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Re: translating an array name to a string

Hi JD--

I agree that the use of ROUTINE_NAMES is has some potential problems
because it is undocumented.  I agree with you also that you need to
protect your usage of ROUTINE_NAMES with a CATCH handler, since there
are a lot of ways for things to go wrong.

But I don't agree with your implementation :-).

First, the awkward use of CALL_FUNCTION can be avoided by using the
FORWARD_FUNCTION declaration.  This is always safe, even if the
function being declared is a built-in one.

Second, your check to see if a variable is undefined is rather
convoluted.  It involves two passes to get it right.  I prefer instead
to use the N_ELEMENTS command to immediately determine whether a
variable is undefined.  Unlike *assigning* an undefined variable,
which does produce an error, simply taking the N_ELEMENTS of an
undefined variable will not cause an error.

Finally, users need to be aware that the capability to use
ROUTINE_NAMES to create new variables at another calling level has
only come with IDL version 5.3.  This is documented at
http://cow.physics.wisc.edu/~craigm/idl/idl.html under Introspection,
by the way.

So here is my revised version of your code :-) It's shorter and the
flow control is primarily linear.

; *******
forward_function routine_names

catch, err
if err NE 0 then begin
  catch, /cancel
  message, 'Assign operation failed'

; Protect against an already-defined variable
if n_elements(routine_names(var_name,fetch=1)) GT 0 then begin
    message,'A variable named '+var_name+' already exists.'

; Still here... we need to export ourself to the main level
catch, /cancel
; *******


"J.D. Smith" <jdsmith@astro.cornell.edu> writes:
> By the way, for those of you using routine_names for heavy magic... you
> might consider examining the following extra-cautions snippet to export
> a variable to the $MAIN$ level:
>    var_free=0
>    catch, err
>    if err ne 0 then begin       
>       ;; An undefvar indicates routine_info ran and 
>       ;; the variable is free
>       if !ERROR_STATE.NAME ne 'IDL_M_UNDEFVAR' then begin 
>         catch,/cancel 
> 	message,"Can't complete operation... Try obj=sp_sel()"   
>       endif 
>       var_free=1
>    endif 
>    ;; If we need to check if the variable name is available, do so.
>    if var_free eq 0 then $
>      rn=call_function('routine_names',var_name,FETCH=1)
>    if n_elements(rn) ne 0 then begin
> 	catch,/cancel
> 	message,'A variable named '+var_name+' already exists.'
>    endif
>    ;; Still here... we need to export ourself to the main level
>    rn=call_function('routine_names',var_name,myvar,store=1)
> basically the idea is to wrap routine_names as a string in
> call_function, to allow your routine to compile even if RSI yanks or
> renames it (it wouldn't compile if you tried to call it directly). 
> You'll get an error, of course, which will be caught.  You have to
> discriminate between errors caused by the successful operation of
> routine_names(), and those caused by incorrect arguments, changed
> keywords (IDL_M_KEYWORD_BAD), or other mutations routine_names() has
> undergone (such as vanishing altogether -- IDL_M_UPRO_UNDEF).  You
> obviously have to have a backup plan too, to tell your users what to do
> in case routine_names() has broken.  But it's better than your program
> not running at all though.
> JD
> -- 
>  J.D. Smith                  |   WORK: (607) 255-6263
>  Cornell Dept. of Astronomy  |         (607) 255-5842
>  304 Space Sciences Bldg.    |    FAX: (607) 255-5875  
>  Ithaca, NY 14853            |

Craig B. Markwardt, Ph.D.         EMAIL:    craigmnet@cow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response