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Re: Modifying an array while conserving memory [message #30872 is a reply to message #30871]

Fri, 24 May 2002 02:06

Randall Skelton
Messages: 169
Registered: October 2000

Senior Member

> Why not using pointer:
>
> ptr1 = PTR_NEW(FINDGEN(1000))
> insert = PTR_NEW(RANDOMU(seed,100))
> a = PTR_NEW([(*ptr1)[0:499], (*insert), (*ptr1)[500:*]])
>
>
> HELP,*a

The problem with using pointers as above is that you are not actually
using the pointer, but copying the data contained within. Take a look at
the heap after doing the above:

IDL> help, /heap
Heap Variables:
# Pointer: 3
# Object : 0

<PtrHeapVar1> FLOAT = Array[1000]
<PtrHeapVar2> FLOAT = Array[100]
<PtrHeapVar3> FLOAT = Array[1100]

This shows that until I physically free the pointers 'ptr1' and 'insert',
I have used exactly double the memory as I now have a copy of each
variable.

Rather than inserting the data into the middle, I would (at this point) be
happy enough just concatenating to arrays...

IDL> ptr1 = PTR_NEW(FINDGEN(1000))
IDL> ptr2 = PTR_NEW(RANDOMU(seed,100))
IDL> a = [ptr1,ptr2]
IDL> print, *a ; fails
IDL> print, *(a) ; fails
IDL> print, *a(*) ; fails
IDL> print, *a[0] ; prints findgen(1000) (i.e. not what I want)
IDL> print, *(a)(*) ; fails... Score: IDL 5 ; Randall 0

Because IDL doesn't keep track of what type of data is in a pointer, the
above is protecting me from doing silly things:

IDL> ptr1 = PTR_NEW(FINDGEN(1000))
IDL> insert = ptr_new('test')
IDL> a = [ptr1, insert]

Perhaps this is something for dlm's. If I pass 'ptr1', 'insert' and the
indices for insertion into C I may be able to resize using 'ptr1' realloc,
shift the data around using pointers and trick the IDL variable structure
when sending the data back. This sounds risky but at this point all my
alternatives read, '% Unable to allocate memory: to make array'.

Cheers,
Randall

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[Message index]

		Re: Modifying an array while conserving memory By: R.Bauer on Fri, 24 May 2002 02:49
		Re: Modifying an array while conserving memory By: Randall Skelton on Fri, 24 May 2002 02:06
		Re: Modifying an array while conserving memory By: R.Bauer on Thu, 23 May 2002 23:44
		Re: Modifying an array while conserving memory By: Craig Markwardt on Thu, 23 May 2002 20:03
		Re: Modifying an array while conserving memory By: David Fanning on Thu, 23 May 2002 19:58
		Re: Modifying an array while conserving memory By: Pavel A. Romashkin on Fri, 24 May 2002 09:33
		Re: Modifying an array while conserving memory By: Randall Skelton on Fri, 24 May 2002 04:45

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