Error existent field - classes

I've coded PSO Simulink, and make particle as a class, main coding as below:

clc;
clear;
close all;
%%Initialization

% Parameters
nPop = 50;
MaxIt = 100;
w = 1;
c1 = 2;
c2 = 2;

% Generate Particle Template
sample_particle = particle();

% Create Population Array
particles(nPop) = particle();

% Array to Hold Best Cost Value on Each Iteration
BestCosts = zeros(MaxIt, 1);

%%Main Loop of PSO

for it=1:MaxIt

      for i=1:nPop

          % Update Velocity
          particle(i).velocity = w*particle(i).velocity ...
              + c1*rand(particle(i).VarSize).*(particle(i).bestPosition - particle(i).position) ...
              + c2*rand(particle(i).VarSize).*(particle(i).globalbest - particle(i).position);

          % Update Position
          particle(i).position = particle(i).position + particle(i).velocity;

          % Evaluation
          particle(i).Cost = CostFunction(particle(i).position);

          % Update Personal Best
          if particle.Cost < particle.Best.Cost

              particle.Best.Position = particle.position;
              particle.Best.Cost = particle.Cost;

              % Update Global Best
              if particle.Best.Cost < GlobalBest.Cost
                  GlobalBest = particle.Best;
              end

          end

      end

      % Store the Best Cost Value
      BestCosts(it) = GlobalBest.Cost;

      % Display Iteration Information
      disp(['Iteration' num2str(it) ': Best Cost = ' num2str(BestCosts(it))]);

end

And Class coding:

classdef particle
    %PARTICLES Summary of this class goes here
    %   Detailed explanation goes here

      properties

          % Parameters
          VarMin = -10;
          VarMax = 10;
          VarSize = 2;

          % Template
          position = [];
          velocity = [];
          cost = [];
          bestPosition = [];
          bestCost = [];

          % Initialize Global Best
          globalbest = inf;

      end

      methods

          function [ obj ] = particle( varargin )

              function circle = CostFunction(x,y)

                  circle = (x.^2)+(y.^2);

              end

              % Generate Random Solution
              obj.position = unifrnd(obj.VarMin, obj.VarMax, obj.VarSize);

              % Initialize Velocity
              obj.velocity = zeros(obj.VarSize);

              % Evaluation
              obj.cost = CostFunction(obj.position,obj.position);

              % Update the Personal Best
              obj.bestPosition = obj.position;
              obj.bestCost = obj.cost;

              % Update Global Best
              if obj.bestCost < obj.globalbest
                  obj.globalbest = obj.bestCost;
              end

          end
      end

end

then, after run got an error:

Reference to non-existent field 'position'.
Error in main (line 35)
        particle(i).position = particle(i).position + particle(i).velocity;

and I want to plot swarm particles on its moving motion.

Answer 1

[Steven Lord]

Don't define a variable in your code with the same name as a function or class you want to use in that code.

Change the variable name particle in your PSO code to particles (which I would avoid as too easy for a human to mistype or misread as particle) or something like listOfParticles and make that an array of particle objects.

[Ali Ali]

I did, but, I had another error.. please, check:

clc; clear; close all; %% Initialization

% Parameters nPop = 50; MaxIt = 100; w = 1; c1 = 2; c2 = 2;

% Generate Particle Template sample_particle = particle();

% Create Population Array particles(nPop) = particle();

% Array to Hold Best Cost Value on Each Iteration BestCosts = zeros(MaxIt, 1);

%% Main Loop of PSO

for it=1:MaxIt

    for i=1:nPop

        % Update Velocity
        particles(i).velocity = w*particles(i).velocity ...
            + c1*rand(particles(i).VarSize).*(particles(i).bestPosition - particles(i).position) ...
            + c2*rand(particles(i).VarSize).*(particles(i).globalbest - particles(i).position);

        % Update Position
        particles(i).position = particles(i).position + particles(i).velocity;

        % Evaluation
        particles(i) = CostFunction(particles(i).position,particles(i).position);

        % Update Personal Best
        if particles(i).Cost < particles(i).Best.Cost

            particles(i).Best.Position = particles(i).position;
            particles.Best.Cost = particles.Cost;

            % Update Global Best
            if particles(i).Best.Cost < GlobalBest.Cost
                GlobalBest = particles.Best;
            end

        end

    end

    % Store the Best Cost Value
    BestCosts(it) = GlobalBest.Cost;

    % Display Iteration Information
    disp(['Iteration' num2str(it) ': Best Cost = ' num2str(BestCosts(it))]);

end

....................

class

classdef particle %PARTICLES Summary of this class goes here % Detailed explanation goes here

    properties

        % Parameters
        VarMin = -10;
        VarMax = 10;
        VarSize = 2;

        % Template
        position = [];
        velocity = [];
        cost = [];
        bestPosition = [];
        bestCost = [];

        % Initialize Global Best
        globalbest = inf;

    end

    methods

        function [ obj ] = particle( varargin )

            function z = CostFunction(x,y)

                z = (x.^2)+(y.^2);

            end

            % Generate Random Solution
            obj.position = unifrnd(obj.VarMin, obj.VarMax, obj.VarSize);

            % Initialize Velocity
            obj.velocity = zeros(obj.VarSize);

            % Evaluation
            obj.cost = CostFunction(obj.position,obj.position);

            % Update the Personal Best
            obj.bestPosition = obj.position;
            obj.bestCost = obj.cost;

            % Update Global Best
            if obj.bestCost < obj.globalbest
                obj.globalbest = obj.bestCost;
            end

        end
    end

end

.....................

function

function z = CostFunction(x,y)

         z = (x.^2)+(y.^2);

end

[Steven Lord]

What is the full text (that means everything displayed in red) of the error message you receive when you execute your code?

Also I recommend eliminating this line from your code:

clc; clear; close all; %%Initialization

That makes it much more difficult to debug your code or compare the results from a run before you make a change to the results after you make the change.

[Ali Ali]

>> main No appropriate method, property, or field 'Cost' for class 'particle'. Error in main (line 38) if particles(i).Cost < particles(i).Best.Cost

[Steven Lord]

That makes sense. The particle class does not have a property Cost. It has a property cost (lower-case) but 'cost' is not the same as 'Cost'. Case matters.

[Ali Ali]

I appreciated this, thank you.

[Ali Ali]

Undefined function or variable 'globalbest'. Error in main (line 55) BestCosts(it) = globalbest;