Re: ob-R, problem with try/catch

["Charles C. Berry" <ccberry@ucsd.edu>]

On Wed, 22 Apr 2015, Thomas S. Dye wrote:

> Aloha all,
>
> Prior to eaa3a761dae, when working in a session, I was able to run this
> R source code block without problems:
>
> ,-----------------------------------------
> | #+header: :file r/adze_wt_log.pdf
> | #+header: :results output graphics
> | #+header: :width 4 :height 3
> | #+begin_src R
> |   g <- ggplot(x, aes(x = weight))
> |   g + geom_histogram(aes(y=..density..))

## Try this:

print( g + geom_histogram(aes(y=..density..)) ) # before rm(g).


> |   rm(g)
> | #+end_src
> `-----------------------------------------
>
> After eaa3a761dae, I get an error and an empty output file.
>

That commit introduced a tryCatch() wrapper for graphics results.

You probably know that ggplot (or ggplot2) relies on printing of objects 
to produce graphics (see R-FAQ 7.22).

tryCatch(expr,...) evaluates expr and returns its value, which is `rm(g)' 
in your case. But `rm(g)' is not autoprinted, and you get an empty file.


> I can work around this error by removing the line "rm(g)",

Right. Then, the expression returned by tryCatch is

 	g + geom_histogram(aes(y=..density..))

which is autoprinted giving the graph.

When in doubt, there is no harm in explicitly print()ing objects that 
would have been autoprinted otherwise.

For reference, here is what org-babel-execute:R produces for your src 
block (lightly formatted for readability):

#+BEGIN_SRC R
   pdf(file=\"r/adze_wt_log.pdf\",width=4,height=3)
   tryCatch({
                g <- ggplot(x, aes(x = weight))
                g + geom_histogram(aes(y=..density..))
                rm(g)
            },
            error=function(e){
                plot(x=-1:1, y=-1:1, type='n', xlab='', ylab='',
 			axes=FALSE)
                text(x=0, y=0, labels=e$message, col='red')
                paste('ERROR', e$message, sep=' : ')})
   dev.off()
#+END_SRC

HTH,

Chuck