| Re: accessing heap variable elements [message #25061] |
Sat, 12 May 2001 10:42 |
Ken Mankoff
Messages: 158
Registered: February 2000
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Senior Member |
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On Sat, 12 May 2001, Ken Mankoff wrote:
> On Sat, 12 May 2001, Jared Crossley wrote:
>
>> Can any one explain this to me?
>> ------------------------------------
>> IDL> a=ptr_new( lonarr(2) )
>> IDL> help, a
>> A POINTER = <PtrHeapVar2229>
>> IDL> help, *a
>> <PtrHeapVar2229>
>> LONG = Array[2]
>> IDL> help, *a[0]
>> <PtrHeapVar2229>
>> LONG = Array[2]
>> IDL> help, *a[1]
>> % Attempt to subscript A with <INT ( 1)> is out of range.
>> ------------------------------------
>> I would expect that *a[0] and *a[1] would be long integers. How do I
>> access the elements in the long integer array heap variable?
>>
>> Thanks, Jared
>
> Hi Jared,
>
> last line should be (*a)[0]
>
> -k.
>
sorry, i realized i should include an explanation, as you requested.
its all about operator precedence. Now, "[" is *not* mentioned in the IDL
5.3 Online Help, but they have this:
Priority Operator
------------------------------------------------------------ ----------
First (highest) ( ) (parentheses, to group expressions)
Second * (pointer dereference)
^ (exponentiation)
Third * (multiplication)
# and ## (matrix multiplication)
etc.
I think its safe to assume that [ ] is parsed as ( ).
so:
( *a[ 0 ] EQ *(a[ 0 ]) )
which means "take subscript of 'a', then dereference it."
But 'a' is a pointer to an array, not an array of pointers. So you can't
subscript it, and even if you could, i'm not sure what you'd be
dereferenceing with your *.
You want the opposite order, or:
(*a)[ 0 ]
which means "dereference 'a', then take subscript."
-k.
--
Ken Mankoff
LASP://303.492.3264
http://lasp.colorado.edu/~mankoff/
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| Re: accessing heap variable elements [message #25062 is a reply to message #25061] |
Sat, 12 May 2001 10:28  |
Ken Mankoff
Messages: 158
Registered: February 2000
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Senior Member |
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On Sat, 12 May 2001, Jared Crossley wrote:
> Can any one explain this to me?
> ------------------------------------
> IDL> a=ptr_new( lonarr(2) )
> IDL> help, a
> A POINTER = <PtrHeapVar2229>
> IDL> help, *a
> <PtrHeapVar2229>
> LONG = Array[2]
> IDL> help, *a[0]
> <PtrHeapVar2229>
> LONG = Array[2]
> IDL> help, *a[1]
> % Attempt to subscript A with <INT ( 1)> is out of range.
> ------------------------------------
> I would expect that *a[0] and *a[1] would be long integers. How do I
> access the elements in the long integer array heap variable?
>
> Thanks, Jared
Hi Jared,
last line should be (*a)[0]
-k.
--
Ken Mankoff
LASP://303.492.3264
http://lasp.colorado.edu/~mankoff/
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