| Re: Problem of precedence with pointer and structure [message #47150] |
Fri, 27 January 2006 12:22 |
L. Testut
Messages: 16
Registered: January 2006
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Junior Member |
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a=ptrarr(5,/allocate_heap)
IDL> help,a
A POINTER = Array[5]
; A is an array of pointer OK
IDL> help,a[0]
<Expression> POINTER = <PtrHeapVar1>
; a[0] is a scalar pointer OK
IDL> help,/heap
Heap Variables:
# Pointer: 5
# Object : 0
<PtrHeapVar1> UNDEFINED = <Undefined>
<PtrHeapVar2> UNDEFINED = <Undefined>
<PtrHeapVar3> UNDEFINED = <Undefined>
<PtrHeapVar4> UNDEFINED = <Undefined>
<PtrHeapVar5> UNDEFINED = <Undefined>
; I've defined 5 differents scalar pointers (included in an array)
pointing on 5 differents heap variable
; I can affect to each of them the heap variable I want
IDL> *a[0]=0.
IDL> *a[1]='I hate pointers'
IDL> *a[2]=indgen(10)
IDL> *a[3]='ok ok we have understoodd !!'
IDL> *a[4]=4.
IDL> for i=0,4 do print,*a[i]
0.00000
I hate pointers
0 1 2 3 4 5 6 7
8 9
ok ok we understand !!
4.00000
; Now if I want to have an array of structure with a field b which is a
pointer
IDL> a=replicate({b:ptr_new(/allocate_heap)},5)
IDL> help,a
A STRUCT = -> <Anonymous> Array[5]
IDL> help,a.b
<Expression> POINTER = Array[5]
; So i think that (a.b) is equivalent to my previous a (e.g an array of
pointer pointing a 5 differents heap variable
; but as i *replicate* my structure I have 5 identical pointer variable
pointing to only ONE heap variable !!!
IDL> *(a[0].b)=0.0
IDL> *(a[1].b)='I hate pointers'
IDL> *(a[2].b)=indgen(10)
IDL> *(a[3].b)='ok ok we have understood !!'
IDL> *(a[4].b)=4.
IDL> for i=0,4 do print,*a[i].b
4.00000
4.00000
4.00000
4.00000
4.00000
; Then can you confirm me that it is stupid to replicate a structure
with a pointer variable field
; or that I am stupid to think that !
> P.S. If I'm not mistaken, I think I recommended that structure
> solution. :-)
Yes you did :-)
Thanks,
Laurent
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| Re: Problem of precedence with pointer and structure [message #47158 is a reply to message #47150] |
Fri, 27 January 2006 08:29  |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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L. Testut writes:
>
> Here is my problem :
> ; If i define a is a 3 elements array of structure with two fields b
> and c
> ; each field is an array
> a=replicate({b:findgen(10),c:findgen(20)},3)
>
> ; and I want to replace the first field b with the constant value 0.5
> ; for the complete array structure, I write:
>
> IDL> a[*].(0)=0.5 ;or a[*].b=0.5 ; and it works
>
> ; Suppose i have now a 3 elements array of structure with two fields b
> and c
> ; each field is *pointer* to an array
>
> a=replicate({b:ptr_new(findgen(10)),c:ptr_new(findgen(20))}, 3)
>
> Is it possible to do the same replacement as above ???
>
> IDL> (*a[*].(0))=0.5 ;doesn't work
> % Expression must be a scalar in this context: <POINTER Array[3]>.
> % Execution halted at: $MAIN$ 1
Yes, I think that is a problem.
IDL> Help, a[*].b
<Expression> POINTER = Array[3]
IDL doesn't allow you to do mathematical operations on pointer arrays:
IDL> *(a[*].b) = *(a[*].b) + 3
Expression must be a scalar in this context: <POINTER Array[3]>.
I think a loop is the only way around this.
Cheers,
David
P.S. If I'm not mistaken, I think I recommended that structure
solution. :-)
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.dfanning.com/
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| Re: Problem of precedence with pointer and structure [message #47159 is a reply to message #47158] |
Fri, 27 January 2006 08:18 |
Foldy Lajos
Messages: 268
Registered: October 2001
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Senior Member |
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Hi,
pointer dereference works only on scalar pointers (it can not be
guaranteed that all the pointers point to heap variables of the
same type/structure in a pointer array).
regards,
lajos
On Fri, 27 Jan 2006, L. Testut wrote:
> a=replicate({b:ptr_new(findgen(10)),c:ptr_new(findgen(20))}, 3)
>
> Is it possible to do the same replacement as above ???
>
> IDL> (*a[*].(0))=0.5 ;doesn't work
> % Expression must be a scalar in this context: <POINTER Array[3]>.
> % Execution halted at: $MAIN$ 1
>
> Thanks,
> Laurent
>
>
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