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Re: update variable in structure [message #66951 is a reply to message #66950]

Tue, 16 June 2009 09:14

David Fanning
Messages: 11724
Registered: August 2001

Senior Member

M. Suklitsch writes:

> So far, so good. Now we do exactly the same, but this time the
> variable is embedded in a structure:
> IDL> my_struct = {my_value:8}
> IDL> update_value, my_value
> IDL> help, my_struct, /STRUC
> ** Structure <8220044>, 1 tags, length=2, data length=2, refs=1:
> MY_VALUE INT 8
>
> And now the rather simple question: how come this doesn't work?

I think you must have meant, why doesn't *this*
work:

IDL> udate_value, my_struck.my_value

And the reason is that structure dereferences, like
expressions, array subscripts, etc., etc. (in fact,
anything *except* a variable) are passed by *value*
and not by *reference*. Things that are passed by
value make a copy of themselves and pass that, rather
than passing the thing itself.

Cheers,

David

--
David Fanning, Ph.D.
Coyote's Guide to IDL Programming (www.dfanning.com)
Sepore ma de ni thui. ("Perhaps thou speakest truth.")

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[Message index]

		Re: update variable in structure By: R.Bauer on Wed, 17 June 2009 10:01
		Re: update variable in structure By: R.G. Stockwell on Tue, 16 June 2009 16:55
		Re: update variable in structure By: Jean H. on Tue, 16 June 2009 10:13
		Re: update variable in structure By: M. Suklitsch on Tue, 16 June 2009 09:38
		Re: update variable in structure By: Michael Galloy on Tue, 16 June 2009 09:19
		Re: update variable in structure By: David Fanning on Tue, 16 June 2009 09:14

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