Re: Array of structures.

["J.D. Smith" <jdsmith(at)astro.cornell.edu>]

Nicolas Decoster wrote:
> 
> Hi.
> 
> I'd like to build an array of structure using one line. Something like
> that:
> 
> arr = [{sig:a, color:'red'}, {sig:b, color:'green'}]
> 
> I don't want to use the replicate function and then fill the array one
> by one:
> 
> arr = replicate({myStruct, sig:a, color:'red'}, 2)
> arr[0] = {myStruct, sig:a, color:'red'}
> arr[1] = {myStruct, sig:b, color:'green'}
> 
> Any suggestions ?
> 
> Thanks.
> 
> Nicolas.
> 
> ps: In fact, I'm trying to find a way to handle lists of anything. In
> the present case I want to pass as one argument a list of signals
> (arrays of value) associated with a color. The list must be of any size,
> we must be able to build it on the fly. Something like that : {{sig1
> 'red'} {sig2 'green'}} in a Tcl-like syntax.

Pointers are your salvation, but the initial request can be solved without them:

arr=[{myStruct,sig:a,color:'red'},{myStruct,b,'green'}]

The key being using a named array, fully filled in on the first element.  If you
don't want to use a named array, you're stuck with replicate.  

As for you p.s., I'd use lots o' pointers... an example in an object oriented
design:

pro myClass__define
	struct={COLSIG,   $
		Color:'', $
		Signals:ptr_new()} ; pointer to a list of signal values

	struct={myClass, $
		colsigs:ptr_new(),$  ;pointer to list of structs of type COLSIG
		blah:0L, $
		...}	
end

So you have the following flexibility:  

1. each colsig record can have any number of signals associated with a given
color.  
2. There can be as many such colsig color-signal pairings (structs) as you like.

Simply make sure to use the appropriate "if ptr_valid(self.colsigs)" and "if
ptr_valid((*self.colsigs)[1].Signals) to test for the validity of a given record
before using it.... though see some prior postings about the advantages of
splitting up such complicated dereferencing/indexing statements.  

You can easily find records for a given color like:

wh=where((*self.colsigs).Color eq 'Green',cnt)

just remember that structure field extraction and array indexing have higher
precedence than pointer derefencing.  This is confusing since the manual lists
pointer derefence as second in Operator Precedence, just below parentheses.  But
"." and "[]" are not included on that list!  So just remember: "*" is weak,
comparatively speaking, which is why you can get away with things like:

a={Foo,b:ptrarr(3)}
c=*a.b[0]

both the "." and the "[0]" are evaluated *before* the "*".  This is why we had
to use parentheses above:  we wanted to take a subscript or a structure field of
the thing *pointed to* by self.colsigs, so we had to empower our "*" operator
with parentheses.

Good luck,

JD

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