Re: passing structure elements... by value?

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

Randall Skelton writes:

> I am seem to be somewhat confused on passing structure... this *almost*
> seems like one of those dreaded 'pass by reference/pass by value'
> problems...
> 
> I have a simple routine that increments the value of a structure:
> 
> pro test, mod_struct
>   for i=0, n_tags(mod_struct)-1 do begin
>     mod_struct.(i) = mod_struct.(i) + 1
>   endfor
> end
> 
> As expected, this will increment all of the values in a passed structure.
> 
> IDL> struct = {A:0, B:0} & big = replicate(struct,3)
> IDL> print, big
> {       0       0}{       0       0}{       0       0}
> IDL> test, big 
> IDL> print, big
> {       1       1}{       1       1}{       1       1}
> 
> But, when I try and increment a single element in the structure it fails?
> 
> IDL> struct = {A:0, B:0} & big = replicate(struct,3)
> IDL> test, big[0]
> IDL> print, big
> {       0       0}{       0       0}{       0       0}
> ^^^^^^^^^^^^^^^^^^
> 
> I expected the ^^ element above to be ones?  Is there any way to force IDL
> to pass this by reference instead of passing by value? It would be nice if
> you could put brackets around the thing you want to pass by reference...
> something like '(big[0])' 

You could easily add the field you want to change as
a second positional parameter:

 pro test, mod_struct, fieldno
   IF N_Elements(filedno) EQ 0 THEN BEGIN
    for I=0, n_tags(mod_struct)-1 do begin
      mod_struct.(I) = mod_struct.(I) + 1
    endfor
   ENDIF ELSE mod_struct.(fieldno) = mod_struct.(fieldno) + 1
 end

Cheers,

David
-- 
David Fanning, Ph.D.
Fanning Software Consulting
Phone: 970-221-0438 E-Mail: davidf@dfanning.com
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