Class constructor that can create either a single object or an object array depending on number of the argument

Hey guys,

Thanks in advance for taking your time to read my question!

I'm currently building a class called "secOrdIdentifier" where I'm trying to write a constructor that can create either a single object or an object array depending on number of the argument, and I used IF statement together with varargin as illustrated below.

A common quantity called Lyapunov Function is needed for both cases. I found it no problem if I solve the Lyapunov function under each case (shown as in Version_0). In order to reduce the redundancy of the code, it would be preferred if I can only write it once, so I tried to write it before the IF statement as indicated in Version_1, and I got an error msg when running Version_1:

Index exceeds the number of array elements (0).

Error in secOrdIdentifier (line 22)
            A = varargin{1};

Error in secOrdIdentifier (line 41)
                obj(1,n) = obj;

and I'm wondering what is the reason and how should I solve it?

PS. In case you want to run my code, here are the arguments

x0 = [0;0];
z0 = [0;0];
theta0 = [3;2];
A = [0,1;-1,-1];
F = 10*rand(6,4);

% To create a single object:
identifier = identifier(A,x0,z0,theta0)
% To create an object array
identifier = identifier(A,F)

Thanks

Lihao

EDIT: I have modified my code according to Mr. Steve Lord's suggestion, and now it works! Here's the edited version:

% Edited Version
classdef secOrdIdentifier
    properties
        x = [0;0] 
        z = [0;0]
        theta (2,1)
        coeMatrix (2,2)
        LyapunovFx
    end
    
    methods
        function obj = secOrdIdentifier(A,var)
            % When var is a double singlet, the input has the following structure:
            % A,theta0
            % When var is a double matrix, the input has the following structure:
            % A, F  
            
            if nargin==0
                
            elseif nargin==2
                % -----------------------------------
                % Start solving the Lyapunov function
                syms x y z
                P = [x,y;y,z];
                eqs = A'*P+P*A==-eye(2);
                sol = solve([eqs(1,1),eqs(1,2),eqs(2,2)],[x,y,z]);
                % End solving the Lyapunov function
                % -----------------------------------
                if ~ismatrix(var) 
                    % Properties assignment
                    obj.theta = var;      
                    obj.coeMatrix = A;
                    obj.LyapunovFx = double([sol.x,sol.y;sol.y,sol.z]);
                else
                    % Array generation
                    n = size(var,2);
                    obj(1,n) = obj;
                    for i=1:n
                        obj(i).theta = var(1:2);
                        obj(i).LyapunovFx = double([sol.x,sol.y;sol.y,sol.z]);
                        obj(i).coeMatrix = A;
                    end
                end
            else
                erorr('Error\nThe number of input must be 0 or 2.')
            end
        end
    end
end

Here's the class definition (version_0 and version_1)

% Version_0
classdef secOrdIdentifier
    properties
        x (2,1)
        z (2,1)
        theta (2,1)
        coeMatrix (2,2)
        LyapunovFx
    end
    
    methods
        function obj = secOrdIdentifier(varargin)
            % When nargin=4, the input has the following structure:
            % A, x0,z0,theta0
            % When nargin=2, the input has the following structure:
            % A, F 
            
            % Case 1: Generate a single object
            if nargin==4  
                % -----------------------------------
                % Start solving the Lyapunov function
                syms x y z
                A = varargin{1};
                P = [x,y;y,z];
                eqs = A'*P+P*A==-eye(2);
                sol = solve([eqs(1,1),eqs(1,2),eqs(2,2)],[x,y,z]);
                obj.LyapunovFx = double([sol.x,sol.y;sol.y,sol.z]);
                % End solving the Lyapunov function
                % -----------------------------------
                
                % Properties assignment
                obj.x = varargin{2};
                obj.z = varargin{3};
                obj.theta = varargin{4};      
                obj.coeMatrix = A;
                
            % Case 2: Generate an object array
            elseif nargin==2
                % -----------------------------------
                % Start solving the Lyapunov function
                syms x y z
                A = varargin{1};
                P = [x,y;y,z];
                eqs = A'*P+P*A==-eye(2);
                sol = solve([eqs(1,1),eqs(1,2),eqs(2,2)],[x,y,z]);
                % End solving the Lyapunov function
                % -----------------------------------
                
                % Array generation
                n = size(varargin{2},2);
                obj(1,n) = obj;
                for i=1:n
                    % Properties Assignment
                    obj(i).x = varargin{2}(1:2);
                    obj(i).z = varargin{2}(3:4);
                    obj(i).theta = varargin{2}(5:6);
                    obj(i).LyapunovFx = double([sol.x,sol.y;sol.y,sol.z]);
                    obj(i).coeMatrix = A;
                end
            end
        end
    end
end

% Version_1
classdef secOrdIdentifier
    properties
        x (2,1)
        z (2,1)
        theta (2,1)
        coeMatrix (2,2)
        LyapunovFx
    end
    
    methods
        function obj = secOrdIdentifier(varargin)
            % When nargin=4, the input has the following structure:
            % A, x0,z0,theta0
            % When nargin=2, the input has the following structure:
            % A, F 
            % -----------------------------------
            % Start solving the Lyapunov function
            syms x y z
            A = varargin{1};
            P = [x,y;y,z];
            eqs = A'*P+P*A==-eye(2);
            sol = solve([eqs(1,1),eqs(1,2),eqs(2,2)],[x,y,z]);
            % End solving the Lyapunov function
            % -----------------------------------
            
            % Case 1: Generate a single object
            if nargin==4 
                % Properties assignment
                obj.x = varargin{2};
                obj.z = varargin{3};
                obj.theta = varargin{4};      
                obj.coeMatrix = A;
                obj.LyapunovFx = double([sol.x,sol.y;sol.y,sol.z]);
            % Case 2: Generate an object array
            elseif nargin==2
                % Array generation
                n = size(varargin{2},2);
                obj(1,n) = obj;
                for i=1:n
                    % Properties Assignment
                    obj(i).x = varargin{2}(1:2);
                    obj(i).z = varargin{2}(3:4);
                    obj(i).theta = varargin{2}(5:6);
                    obj(i).LyapunovFx = double([sol.x,sol.y;sol.y,sol.z]);
                    obj(i).coeMatrix = A;
                end
            end
        end
    end
end

Answer 1

[Steven Lord]

Your constructor needs to accept the case where it is called with 0 input arguments. See the section "No Input Argument Constructor Requirement" on this documentation page for more information.

[Lihao Zheng]

Aha, I see, thank you so much! Problem solved!