Catch Error Handling Traceback

[davidf(at)dfanning.com (David Fanning)]

Folks,

One of the main things that makes it impossible for me
to give up IDL is that I learn something new about it
almost every week. Sometimes what I learn is subtle and
like a fine distinction. Sometimes it's more like a Mac
truck. But I'm always humbled by how much I don't know about
it and how much more there is to learn.

Occasionally, what I learn surprises me very much. That is
the case with what I report today, which could--for all
I know--be common knowledge for all of you, since it 
appears to be a feature that has been in IDL for some time.
(I confess that I have even read the appropriate documentation
for this feature several times without understanding the
significance of what I read. In fact, I'm quite sure I
could have read the documentation several thousand times
without understanding the significance of what I read,
but that probably has more to do with me than with the
documentation.)

In any case, what I have wanted for some time is the
ability to write a CATCH error handler that could
provide an error trace. In particular, I want to indicate
to the user exactly what the error was and which line
in the code caused the error. For the life of me, I could
not figure out how to get the line number. My error handlers
could provide a traceback, but the line number was always
pointing to the line in the CATCH error handling code and
not to the line that actually caused the error.

But thanks to Pavel Romashkin (who sensibly ignored my
learned advice in this matter and proceeded to find out how
things *really* worked) and the technical support
people at RSI I now know that IDL has had the ability to 
provide the information I wanted all the time. It is
accessed through the LAST_MESSAGE keyword to the HELP
command.

Here is a little test program that shows how this
is done:

   PRO TEST

   Catch, theError
   IF theError NE 0 THEN BEGIN
      Catch, /Cancel
      Help, /Last_Message, Output=theErrorMessage
      FOR j=0,N_Elements(theErrorMessage)-1 DO BEGIN
         Print, theErrorMessage[j]
      ENDFOR
      RETURN
   ENDIF

   Print, xxx
   END

Running the program results in this output:

   IDL> test
   % PRINT: Variable is undefined: XXX.
   % Execution halted at:  TEST  13 C:\RSI\IDL52\DAVID\test.pro
   %                       $MAIN$                 

This was the last piece of information I needed to make my
little device-independent ERROR_MESSAGE program I posted here
the other day complete. I've updated that program and made
it available on my web page for those who are interested. You
can find it at:

   http://www.dfanning.com/programs/error_message.pro

Happy Programming!

David

-- 
David Fanning, Ph.D.
Fanning Software Consulting
Phone: 970-221-0438 E-Mail: davidf@dfanning.com
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Toll-Free IDL Book Orders: 1-888-461-0155