cell array of classes

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Jean-Luc
Jean-Luc 2012-1-13
Hi,
Consider a simple class Aclass:
classdef Aclass
properties
w
end
methods
function a = Aclass(b)
a.w = b;
end
end
end
The only data member is a vector. Now say I have a very large matrix that I'd like to turn into a cell array of Aclass objects:
n = 60000; k = 20;
l = zeros(n,k);
% Convert to a cell array of Aclass objects,
sz = size(l,1);
l2 = mat2cell(l,ones(1,sz),k);
l2 = reshape(cellfun(@Aclass,l2,'UniformOutput',0),[sz 1]);
Here, the matrix l with n rows is converted into a cell array with n members, each of which is an Aclass.
My question is: is this the best way to do this? The problem is that it's quite slow when n becomes large. Is there a way to make a "custom" constructor that would return a cell array of Aclass rapidly, perhaps using a MEX file?
Many thanks,
JLT

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采纳的回答

per isakson
per isakson 2012-1-13
R2010b doc says:
Building Arrays in the Constructor
A constructor function can return an object array by making the necessary assignments when initializing the output argument. ...
See also: Creating Object Arrays

更多回答（1 个）

David Young
David Young 2012-1-13
Is there any reason to use a cell array rather than an array of Aclass objects? The latter should be more efficient in both time and storage.
You can make such an array in various ways, but probably the neatest is to provide the functionality in the class constructor. Per isakson's link gives the framework. Here's what it might look like for your case:
classdef Aclass
properties
w
end
methods
function a = Aclass(b)
if nargin > 0
a(size(b,1)) = Aclass; % preallocate array
for k = 1:size(b,1)
a(k).w = b(k,:);
end
end
end
end
end
and to create the array you simply do
arr = Aclass(l);
I think this goes faster than your cell-based code, but I haven't checked very carefully.
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Jean-Luc
Jean-Luc 2012-1-17
Update: cell2mat({arr.w}') is about the same speed (and maybe even slightly faster) that vertcat(arr.w). Weird.
Also, I don't see how one could use get.w instead of getw:
function b = get.w(a)
b = vertcat(a.w);
end
leads to an infinite recursion, I think, since a.w calls get.w.
jL
David Young
David Young 2012-1-17
In fact, it looks like get.w does not work, but for a different reason.
There is no recursion, because within get.w a reference to a.w goes directly to the property. (Oddly enough, the documentation doesn't seem to say this for get methods, though it does say it for set methods. However, it has to be the case that recursion is avoided, or get methods just wouldn't be useful.)
The reason get.w fails is that it is called once for each element of the array, rather than being passed the whole array. (I don't understand the rationale for this.) What it boils down to is that it looks like you have to use an ordinary method like getw.

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