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Re: pointer to structures [message #19562 is a reply to message #19558]

Tue, 04 April 2000 00:00

John-David T. Smith is currently offline

John-David T. Smith
Messages: 384
Registered: January 2000

Senior Member

David Fanning wrote:
>
> eeeyler@my-deja.com (eeeyler@my-deja.com) writes:
>
>> suppose I wish to create a structure and wish to reference that
>> structure and its contents via a pointer:
>> filter=ptr_new({points:['a','b'],pt1_value:200, pt2_value:'X_WHYLOG'})
>> how do I reference the points array? I thought it would be as:
>> print, *filter.points
>> but I get the message
>> %Expression must be a structure in this context: Filter
>
> The problem here is that a pointer dereference has the lowest
> order of precedence. Lower, even, than a structure dereference.
> So, you must first dereference the structure (by using
> parentheses), and *then* dereference the pointer, like this:
>
> filter=ptr_new({points:['a','b'],pt1_value:200, pt2_value:'X_WHYLOG'})
> Print, *(filter.points)
>

Uh oh... forgot to test your code I'm afraid. You have the precendence correct,
but the solution reversed. You must force the pointer dereference to occur
*before* structure dereference... see the other posts.

If you had something like this:

filter={points:ptr_new(['a','b']),pt1_value:200, pt2_value:'X_WHYLOG'}

You can use the precendence to your advantage, visa vis:

print,*filter.points

Keeping the precendence in mind can eliminate extraneous parentheses, which, for
everyone except lisp programmers, often add confusion.

JD

--
J.D. Smith |*| WORK: (607) 255-5842
Cornell University Dept. of Astronomy |*| (607) 255-6263
304 Space Sciences Bldg. |*| FAX: (607) 255-5875
Ithaca, NY 14853 |*|

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[Message index]

		Re: pointer to structures By: John-David T. Smith on Wed, 05 April 2000 00:00
		Re: pointer to structures By: Liam E. Gumley on Wed, 05 April 2000 00:00
		Re: pointer to structures By: John-David T. Smith on Tue, 04 April 2000 00:00
		Re: pointer to structures By: Liam E. Gumley on Tue, 04 April 2000 00:00
		Re: pointer to structures By: davidf on Tue, 04 April 2000 00:00
		Re: pointer to structures By: Ben Tupper on Tue, 04 April 2000 00:00
		Re: pointer to structures By: John-David T. Smith on Tue, 04 April 2000 00:00
		Re: pointer to structures By: John-David T. Smith on Tue, 04 April 2000 00:00
		Re: pointer to structures By: davidf on Tue, 04 April 2000 00:00
		Re: pointer to structures By: wbiagiot on Tue, 04 April 2000 00:00
		Re: pointer to structures By: Liam E. Gumley on Tue, 04 April 2000 00:00
		Re: pointer to structures By: Ben Tupper on Tue, 04 April 2000 00:00

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