Re: Structure field concatenation

["Liam E. Gumley" <Liam.Gumley(at)ssec.wisc.edu>]

David Fanning wrote:
> 
> Amara Graps (Amara.Graps@mpi-hd.removethis.mpg.de) writes:
> 
> > I appreciate your answer, but then I am back to the same error
> > I inquired about a couple of weeks ago, i.e.:
> >
> > If I do this:
> > thisstruc = {orbit:'',freq:ptr_new()}
> > instead of this:
> > thisstruc = {orbit:'',freq:ptr_new(/allocate_heap)}
> >
> > I get this error when I start to create an array of structures
> > and fill it:
> >
> > periodcube = replicate(thisstruc,1)
> > periodcube(0).orbit = 'G2'
> > *periodcube(0).freq=DINDGEN(100)  ;first pointer array is len 100
> >
> > % Unable to dereference NULL pointer: <POINTER  (<NullPointer>)>.
> 
> Exactly. A NULL pointer is an *invalid* pointer. Hence,
> it cannot be dereferenced. Only valid pointers can be
> dereferenced. A pointer to an undefined variable *is*
> a valid pointer and can be dereferenced, but if you
> replicate the same pointer in a bunch of structures
> all the pointers are to the same variable. It is
> indeed an oscillating universe. :-)
> 
> The solution, I think, is to check to see (if you have
> no other way of knowing in your code) if you have
> a valid pointer reference before trying to fill the
> field with data. Something like this:
> 
>    thisstruc = {orbit:'',freq:ptr_new()}
>    structs = Replicate(thisStruc, 10)
>    IF Ptr_Valid(structs[5].freq) THEN $
>       *structs[5].freq = FLTARR(100) ELSE $
>       structs[5].freg = Ptr_New(FLATARR(100))

Or you could create the valid pointers first:

;- Create template for one record which contains a single NULL pointer
thisstruc = {orbit:'',freq:ptr_new()}

;- Make an array which has the same NULL pointer in 10 places
structs = replicate(thisstruc, 10)

;- Replace the null pointer with an array of 10 valid pointers
structs.freq = ptrarr(10, /allocate_heap)

;- Store your data
*(structs[0].freq) = findgen(25)
*(structs[1].freq) = dist(256)

Cheers,
Liam.
http://cimss.ssec.wisc.edu/~gumley