HomeAIParticle Swarm Optimization (PSO) from scratch. Easiest clarification in python | by...

Particle Swarm Optimization (PSO) from scratch. Easiest clarification in python | by Aleksei Rozanov | Feb, 2024


To begin with, let’s outline our hypoparameters. Like in lots of different metaheuristic algorithms, these variables ought to be adjusted on the best way, and there’s no versatile set of values. However let’s stick to those ones:

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POP_SIZE = 10 #inhabitants dimension 
MAX_ITER = 30 #the quantity of optimization iterations
w = 0.2 #inertia weight
c1 = 1 #private acceleration issue
c2 = 2 #social acceleration issue

Now let’s create a perform which might generate a random inhabitants:

def populate(dimension):
x1,x2 = -10, 3 #x1, x2 = proper and left boundaries of our X axis
pop = rnd.uniform(x1,x2, dimension) # dimension = quantity of particles in inhabitants
return pop

If we visualize it, we’ll get one thing like this:

x1=populate(50) 
y1=perform(x1)

plt.plot(x,y, lw=3, label='Func to optimize')
plt.plot(x1,y1,marker='o', ls='', label='Particles')
plt.xlabel('x')
plt.ylabel('y')
plt.legend()
plt.grid(True)
plt.present()

Picture by creator.

Right here you’ll be able to see that I randomly initialized a inhabitants of fifty particles, a few of that are already near the answer.

Now let’s implement the PSO algorithm itself. I commented every row within the code, however in case you have any questions, be at liberty to ask within the feedback under.

"""Particle Swarm Optimization (PSO)"""
particles = populate(POP_SIZE) #producing a set of particles
velocities = np.zeros(np.form(particles)) #velocities of the particles
features = -np.array(perform(particles)) #calculating perform values for the inhabitants

best_positions = np.copy(particles) #it is our first iteration, so all positions are the perfect
swarm_best_position = particles[np.argmax(gains)] #x with with the best acquire
swarm_best_gain = np.max(features) #highest acquire

l = np.empty((MAX_ITER, POP_SIZE)) #array to gather all pops to visualise afterwards

for i in vary(MAX_ITER):

l[i] = np.array(np.copy(particles)) #gathering a pop to visualise

r1 = rnd.uniform(0, 1, POP_SIZE) #defining a random coefficient for private habits
r2 = rnd.uniform(0, 1, POP_SIZE) #defining a random coefficient for social habits

velocities = np.array(w * velocities + c1 * r1 * (best_positions - particles) + c2 * r2 * (swarm_best_position - particles)) #calculating velocities

particles+=velocities #updating place by including the speed

new_gains = -np.array(perform(particles)) #calculating new features

idx = np.the place(new_gains > features) #getting index of Xs, which have a larger acquire now
best_positions[idx] = particles[idx] #updating the perfect positions with the brand new particles
features[idx] = new_gains[idx] #updating features

if np.max(new_gains) > swarm_best_gain: #if present maxima is greateer than throughout all earlier iters, than assign
swarm_best_position = particles[np.argmax(new_gains)] #assigning the perfect candidate resolution
swarm_best_gain = np.max(new_gains) #assigning the perfect acquire

print(f'Iteration {i+1} tGain: {swarm_best_gain}')

After 30 iteration we’ve acquired this:

PSO (w=0.2, c1=1, c2=2). Picture by creator.

As you’ll be able to see the algorithm fell into the native minimal, which isn’t what we needed. That’s why we have to tune our hypoparameters and begin once more. This time I made a decision to set inertia weight w=0.8, thus, now the earlier velocity has a larger affect on the present state.

PSO (w=0.9, c1=1, c2=2). Picture by creator.

And voila, we reached the worldwide minimal of the perform. I strongly encourage you to mess around with POP_SIZE, c₁ and c₂. It’ll will let you acquire a greater understanding of the code and the thought behind PSO. Should you’re you’ll be able to complicate the duty and optimize some 3D perform and make a pleasant visualization.

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[1]Shi Y. Particle swarm optimization //IEEE connections. — 2004. — Т. 2. — №. 1. — С. 8–13.

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