# Resampling Wheel Algorithm

I've been taking the Udacity CS373 (Robotic Car) class, and this week the topic was Particle filters. Particle filters are basically a localization algorithm that accounts for error in measurements and sensors.

Anyway, part of the Particle Filter algorithm requires the generation of a new set of these things called "particles" based on the particles' weights. So to accomplish this task, the Resample Wheel algorithm was presented in class. It is a particularly elegant method of generating a new set of particles by randomly drawing from an old set of particles (with replacement). The particle weight determines the likelihood of it being picked.

Here is the algorithm (with print statements - I used these to help me understand how the algorithm works):

import random

def generate_new_particles(old_particles, weights):
N = len(old_particles)
new_particles = []
index = int(random.random() * N)
beta = 0.0
mw = max(weights)
for i in range(N):
beta += random.random() * 2.0 * mw
print "beta =", beta
while beta > weights[index]:
beta -= weights[index]
index = (index + 1) % N
print "\tbeta= %f, index = %d, weight = %f" % (beta, index, weights[index])
new_particles.append(old_particles[index])
return new_particles

if __name__ == "__main__":
old_particles = [1, 2, 3, 4]
weights = [.3, 0, .4, .3]
new_particles = generate_new_particles(old_particles, weights)

print "old particles =", old_particles
print "weights =", old_particles
print "new particles =", new_particles


And here is a sample run (with my debug print statements):

beta = 0.536313558942
beta= 0.236314, index = 0, weight = 0.300000
beta = 0.510275914962
beta= 0.210276, index = 1, weight = 0.000000
beta= 0.210276, index = 2, weight = 0.400000
beta = 0.954824882947
beta= 0.554825, index = 3, weight = 0.300000
beta= 0.254825, index = 0, weight = 0.300000
beta = 0.678700538951
beta= 0.378701, index = 1, weight = 0.000000
beta= 0.378701, index = 2, weight = 0.400000
old particles = [1, 2, 3, 4]
weights = [1, 2, 3, 4]
new particles = [1, 3, 1, 3]