MATLAB: Object array implemenation in MATLAB OOP

MATLABoop

Suppose that we have this class in MATLAB R2014b :
classdef Pre
properties
    Value
end
methods
    function obj = Pre(F)
          if nargin ~= 0
              m = size(F,1);
              n = size(F,2);
              obj(m,n) = Pre;
              for i = 1:m
                  for j = 1:n
                      obj(i,j).Value = F(i,j);
                  end
              end
          end
      end
  end
  end
1) If we erase if nargin~=0 from this code we have this error :
Error using Pre (line 13)
Not enough input arguments.
Error in Pre (line 15)
                obj(m,n) = Pre;
Why? I think this is only checking for number of input arguments !
2) what is obj(m,n) = Pre;? what this line is doing in this code? This is for pre-allocating but how this line can do that?
I checked this class with this syntax: az = Pre([2 3 5;5 3 0])

Best Answer

Jack - the error message, Not enough input arguments, makes sense since the code is calling the class constructor at the line
 obj(m,n) = Pre;
without any input arguments. So when the line
 m = size(F,1);
is executed, there is no F and the error appears. Try putting a breakpoint at this line, and then create an instance of this class to verify when this error is being generated.
As for what the line
 obj(m,n) = Pre;
is doing, it is just creating an mxn array of objects of the type Pre. This is typical MATLAB code where you want to initialize an array (or matrix) with an object of a data type. For example,
 array1(3,5) = uint8(0)
creates a 3x5 array of 8-bit unsigned integers, whereas
 array2(12,3) = single(0)
creates a 12x3 array of elements of the single data type.
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