How to access multiple subfields of an object array?

I have an array of objects ("g") each with a field "location". Location is a class with fields "x", "y", "z", etc. Is it possible to retrieve an array of the "x", "y" or "z" fields directly from the array "g"?

What I've done so far:

I first tried (see comment by per isakson)

g.location.x

which gives a "Dot name reference on non-scalar structure" error, even though g.location is perfectly valid for a non-scalar g.

Then I tried the following code which returns an array of location classes:

[g.location]

However, I cannot get the array of x fields as such:

[[g.location].x]

(due to an unexpected operator error at the ".x" part) without first defining

l = [g.location]

then evaluating

[l.x]

I have more subfields in x, y, and z to access and this method would be tedious to code out for all subfields.

So is there any syntax that will allow me to evaluate the array [g.location] then evaluate the array [[g.location].x] and so on for all subfields in a single line?

Answer 1

[per isakson]

Try

    z = g.location.x;

and if g is scalar

    z = [ g.location.x ];

as you would do with structures

.

Added: 2012-12-14

I fail to find a concise description on how Matlab is supposed to behave in these cases (R2012a). However, I found some "tips" and "examples". I need something better.

The statement

    z = [ g(:).location(:).x ];

or short

    z = [ g.location.x ];

consists of two operations: list expansion and concatenation. I don't find the documentation on "list expansion". However, I'm positive that "list expansion" only works with the rightmost index in a train-wreck like g.location.x

Here is an experiment with list expansion on objects. I think this illustrates pretty much what is possible to achieve.

>> g34 = ArrayClass( 3, 4 );
>> g14 = ArrayClass( 1, 4 );
>> g41 = ArrayClass( 4, 1 );

where

    classdef    ArrayClass < handle
        properties
            location
        end
        methods
            function this = ArrayClass( n, m )
                if nargin == 0,     return, end
                this( 1, n ) = ArrayClass();
                for ii = 1 : n
                   this( ii ).location = XYZ( m ); 
                end
            end
        end
    end

and where

    classdef    XYZ < handle
        properties
            x
            y
            z
        end
        methods
            function this = XYZ( m )
                if nargin == 0,     return, end

                this( 1, m ) = XYZ();
                for ii = 1 : m
                   this( ii ).x = 10 + rand; 
                   this( ii ).y = 20 + rand; 
                   this( ii ).z = 30 + rand; 
                end
            end
        end
    end

and some calls

>> [ g14.location.x ]
ans =
   10.5211   10.6241   10.3674   10.8852

>> [ g34(3).location.x ]
ans =
   10.0855   10.0292   10.4886   10.4588

>> [ g41.location.x ]
Dot name reference on non-scalar structure. 

>> [ g41(2).location.x ]
ans =
   10.6797

.

"[ g41.location.x ]" breaks the the rightmost index rule.

[Brendan]

Forgot to mention in the question I had tried these already. They both give the error:

Dot name reference on non-scalar structure.

[per isakson]

See above

Answer 2

[Samuel Harry]

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