[R] getting informative error messages

Spencer Graves spencer.graves at pdf.com
Tue May 8 16:40:25 CEST 2007

Hi, Tony: 

      Are you familiar with the 'debug' command?  I agree that more 
informative error messages and 'traceback' would be nice, but I've found 
the 'debug' facility quite useful.  [I even sometimes prepare a shell of 
a function 'fn', then say debug(fn) and fn(), and complete writing the 
function in its native environment where I can more easily check what 
each step does.]  I've heard that 'debug' does not work will with S4 
class generics, but I have not so far had to deal with that.  {There is 
also a 'debug' package, which is completely separate from the debug 
command in the 'base' package.  I've heard that it has more extensive 
capabilities, but I've never used it.}

      I suspect you may already know 'debug', but for those who don't, I 
think it's worth noting its utility for this kind of thing. 

      Hope this helps. 
      Spencer Graves

Tony Plate wrote:
> Certain errors seem to generate messages that are less informative than 
> most -- they just tell you which function an error happened in, but 
> don't indicate which line or expression the error occurred in.
> Here's a toy example:
>  > f <- function(x) {a <- 1; y <- x[list(1:3)]; b <- 2; return(y)}
>  > options(error=NULL)
>  > f(1:3)
> Error in f(1:3) : invalid subscript type
>  > traceback()
> 1: f(1:3)
>  >
> In this function, it's clear that the error is in subscripting 'x', but 
> it's not always so immediately obvious in lengthier functions.
> Is there anything I can do to get a more informative error message in 
> this type of situation?  I couldn't find any help in the section 
> "Debugging R Code" in "R-exts" (or anything at all relevant in "R-intro").
> (Different values for options(error=...) and different formatting of the 
> function made no difference.)
> -- Tony Plate
>  > sessionInfo()
> R version 2.5.0 (2007-04-23)
> i386-pc-mingw32
> locale:
> LC_COLLATE=English_United States.1252;LC_CTYPE=English_United 
> States.1252;LC_MONETARY=English_United 
> States.1252;LC_NUMERIC=C;LC_TIME=English_United States.1252
> attached base packages:
> [1] "stats"     "graphics"  "grDevices" "utils"     "datasets"  "methods"
> [7] "base"
> other attached packages:
> tap.misc
>     "1.0"
>  >
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