at r-project.org
> > Subject: Re: [R] Help understanding environments
> >
> > On 13-06-17 5:02 PM, Davis, Brian wrote:
> >> I have a collection of .RData files that have result objects that I would like to combine.
> Specifically, skatCohort objects from the skatMeta package, but I've run into a similar
> issue with simple data.frames also.
> >>
> >> If I run something like
> >>
> >> FILES <- list.files(path="/path/to/my/results", pattern=".RData$",
> >> full.names=TRUE) combined <- NULL
> >>     for (FILE in FILES) {
> >>       load(FILE)
> >>       if (!is.null(combined)) {
> >>         combined <- c(combined, res)
> >>       } else {
> >>         combined <- res
> >>       }
> >>     }
> >>
> >> I get all my objects combined.  However, if I wrap this into a
> >> function I get the following error
> >>
> >> c_objects <- function(FILES) {
> >>     combined <- NULL
> >>     for (FILE in FILES) {
> >>       load(FILE)
> >>       if (!is.null(combined)) {
> >>         combined <- c(combined, res)
> >>       } else {
> >>         combined <- res
> >>       }
> >>     }
> >>     return(combined)
> >> }
> >>
> >> combined_results <- c_objects(FILES)
> >> Error in eval(expr, envir, enclos) : object 'combined' not found
> >>
> >> How should I write this function such that it can find "combined".   I've tried reading
> the help on envirnaments, and the exisits function but I haven't been able to figure this
> out.  Are there any other resources to read up on this?
> >
> > You are doing something that you aren't showing us:  I don't see any calls to eval(), but
> it's eval() that generated the error.
> >
> > Calling traceback() after the error might be informative.
> >
> > Duncan Murdoch
> >
> 
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