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Lab 4: EC (ACO)

Ant colony optimisation

Objective

  • to develop a Python function to perform ant colony optimisation on a problem

Problem to solve

We will use ant colony optimisation to solve the Nick's route-finding problem in Romania. The problem is a route finding problem to identify the best (cheapest) route to travel from Arad to Bucharest.

The road map of Romania is provided as follows:

75 71 151 140 118 111 70 75 120 146 80 99 97 138 101 211 90 85 98 86 142 92 87 Arad Zerind Oradea Sibiu Fagaras Rimnicu Vilcea Pitesti Craiova Drobeta Mehadia Lugoj Timisoara Bucharest Giurgiu Urziceni Hirsova Eforie Vaslui Iasi Neamt

Problem formulation

  1. The coordinates of each town are provided as follows. This will be used later for the purpose of visualisation.

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    location_list = [ # [x,y,name]
  [75, 125, 'Arad'],
  [100, 75, 'Zerind'],
  [125, 25, 'Oradea'],
  [265, 175, 'Sibiu'],
  [425, 175, 'Fagaras'],
  [320, 230, 'Rimnicu Vilcea'],
  [475, 310, 'Pitesti'],
  [350, 465, 'Craiova'],
  [185, 450, 'Drobeta'],
  [190, 390, 'Mehadia'],
  [185, 335, 'Lugoj'],
  [85, 280, 'Timisoara'],
  [640, 390, 'Bucharest'],
  [575, 485, 'Giurgiu'],
  [745, 340, 'Urziceni'],
  [875, 340, 'Hirsova'],
  [935, 440, 'Eforie'],
  [850, 225, 'Vaslui'],
  [760, 120, 'Iasi'],
  [625, 60, 'Neamt']
]

  2. Then define the travel cost between connected cities. 

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    step_cost = [
  ['Arad', 'Zerind', 75],
  ['Zerind', 'Oradea', 71],
  ['Oradea', 'Sibiu', 151],
  ['Sibiu', 'Arad', 140],
  ['Sibiu', 'Fagaras', 99],
  ['Sibiu', 'Rimnicu Vilcea', 80],
  ['Fagaras', 'Bucharest', 211],
  ['Bucharest', 'Giurgiu', 90],
  ['Bucharest', 'Pitesti', 101],
  ['Pitesti', 'Rimnicu Vilcea', 97],
  ['Rimnicu Vilcea', 'Craiova', 146],
  ['Craiova', 'Pitesti', 138],
  ['Craiova', 'Drobeta', 120],
  ['Drobeta', 'Mehadia', 75],
  ['Mehadia', 'Lugoj', 70],
  ['Lugoj', 'Timisoara', 111],
  ['Arad', 'Timisoara', 118],
  ['Bucharest', 'Urziceni', 85],
  ['Urziceni', 'Vaslui', 142],
  ['Vaslui', 'Iasi', 92],
  ['Iasi', 'Neamt', 87],
  ['Urziceni', 'Hirsova', 98],
  ['Hirsova', 'Eforie', 86]
]

  3. We will define two class, City and Road.

  4. An object of class City has the attributes of name (the name of the city), roads (an array of references to the roads connected to the current city), and coordinates (coordinates of the cities).

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    class City:
  def __init__(self, name):
    self.name = name
    self.roads = []
    self.coordinates = []

  def set_coordinates(self, coordinates):
    self.coordinates = coordinates

  def add_road(self, road):
    if road not in self.roads:
      self.roads.append(road)

  5. An object of class Road has the attributes of connected_cities (an array of references to the cities connected through this road), cost (the step cost of this road), and pheromone (the pheromone on this road).

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    class Road:
  def __init__(self, connected_cities, cost, pheromone=0):
    self.connected_cities = connected_cities
    self.cost = cost
    self.pheromone = pheromone

  6. We will construct the list of City objects and Road objects from the information provided by the question, i.e. information in location_list and step_cost. The following code block should be in the main code block.

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    cities = {}
for coord1, coord2, name in location_list:
  cities[name] = City(name)
  cities[name].set_coordinates([coord1, coord2])
roads = []
for city1, city2, cost in step_cost:
  road = Road([cities[city1], cities[city2]], cost)
  cities[city1].add_road(road)
  cities[city2].add_road(road)
  roads.append(road)

  7. In the main code block, define the origin and destination cities.

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    origin = cities['Arad']
destination = cities['Bucharest']

Initiating ACO algorithm

  1. We then define the parameters for ACO, i.e. number of ants, n_ant, pheromone influence constant, alpha, and evaporation rate, rho.

