how to preallocate an object array?

2020 11 17
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更新时间：2020 11 18
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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

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Walter Roberson
Walter Roberson 2020-11-17

0 个投票

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);

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Mark Ricard
Mark Ricard 2020-11-18
编辑：Walter Roberson 2020-11-18
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
Walter Roberson 2020-11-18
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.

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