Solving cars moving in multiple lanes simulation problem - python-3.x

Question

I am trying to simulate cars moving in multiple lanes in python. The problem is like this:
The number of cars, the roadlength, the probability and vmax are all input values.
Rules:
1. If vi < vmax, increase the velocity vi of car i by one unit, that is, vi → vi + 1. This change models the process of acceleration to the maximum velocity.
2. Compute the distance to the next car in the same lane and the distance to the cars in both (if there are 2) lanes next to the car.
If d=max([d1,d2,d3]) and vi ≥ d, then reduce the velocity to vi = d − 1 to prevent crashes and switch lane to the lane where the distance to the next car is d (if there are multiple choose one at random or whichever you want).
Else (meaning there is at least one lane next to the car's lane or it could be the same lane that the car is in where d > vi) go in that lane and don't change the velocity of the car if there is more than one lane, pick one at random.
3. With probability p, reduce the velocity of a moving car by one unit: vi → vi − 1, only do this when v > 0 to avoid negative velocities
4. Update the position xi of car i so that xi(t + 1) = xi(t) + vi
Also the path of the cars is circular, meaning there will be cars in front and behind.
Below is my attempt to solve the problem. Don't get confused over the variables theta and r. theta is just the position and r is the lane.
My attempt:
from matplotlib import pyplot as plt
import random
import math
from matplotlib import animation
import numpy as np
from operator import attrgetter
roadLength = 100
numFrames = 200
nlanes = 3
numCars = 20
posss =[]
theta = []
r = []
color = []
probability = 0.5
vmax = 1
cars=[]
class Car:
def __init__(self, position, velocity, lane):
self.position = position
self.velocity = velocity
self.lane = lane
def pos(car,k):
rand = random.uniform(0,1)
if car[k].velocity < vmax:
car[k].velocity += 1
dist = 0
if car[k].lane == 1:
temp_lanes_between = [0,1]
if car[k].lane == nlanes and nlanes != 1:
temp_lanes_between = [-1 ,0]
if 1 < car[k].lane < nlanes:
temp_lanes_between = [-1 ,0, 1]
iterator = []
for p in range(k+1, numCars):
iterator.append(p)
#if car[k+1].position - car[k].position <= car[k].velocity and car[k].lane == car[k+1].lane:
for p in range(k):
iterator.append(p)
for s in iterator:
if car[s].lane - car[k].lane in temp_lanes_between:
temp_lanes_between.remove(car[s].lane - car[k].lane)
distance = min([abs((car[s].position - car[k].position) % roadLength), roadLength - abs((car[s].position - car[k].position) % roadLength)])
if dist < distance:
dist = distance
l = car[s].lane
if dist <= car[k].velocity:
break
if temp_lanes_between:
j=random.randrange(0, len(temp_lanes_between))
car[k].lane += temp_lanes_between[j]
if temp_lanes_between == [] and dist <= car[k].velocity:
car[k].velocity = dist - 1
car[k].lane = l
if rand < probability and car[k].velocity > 0:
car[k].velocity = car[k].velocity - 1
car[k].position = car[k].position + car[k].velocity
return car[k].position
for i in range(numCars):
cars.append(Car(i, 0, 1))
theta.append(0)
r.append(1)
color.append(i)
posss.append(i)
fig = plt.figure()
ax = fig.add_subplot(111)
point, = ax.plot(posss, r, 'o')
ax.set_xlim(-10, 1.2*numFrames)
ax.set_ylim(-2, nlanes + 3)
def animate(frameNr):
sort_cars = sorted(cars, key=attrgetter("position"))
for i in range(numCars):
pos(sort_cars,i)
for k in range(numCars):
theta[k]=cars[k].position
r[k]=cars[k].lane
print(theta)
print(r)
point.set_data(theta, r)
return point,
def simulate():
anim = animation.FuncAnimation(fig, animate,
frames=numFrames, interval=100, blit=True, repeat=False)
plt.show()
simulate()
I get error saying: "local variable 'l' referenced before assignment" in the line where car[k].lane = l . I know that they mean that l doesn't have any value and therefore I get this error. But I don't see how this is possible. Every time pos() is run it should always go through the line l = car[s].lane and there it gets assigned a value. Maybe there are more errors in the code above but I have really given it my best shot and I don't know what to do.
Thanks in advance!