How to properly overload horzcat and vertcat?

Question

Eugenio Grabovic 2019-1-24

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/441376-how-to-properly-overload-horzcat-and-vertcat

编辑： per isakson 2019-1-25

Hi, im having some trouble overloading horzcat/vertcat for my class with 2 properties:

classdef myClass
    properties
        a
        b
    end
    methods
        function obj = myClass(u,v)
            obj.a = u;
            obj.b = v;
        end
        function out = horzcat(varargin)
            ...
        end
    end
end

As a result of the concatenation i want to achieve just the concatenations of the two proprieties, for example concatenating those objects:

x = myClass(2,3);
y = myClass(4,5);
ConcArray = [x,y];

i would like to have as a result a single object with as properties the concatenation of each property. Basically having the equivalent of:

EquivalentObj = myClass([2,4],[3,5])

I came up with this solution:

function out = horzcat(varargin)
    
    n = length(varargin);
    aProp = [];
    bProp = [];
    
    for i = 1 : n
        var = varargin{i};
        aProp = [aProp var.a];
        bProp = [bProp var.b];
    end
    out = myClass(aProp,bProp);
end        

but it looks really bad since i couldn't preallocate the concatenating arrays, im using a for loop and in each loop im using standard horzcat recursively.

Any ideas on how to improve that ? Thank you in advance.

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Walter Roberson 2019-1-25

在 MATLAB Online 中打开

I do not see any property data type validation in your constructor, so we must assume that you want to be able to handle inputs of different data types, like

[myClass(2,3), myClass(11, tf([17 0], [1 9]))]

But to handle this you are going to have to define what the output should be. Your question suggests that you would want property a to become [2 11], but [3 tf([17 0], [1 9]))] cannot be directly mixed, and we don't know what you want to have happen in this situation.

Eugenio Grabovic 2019-1-25

Yeah sorry i didn't explained myself correctly: the property data type validation is implemented already, my main focus was on how to properly ( in a performant way) to overload concatenations assuming i always concatenate the same data types properties.

Mr. Isakson showed me what i needed.

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Answer 1

per isakson 2019-1-25

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/441376-how-to-properly-overload-horzcat-and-vertcat#answer_357961

编辑：per isakson 2019-1-25

在 MATLAB Online 中打开

This looks more Matlabish, but whether it's better ...

classdef    myClass
    properties
        a@double
        b@double
    end
    methods
        function obj = myClass(u,v)
            obj.a = u;
            obj.b = v;
        end
        function out = horzcat(varargin)
            obj = cat( 2, varargin{:} );
            as  = [ obj.a ];
            bs  = [ obj.b ];
            out = myClass( as, bs );
        end
    end
end

Test

%%
x = myClass(2,3);
y = myClass(4,5);
ConcArray = [x,y]

outputs

ConcArray = 
  myClass with properties:
    a: [2 4]
    b: [3 5]
>>

and try @Walter's expression

>> [myClass(2,3), myClass(11, tf([17 0], [1 9]))]
Error setting property 'b' of class 'myClass':
Value must be 'double'.
Error in myClass (line 9)
            obj.b = v;  
            

"I couldn't preallocate" why not just

    n = length(varargin);
    aProp = nan(1,n);
    bProb = nan(1,n);

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Eugenio Grabovic 2019-1-25

编辑：Eugenio Grabovic 2019-1-25

在 MATLAB Online 中打开

That's exactly what i was looking for! Thank you.

"aProp = nan(1,n);"
"bProb = nan(1,n);"

Actually its like preallocating with zeroes or ones, problem is i couldn't just "add" columns

aProp = [aProp var.a];

concatenating previous ones like this. Anyway it doesn't matter since you overload suggestion is just fine as it is.

per isakson 2019-1-25

编辑：per isakson 2019-1-25

I use NaN when preallocating, because if I fail to overwrite all preallocated values NaN will mostly likely quicker make me aware of the problem. Zero and one are often legal values.

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