how to preallocate an object array?

I am trying to preallocate an object array of a class. Below is the class that I am using in MyobjectArray class

classdef MyData
    properties
       x = [];  % a column vector of data
    end
        methods
        function obj = MyData(nRows)
            if nRows > 0
                obj.x = zeros(nRows,1);
            end
        end
    end
end

In the code below I am using obj.value = zeros(nArrays,1); to preallocate. What is the correct way to preallocate an object array?

classdef MyobjectArray
    properties
        value = [];  % an array of MyData objects
    end
     methods
        function obj = MyobjectArray(nArrays, nRows)
            if nargin ~= 0
                obj.value = zeros(nArrays,1);
                for i = 1:nArrays
                    obj(i).value = MyData(nRows);
                end
            end
        end
    end
end

Answer 1

[Walter Roberson]

Do not initialize value to [] if the property is not going to have type "double" .

If it is going to have type that is one of the built-in types, use empty() to create an empty array of the correct type

https://www.mathworks.com/help/matlab/ref/empty.html#mw_204d42d9-5615-4d04-b95e-4c647e4ca3bd

See also https://www.mathworks.com/help/matlab/matlab_oop/creating-object-arrays.html

If MyData is not going to return double, then do not assign zeros to initialize the property . If MyData(nRows) returns a constant value you would be better with

[obj(1:nArrays).value] = MyData(nRows);

[Mark Ricard]

This runs but I am not correctly prellocating the array of MyData objects.

The goal is to create 3 arrays of MyData objects, each MyData object is a column vector

of 10 doubles.

classdef MyobjectArray

    properties
        value;  % an array of MyData objects
    end
    
    methods
        function obj = MyobjectArray(nArrays, nRows)
            if nargin ~= 0
                for i = 1:nArrays
                    obj(i).value = MyData(nRows);
                end
            end
        end

    end
end

classdef MyData

    properties
        x = [];  % a column vector of data
    end
    
    methods
        function obj = MyData(nRows)
            if nRows > 0
                obj.x = linspace(1,nRows,nRows)';
            end
        end
    end
end

[Walter Roberson]

        function obj = MyobjectArray(nArrays, nRows)
            if nargin ~= 0
              [obj(1:nArrays).value] = deal(MyData(nRows));
            end
        end

Tested.

This does assume that myData(nRows) returns constant output.

The property will be set to a vector in which each item has the same data pointer -- one original copy of the MyData object being pointed to multiple times. However, as these are value class objects and not handle class objects, that is not a problem unless you plan to use mex functions that overwrite the input without properly checking first to see if they are dealing with shared copies or not: any conformant function will either automatically or deliberately use copy-on-write.