Re: passing structure elements... by value?

[Jaco van Gorkom <j.c.van.gorkom(at)fz-juelich.de>]

Randall Skelton wrote:
> 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])'

Note that in your example you are passing one *array* element of an array of
structures. Indexed array elements are also passed by value - see Liam's list.
One workaround would be to copy the array element(s) into a temporary variable:
working_elements = big[range]
test, working_elements
big[range] = working_elements
, with range being a vector of indices, just [0] in the example.
Another workaround involves passing the indices-of-interest into the routine.

cheers,
  Jaco

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Jaco van Gorkom                               gorkom@rijnh.nl
FOM-Instituut voor Plasmafysica `Rijnhuizen', The Netherlands