Q: pointers inside of an array of structures?

[Amara Graps <Amara.Graps(at)mpi-hd.removethis.mpg.de>]

Hi Folks,

I am trying to create and build a flexible data structure
in IDL for a situation where we have fields of different types
in a structure, and then that structure is made into an array
of structures using "replicate". And I extend the
array of structures by concantenating, because I don't
know how many structures I need.

My question: Can a field inside of the array-of-structures be 
a pointer ?

For example --

I know how to flexibly extend a structure,
with each of the fields defined the same way:

-------
Create an array of an anonymous structure
IDL> thisstruc = {orbit:'', len:0,freq:DBLARR(100)}
IDL> periodcube = replicate(thisstruc,1)

;Put values into the structure
IDL> periodcube(0).orbit='G2'
IDL> periodcube(0).len=50    ;to store length of array : freq 
IDL> periodcube(0).freq=DINDGEN(50)

;Create a temporary structure like the original
IDL> tempperiod = thisstruc 

;Put values into the temporary structure
IDL> tempperiod.orbit='G3'
IDL> tempperiod.len=75
IDL> tempperiod.freq=DINDGEN(75)

;Now extend it by concatenating
IDL> newperiodcube=[periodcube,tempperiod]

;Take a look
IDL> help, newperiodcube, /st
** Structure <89af0>, 3 tags, length=816, refs=4:
   ORBIT           STRING    'G2'
   LEN             INT             50
   FREQ            DOUBLE    Array[100]
IDL> print, newperiodcube(0).orbit, ' ', newperiodcube(1).orbit
G2 G3
IDL> print, newperiodcube(0).len, ' ', newperiodcube(1).len    
      50       75
-------

But what about varying data lengths inside of the array of 
structures, for example, if my freq arrays are each a different
length:

IDL> thisstruc = {orbit:'', freq:ptr_new()} 
IDL> periodcube = replicate(thisstruc,1)

;Put values into the structure
IDL> periodcube(0).orbit='G2'
IDL> *periodcube(0).freq=DINDGEN(50)
% Unable to dereference NULL pointer: <POINTER  (<NullPointer>)>.
% Execution halted at:  $MAIN$                 

What am I doing wrong?

Is it possible to have pointers inside of an array of structures?


My pointer programming experiences were 15 yrs ago in a Pascal
class.. it's quite possible I'm missing something big, with how
the pointers work...

Thanks very much in advance,

Amara

P.S. IDL Version 5.3 (sunos sparc)
-- 

***************************************************************
Amara Graps               | Max-Planck-Institut fuer Kernphysik
Interplanetary Dust Group | Saupfercheckweg 1   
+49-6221-516-543          | 69117 Heidelberg, GERMANY
                          * http://galileo.mpi-hd.mpg.de/~graps  
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      "Never fight an inanimate object." - P. J. O'Rourke