OOP- instantiate a class within a method of a different class

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Hadar 2018-3-20

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https://ww2.mathworks.cn/matlabcentral/answers/389457-oop-instantiate-a-class-within-a-method-of-a-different-class

评论： Hadar 2018-3-25

Hi, I have 2 class files, both stored under the same folder. I want to instantiate class A in one of the method of class B but I get an error message: Undefined function or variable 'A'. how can "connect" between the 2 class files so that they know each other? Thanks!

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Hadar 2018-3-21

编辑：per isakson 2018-3-22

在 MATLAB Online 中打开

Hi Adam, thanks for your reply. I'm new in matlab but familiar with c++. Im trying to practice matlab by implementing algorithms Ive already implemented in c++.

Im have the following class:

classdef MyGraph
      properties
          nodes_num
          vector_of_adj
      end
      methods
          function obj = MyGraph(inputArg1)
              obj.nodes_num = inputArg1;
              obj.vector_of_adj = cell(1,inputArg1);
          end
          function obj = AddEdgeInGraph(obj, src, dest)
               obj.vector_of_adj{src} = [obj.vector_of_adj{src}, dest]
          end
          function DFSutil(obj, start, visited)
              disp(start)
              visited(start) = 1;
              for i = drange(1:numel(obj.vector_of_adj{start}))
                  if visited(obj.vector_of_adj{start}(i)) == 0
                      DFSutil(obj, obj.vector_of_adj{start}(i), visited);
                  end
              end
          end
          function DFS(obj)
              visited_size = obj.nodes_num + 1;
              visited = zeros(1, visited_size);
              for i = 1:obj.nodes_num
                   if visited(i) == 0
                       DFSutil(obj, i, visited);
                  end
              end
          end
      end
  end

I have a problem with the array: visited which is a local variable in function DFS. when I pass it to function DFSutil, it is passed by value and not by reference so its updated values are not saved.

I read and found that I can declare a handle class, like this:

classdef Visited < handle

and to store the array: visited as a property of that class so any change done on its instantiated objects will be saved. So I saved another file for the class above and tried to instantiate an object in the DFS function but I got an error: Undefined function or variable 'Visited'.

so my questions are as follows:

1. is there any way I can pass array by reference to a function in matlab?

if not and I have to save the array as a property of a handle class, then:

2. how can I instantiate a class in a method or as a property of a different class (assuming both classes are saved in their own files but under the same folder)?

Thank you so much for your help!

Hadar 2018-3-21

在 MATLAB Online 中打开

I dont understand the following question:

So what does

which Visited

show on the command line?

----------------------------

anyway, I changed my code so now visited is a property of MyGraph class and I initialize it in the constructor of MyGraph. therefore, I removed it from argument list of DFSutil so now it can not(!) be called externally with a different 'visited' array. its working now!

here are the 2 modified classes:

classdef MyGraph

    properties
        nodes_num
        vector_of_adj
        visited
    end
    methods
        function obj = MyGraph(inputArg1)
            obj.nodes_num = inputArg1;
            obj.vector_of_adj = cell(1,inputArg1);
            obj.visited = Visited(inputArg1);
        end
        function obj = AddEdgeInGraph(obj, src, dest)
             disp('hi')
             obj.vector_of_adj{src} = [obj.vector_of_adj{src}, dest]
        end
        function DFSutil(obj, start)
            disp(start)
            obj.visited.visited(start) = 1;
            for i = drange(1:numel(obj.vector_of_adj{start}))
                if obj.visited.visited(obj.vector_of_adj{start}(i)) == 0
                    DFSutil(obj, obj.vector_of_adj{start}(i));
                end
            end
        end
        function DFS(obj)
%             
            for i = 1:obj.nodes_num
                 if obj.visited.visited(i) == 0
                     DFSutil(obj, i);
                end
            end
        end
    end
end

classdef Visited < handle here

    properties
        visited;
    end
    methods
        function obj = Visited(input)
            obj.visited = zeros(1, input);
%                 
        end
    end
end

for practice, I intend to also try your other suggestion: "Alternatively you can just have it passed by value and be an output argument of DFSutil".

Thanks alot, you've been very helpful!

Hadar 2018-3-21

ohh cool, like in bash terminal.

I get the path:

/home/hadar/MATLAB/R2017b/bin/Visited.m % Visited constructor

Adam 2018-3-21

So it should be able to see it whether inside or outside of your other class, in that case.

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Answer 1

Guillaume 2018-3-22

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/389457-oop-instantiate-a-class-within-a-method-of-a-different-class#answer_311462

在 MATLAB Online 中打开

For somebody coming from C++, I'm surprised you're not using private/public qualifiers for your class methods, rather than having everything public.

The problem with the visited class not being visible is puzzling since which can find it. It's surprising that it is in a bin directory but that doesn't matter.

Irrespective of that issue however, the whole concept of creating a handle class just to emulate a pass-by-reference array is extremely misguided. Matlab is not C++. If you need a mutable object in and out of a function, then you pass it as input and output. Therefore, the proper way to write your DFS would be:

      methods
          %... constructor, etc.
          %DFS method. Note: I prefer calling the object 'this' rather than 'obj'
          function DFS(this)
              visited_size = this.nodes_num + 1;
              visited = zeros(1, visited_size);
              for i = 1:this.nodes_num
                   if visited(i) == 0
                       visited = DFSutil(this, i, visited);
                   end
              end
          end
      end
      methods (Access = private)
          function visited  = DFSutil(this, start, visited)
              disp(start)
              visited(start) = 1;
              for i = drange(1:numel(this.vector_of_adj{start}))
                  if visited(this.vector_of_adj{start}(i)) == 0
                      visited = DFSutil(this, this.vector_of_adj{start}(i), visited);
                  end
              end
          end
     end

Note that in the above case, matlab's JIT compiler should be able to detect that input and output are the same and pass the visited array by reference anyway.

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Hadar 2018-3-25

Hi, I'm familiar with public/private qualifiers in c++ and know this option exists in matlab too. I'm new in matlab, therefore less familiar with its syntax but during learning I intend to add these qualifiers to my classes in addition to pass visited as input and output just like adam previously suggested.

Now Visited class is visible and I have no problem to instantiate it. Regarding bin directory, Ive erroneously saved it there.

Thank you for your help!

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OOP- instantiate a class within a method of a different class

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