| Re: trouble with pointers within array of structures [message #94444 is a reply to message #94443] |
Tue, 23 May 2017 23:49   |
Helder Marchetto
Messages: 520
Registered: November 2011
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Senior Member |
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On Wednesday, May 24, 2017 at 3:12:15 AM UTC+2, wlandsman wrote:
> When you replicate a scalar pointer, you are making duplicate copies of the *same* pointer
>
> IDL> p = ptr_new(dist(256))
> IDL> pp = replicate(p,10)
> IDL> help,pp[0],pp[1],pp[2]
> <Expression> POINTER = <PtrHeapVar23>
> <Expression> POINTER = <PtrHeapVar23>
> <Expression> POINTER = <PtrHeapVar23>
>
> To make an array of distinct pointers, use ptrarr()
>
> IDL> p = ptrarr(3,/all)
> IDL> help,p[0],p[1],p[2]
> <Expression> POINTER = <PtrHeapVar24>
> <Expression> POINTER = <PtrHeapVar25>
> <Expression> POINTER = <PtrHeapVar26>
>
> In your structure example, I think what you want is
>
> main = { name:'', image : ptrarr(10,/allocate) }
> *main.image[5] = world()
> *main.image[4] = ct()
>
> --Wayne
>
> On Tuesday, May 23, 2017 at 8:12:58 PM UTC-4, Ann Nonymous wrote:
>> I apologize for what I'm sure is a dumb question but I've looked all through the documentation for three days and I apparently am missing something.
>>
>> I'm trying to store a pointer to a vector as an entry within a structure of arrays. This is the basic idea:
>>
>>
>> a = { name:'', image : ptr_new(/allocate) }
>> main = replicate(a,10)
>>
>> ;world() returns a 256x256 floating point image of the world,
>> ;ct() returns a 256x256 floating point CT image
>>
>>
>> *main[5].image = world()
>> *main[4].image = ct()
>>
>> tv, *main[4].image ;gives an image of the CT scan, which is what I expected
>> tv, *main[5].image ;gives an image of the CT scan
>> tv, *main[0].image ; ditto
>> tv, *main[9].image ; ditto
>>
>> Obviously, I'm missing something fundamental about the syntax here but I've tried every permeation of parentheses and indices I can think of, and no matter what, the last pointer assigned overwrites every other pointer in the structure array.
>>
>> Can anyone tell me what I should be doing here?
>>
>> Thanks very much,
I've came across this problem once and the reasoning of the way around it is as follows:
Define a structure with a single pointer:
a = { name:'', image : ptr_new()}
Replicate the structure:
main = replicate(a,10)
Now the pointers don't point to anything (no allocation yet).
help, main[0].image
When you declare the pointers now, you are each time generating a new one:
for i=0,9 do main[i].image = ptr_new(i)
Now you can check the contents:
for i=0,9 do print, *main[i].image
0
1
2
3
4
5
6
7
8
9
And everything is working again.
The reason why I suggest this solution, is that when defining a structure, you often want to think about it as a representation of "something" (data). This something might have one dynamic value (one pointer) or an array of dynamic values (array of pointers).
I hope this helps and is easy to understand.
Cheers,
Helder
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