On Nov 23, 2011, at 11:06 PM, Henrik Bengtsson wrote:
> Thanks for the quick answer. I didn't know about force() function.
>
It doesn't matter how you force the argument, anything - e.g.
if(is.environment(envir)) capture.output(...) would do - I used force() just to
make the point that it is what is causing it.
A more simple example illustrating what happens here:
> f = function(e=parent.frame()) local(print(e))
> f()
<environment: 0x102f1f470>
> f = function(e=parent.frame()) { force(e); local(print(e)) }
> f()
<environment: R_GlobalEnv>
> f = function(e=parent.frame()) if (is.environment(e)) local(print(e))
> f()
<environment: R_GlobalEnv>
Cheers,
Simon
> Cheers,
>
> Henrik
>
> On Wed, Nov 23, 2011 at 6:56 PM, Simon Urbanek
> <simon.urba...@r-project.org> wrote:
>> 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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