IMHO this has nothing to do with capture.output() per se - it's simply lazy
evaluation that gets you. Add force(envir) before capture.output and it works
as you expected - the parent.frame() will be different inside capture.output
than outside.
Cheers,
Simon
On Nov 23, 2011, at 9:36 PM, Henrik Bengtsson wrote:
> I've noticed the following oddity where capture.output() prevents
> eval() from evaluating an expression in the specified environment.
> I'm not sure if it is an undocumented feature or a bug. It caused me
> many hours of troubleshooting. By posting it here, it might save
> someone else from doing the same exercise.
>
> Start by defining foo() which evaluates an expression locally in a
> given environment and catches the output via capture.output():
>
> foo <- function(..., envir=parent.frame()) {
> capture.output({
> eval(substitute({x <- 1}), envir=envir)
> })
> } # foo()
>
> Then call:
>
>> suppressWarnings(rm(x)); foo(envir=globalenv()); print(x);
> character(0)
> [1] 1
>
> This works as expected. However, if argument 'envir' is not specified
> explicitly, you get:
>
>> suppressWarnings(rm(x)); foo(); str(x);
> character(0)
> Error in str(x) : object 'x' not found
>
> which shows that the internal expression of foo() is *not* evaluated
> in the parent.frame(), i.e. the caller of foo(), which here should be
> globalenv(). It appears that capture.output() prevents this, because
> by dropping the latter:
>
> foo <- function(..., envir=parent.frame()) {
> eval(substitute({x <- 1}), envir=envir)
> } # foo()
>
> it works:
>
>> suppressWarnings(rm(x)); foo(); str(x);
> [1] 1
>
> The workaround when still using capture.output() is to force an
> explicit evaluation of argument 'envir' inside of foo() before:
>
> foo <- function(..., envir=parent.frame()) {
> stopifnot(is.environment(envir)) # Workaround
> capture.output({
> eval(substitute({x <- 1}), envir=envir)
> })
> } # foo()
>
> which gives:
>> suppressWarnings(rm(x)); foo(); str(x);
> character(0)
> num 1
>
> This occurs with R v2.14.0 patched and R devel:
>
>> sessionInfo()
> R version 2.14.0 Patched (2011-11-20 r57720)
> Platform: x86_64-pc-mingw32/x64 (64-bit)
>
> locale:
> [1] LC_COLLATE=English_United States.1252
> [2] LC_CTYPE=English_United States.1252
> [3] LC_MONETARY=English_United States.1252
> [4] LC_NUMERIC=C
> [5] LC_TIME=English_United States.1252
>
> attached base packages:
> [1] stats graphics grDevices utils datasets methods base
>
>> sessionInfo()
> R Under development (unstable) (2011-11-20 r57720)
> Platform: x86_64-pc-mingw32/x64 (64-bit)
>
> locale:
> [1] LC_COLLATE=English_United States.1252
> [2] LC_CTYPE=English_United States.1252
> [3] LC_MONETARY=English_United States.1252
> [4] LC_NUMERIC=C
> [5] LC_TIME=English_United States.1252
>
> attached base packages:
> [1] stats graphics grDevices utils datasets methods base
>
> /Henrik
>
> ______________________________________________
> R-devel@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
>
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