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
