| trouble with pointers within array of structures [message #94442] |
Tue, 23 May 2017 17:12  |
astroboy.20000
Messages: 39
Registered: August 2012
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Member |
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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,
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| Re: trouble with pointers within array of structures [message #94443 is a reply to message #94442] |
Tue, 23 May 2017 18:12  |
wlandsman
Messages: 743
Registered: June 2000
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Senior Member |
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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,
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| 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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| Re: trouble with pointers within array of structures [message #94445 is a reply to message #94444] |
Wed, 24 May 2017 05:32 |
Markus Schmassmann
Messages: 129
Registered: April 2016
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Senior Member |
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On 05/24/2017 08:49 AM, Helder wrote:
> 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
>
Wayne's explanation of the underlying problem and Helder's solution are
correct.
However, if you want to avoid unnecessary looping and for code
readability prefer to use
*main[5].image=world()
instead of
main[5].image=ptr_new(world())
then create the structure with
a = { name:'', image : ptr_new()}
main = replicate(a,10)
main.image=ptrarr(10,/allocate_heap)
---Markus
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