| Re: Pointers to a variable... [message #77372 is a reply to message #77371] |
Fri, 26 August 2011 06:44   |
H. Evans
Messages: 18
Registered: December 2009
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Junior Member |
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On Aug 26, 3:36 pm, David Fanning <n...@idlcoyote.com> wrote:
> H. Evans writes:
>> In other less friendly languages, e.g. C, the pointer points to an
>> area of memory, which can coincide with a variable. This gives two
>> methods to access the contents of the variable:
>
>> #include <stdio.h>
>> main() {
>> int a=5;
>> int *p;
>
>> p = &a;
>> printf("a=%i, *p=%i\n", a, *p);
>> a= 10;
>> printf("a=%i, *p=%i\n", a, *p);
>> }
>
>> outputs:
>> a=5, *p=5
>> a=10, *p=10
>
>> So, now that IDL has pointers...can a pointer be set to point to a
>> variable in the same way, i.e. to reference exactly the same memory
>> space as the variable?
>
> No, IDL pointers are NOT like C pointers.
>
>> From the examples, I am under the impression that these pointers don't
>> quite work in the same way, i.e. the pointers don't point to the same
>> memory space as the variables.
>
> This is correct.
I suspected as much. Oh well. off to code the scope_varfetch method...
>
>> The reason I ask is that there are some very large variables that I'd
>> rather not duplicate (waste of memory), but would like to group
>> serially via a pointer array.
>
>> As a trivial example:
>> a = FINDGEN(10000000L)
>> b = DINDGEN(200000L)
>> c = REPLICATE( !P, 10000L)
>> p = PTRARR(3, /ALLOC)
>> *p[0] = a
>> *p[1] = b
>> *p[2] = c
>
>> for i=0,n_ELEMENTS(p)-1 DO print,N_ELEMENTS(*p[i])
>
>> Is the only solution to create a,b, and c as heap variables in the
>> first instance and then point p[i] to the heap variable?
>
> IDL pointer variables are *exactly* like any other IDL
> variable:
>
> http://www.idlcoyote.com/misc_tips/pointers.html
>
> To transfer without duplicating, you could do this:
>
> a = FINDGEN(10000000L)
> b = DINDGEN(200000L)
> c = REPLICATE( !P, 10000L)
> p = PTRARR(3, /ALLOC)
> *p[0] = Temporary(a)
> *p[1] = Temporary(b)
> *p[2] = Temporary(c)
>
> This will undefine the variables a, b, and c in your program.
Unfortunately, I want to preserve them as they are used later in the
code.
For now, I'll just use
vars = ['a','b','c']
for i=0,n_elements(vars)-1 do
print,n_elements(scope_varfetch(vars[i]))
Oh well...
Ta.
Hugh
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