PSO algorithm value problem

Hi

I have a pso code, but i have a little problem. Here is a code

clc;
clear;
close all;

%% Problem Definition

CostFunction = @(x) Sphere(x); % Cost Function

nVar = 5; % Number of Unknown (Deceision) Variables

VarSize = [1 nVar]; % Matrix Size of Decision Variables

VarMin = -10; % Lower bound of Decesion Variable
VarMax = 10; % Upper bound of Decesion Variable

%% Parameters of PSO

MaxIt = 100; % Maximum Number of Iterations

nPop = 50; % Population Size (Swarm Size)
wdamp = 0.99; % Damping Ratio of Inertia Coefficient
w = 1; % Inertia Coefficient
c1 = 2; % Personal Acceleration Coefficient
c2 = 2; % Social Acceleration Coefficient

%% Initialization

% The Particle Template
empty_particle.Position = [];
empty_particle.Velocity = [];
empty_particle.Cost = [];
empty_particle.Best.Position = [];
empty_particle.Best.Cost = [];

% Create Population Array
particle = repmat(empty_particle, nPop, 1);

% Initialize Global Best
GlobalBest.Cost = inf;

% Initialize Population Members
for i=1:nPop

% Generate Random Solution
particle(i).Position = unifrnd(VarMin, VarMax, VarSize);

% Initialize Velocity
particle(i).Velocity = zeros(VarSize);

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

% Update the Personal Best
particle(i).Best.Position = particle(i).Position;
particle(i).Best.Cost = particle(i).Cost;

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

end
%
% 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(VarSize).*(particle(i).Best.Position - particle(i).Position) ...
+ c2*rand(VarSize).*(GlobalBest.Position - particle(i).Position);

% Update Position
particle(i).Postion = particle(i).Position + particle(i).Velocity;

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

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

particle(i).Best.Position = particle(i).Position;
particle(i).Best.Cost = particle(i).Cost;

% Update Global Best
if particle(i).Best.Cost < GlobalBest.Cost
GlobalBest = particle(i).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))]);


% Damping Interia Coefficient
w = w * wdamp;
end

figure;
 plot(BestCosts, 'LineWidth', 2);

xlabel('Iteration');
ylabel('Best Cost');
grid on;

Value of BestCosts is alwyas the same, but shouldnt. I dont know how to fix it. Please help me.

Answer 1

[Ayush]

Hi Jakub,

I find that the issue you're facing with the PSO code is likely due to a typo in the update of the particle's position. In the line where you update the position of each particle, you have a typo: "Postion" instead of "Position". This typo means that the position is never actually updated, which would indeed result in the "BestCosts" array having the same value across iterations.

Here's the corrected line of code:

% Update Position
particle(i).Position = particle(i).Position + particle(i).Velocity;

Hope it helps!