Matlab's arrayfun for uniform output of class objects

Question

I need to build an array of objects of class ID using arrayfun:

% ID.m
classdef ID < handle
    properties
        id
    end
    methods
        function obj = ID(id)
            obj.id = id;
        end
    end
end

But get an error:

>> ids = 1:5;
>> s = arrayfun(@(id) ID(id), ids) 
??? Error using ==> arrayfun
ID output type is not currently implemented.

I can build it alternatively in a loop:

s = [];
for k = 1 : length(ids)
    s = cat(1, s, ID(ids(k)));
end

but what is wrong with this usage of arrayfun?

Edit (clarification of the question): The question is not how to workaround the problem (there are several solutions), but why the simple syntax s = arrayfun(@(id) ID(id), ids); doesn't work. Thanks.

robince · Accepted Answer · 2012-06-05 13:24:56Z

5

Perhaps the easiest is to use cellfun, or force arrayfun to return a cell array by setting the 'UniformOutput' option. Then you can convert this cell array to an array of obects (same as using cat above).

s = arrayfun(@(x) ID(x), ids, 'UniformOutput', false);
s = [s{:}];

answered Jun 5, 2012 at 13:24

robince

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3 Comments

Serg Over a year ago

Thanks, but it seems as a patch, like the loop method. My question was why arrayfun(@(id) ID(id), ids) doesn't work. Is it a Matlab's bug, or wrong usage?

hugovdberg Over a year ago

as I sometimes miss the python functionality to endlessly extend commands, and frequently use your cell to array construction I often declare an anonymous function to do it in one go: dealcell = @(x)x{:};, which can then be used like s = [dealcell(arrayfun(...))];

Eric Over a year ago

You need s = reshape(s, size(ids)) to make that truly match arrayfun

tmpearce · Accepted Answer · 2012-06-05 15:26:43Z

3

You are asking arrayfun to do something it isn't built to do.

The output from arrayfun must be:

scalar values (numeric, logical, character, or structure) or cell arrays.

Objects don't count as any of the scalar types, which is why the "workarounds" all involve using a cell array as the output. One thing to try is using cell2mat to convert the output to your desired form; it can be done in one line. (I haven't tested it though.)

s = cell2mat(arrayfun(@(id) ID(id), ids,'UniformOutput',false));

answered Jun 5, 2012 at 15:26

tmpearce

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3 Comments

Amro Over a year ago

actually CELL2MAT will fail with the error Cannot support cell arrays containing cell arrays or objects.

tmpearce Over a year ago

@Amro Ah, interesting. Thanks for pointing that out, like I said I hadn't tested it. Is there a way to get handles to the objects into a vector?

Amro Over a year ago

you can create object arrays (I posted an answer), just not with ARRAYFUN. In fact ARRAYFUN can iterate over array of objects (as input), just not return them as output (must return scalars of cells as you pointed out). You can also do it as @robince showed

Amro · Accepted Answer · 2012-06-05 22:34:19Z

2

This is how I would create an array of objects:

s = ID.empty(0,5);
for i=5:-1:1
    s(i) = ID(i);
end

It is always a good idea to provide a "default constructor" with no arguments, or at least use default values:

classdef ID < handle
    properties
        id
    end
    methods
        function obj = ID(id)
            if nargin<1, id = 0; end
            obj.id = id;
        end
    end
end

answered Jun 5, 2012 at 22:34

Amro

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