| Re: Help: Pointer [message #14865] |
Sat, 03 April 1999 00:00 |
steinhh
Messages: 260
Registered: June 1994
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Senior Member |
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Vu Khac Tri wrote:
> But in C, when c is modified, (*pc) is also modified because
> pc points to the memory where c is. That mean, if c = 5, *pc = 5.
You're right. IDL doesn't have pointers in the same sense
as C does - there is no "address of" operator, and there
is no way to make a pointer that points to a normal variable.
The closest thing (and it's close enough to emulate
the C behaviour, but not it's notation) is:
IDL> c=ptr_new(4)
IDL> pc=c
IDL> print,*pc
4
IDL> *c = 5
IDL> print,*pc
5
I.e., you have two pointers, pointing to the same heap
variable. The only difference from C notation is that
you have to dereference *both* variables as pointers.
Regards,
Stein Vidar
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| Re: Help: Pointer [message #14885 is a reply to message #14865] |
Fri, 02 April 1999 00:00  |
VU KHAC Tri
Messages: 25
Registered: March 1999
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Junior Member |
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>> In C, I can write:
>>
>> void XXX ()
>> {
>> int C = 4;
>> int *pC;
>> pC = &C; /*pC points to C*/
>> }
>>
>> I cannot find out how to do this in IDL ?
>
> IDL> c = 4
> IDL> pC = Ptr_New(/Allocate_Heap)
> IDL> *pC = c
>
I found this :
IDL> c = 4
IDL> pc = PTR_NEW(/ALLOCATE_HEAP)
IDL> *pc = c
IDL> print, *pc
4
IDL> c = 5
IDL> print, *pc
4
But in C, when c is modified, (*pc) is also modified because pc points
to the memory where c is. That mean, if c = 5, *pc = 5.
Thank you so much for your book.
Best regards,
Tri.
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| Re: Help: Pointer [message #14890 is a reply to message #14885] |
Fri, 02 April 1999 00:00 |
davidf
Messages: 2866
Registered: September 1996
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Senior Member |
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VU KHAC Tri (tvk@info.fundp.ac.be) writes:
> In C, I can write:
>
> void XXX ()
> {
> int C = 4;
> int *pC;
> pC = &C; /*pC points to C*/
> }
>
> I cannot find out how to do this in IDL ?
IDL> c = 4
IDL> pC = Ptr_New(/Allocate_Heap)
IDL> *pC = c
But most people would just to this:
IDL> pC = Ptr_New(c)
To destroy the pointer (and the thing it points to),
be sure to do this when you are finished:
IDL> Pointer_Free, pC
If you are writing a widget program you can clean up
your pointers in the CLEANUP procedure you assign
to your top-level base with the XMANAGER command.
Cheers,
David
P.S. People may be interested to know that a book is
winging its way to Tri today. :-)
--
David Fanning, Ph.D.
Fanning Software Consulting
Phone: 970-221-0438 E-Mail: davidf@dfanning.com
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Toll-Free IDL Book Orders: 1-888-461-0155
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