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    n_ant = 10
alpha = 1
rho = 0.1

  2. Add the method set_pheromone to the class Road. 

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    class Road:
  def __init__(...):
    ...

  def set_pheromone(self, pheromone):
    self.pheromone = pheromone

  3. Set the initial pheromone of each road to 0.01. 

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    # in main block
initial_pheromone = 0.01
for road in roads:
  road.set_pheromone(initial_pheromone)

  4. Define the class Ant. 

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    class Ant:
  def __init__(self):
    self.cities = [] # cities the ant passes through, in sequence
    self.path = [] # roads the ant uses, in sequence

  5. Initiate n_ants ants. 

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    ants = [Ant() for _ in range(n_ant)]

Identify path of each ant

  1. In the Ant class, define a method to identify the path by taking the inputs of the available roads, the origin, the destination, and the pheromone influence constant α.

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    class Ant:
  def __init__(...):
    ...

  def get_path(self, origin, destination, alpha):
    # 1. append origin to the self.cities
    # 2. if the last city is not destination, search for the next city to go
    # 3. after getting to the destination, remove the loop within the path, i.e. if there are repeated cities in self.cities, remove the cities and the roads in between the repetition

  2. Define a method to calculate the path length.

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    class Ant:
  def __init__(...):
    ...

  def get_path(...):
    ...

  def get_path_length(self):
    # calculate path length based on self.path
    return path_length

  3. As the path of each ant will be reset every iteration, define a method that reset the path and cities.

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    class Ant:
  def __init__(...):
    ...

  def get_path(...):
    ...

  def get_path_length(...):
    ...

  def reset(self):
    self.path = []
    self.cities = []

Evaporation

  1. In the Road class, define a method to evaporate the pheromone by taking the input of evaporation rate ρ.

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    class Road:
  def __init__(...):
    ...

  def set_pheromone(...):
    ...

  def evaporate_pheromone(self, rho):
    # update the pheromone of the road

Deposition

  1. In the Road class, define a method to calculate the updated pheromone after pheromone deposition by taking the input of all the ants. We will use the following pheromone deposition formula for ant k on road i:

    Δτi,k = 1/Lk

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    class Road:
  def __init__(...):
    ...

  def set_pheromone(...):
    ...

  def evaporate_pheromone(...):
    ...

  def deposit_pheromone(self, ants):
    # 1. search for ants that uses the raod
    # 2. deposit pheromone using the inversely proportionate relationship between path length and deposited pheromone

Termination conditions

  1. We will use the following conditions as the termination conditions:

    • maximum iteration of 200
    • if ≥90% of the ants use the same path

  2. Define a function to calculate the percentage of the most dominant path.

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    def get_percentage_of_dominant_path(ants):
  ...
  return percentage

Loop until termination

  1. Create a loop to iterate until termination.

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    # termination threshold
max_iteration = 200
percentage_of_dominant_path = 0.9

iteration = 0
while ...: # termination conditions
  # loop through all the ants to identify the path of each ant
  for ant in ants:
    # reset the path of the ant
    ant.reset()
    # identify the path of the ant
    ant.get_path(origin, destination, alpha)
  # loop through all roads
  for road in roads:
    # evaporate the pheromone on the road
    road.evaporate_pheromone(rho)
    # deposit the pheromone
    road.deposit_pheromone(ants)
  # increase iteration count
  iteration += 1
# after exiting the loop, return the most occurred path as the solution
...

Visualisation

  1. Define the following functions:

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    import matplotlib.pyplot as plt
...
def create_graph(cities):
  fig = plt.figure()
  ax = fig.add_subplot(1,1,1)
  cities_x = [city.coordinates[0] for key, city in cities.items()]
  cities_y = [city.coordinates[1] for key, city in cities.items()]
  ax.scatter(cities_x, cities_y)
  ax.set_aspect(aspect=1.0)
  return ax

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    def draw_pheromone(ax, roads):
  lines = []
  for road in roads:
    from_coord = road.connected_cities[0].coordinates
    to_coord = road.connected_cities[1].coordinates
    coord_x = [from_coord[0], to_coord[0]]
    coord_y = [from_coord[1], to_coord[1]]
    lines.append(ax.plot(coord_x, coord_y, c='k', linewidth=road.pheromone**2))
  return lines

  2. Add the following lines to the main code block just before the while loop (loop until termination).

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    ax = create_graph(cities)
lines = draw_pheromone(ax, roads)

  3. Add the following lines to after iteration += 1.

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    # visualise
for l in lines:
  del l
lines = draw_pheromone(ax, roads)
plt.pause(0.05)

Evaluate effect of parameters

  1. Modify the pheromone depositing formula to

    Δτi,k = 1/Lk1.5

    What is the effect of this?

  2. Modify the pheromone depositing formula to

    Δτi,k = 5/Lk

    What is the effect of this?

  3. Investigate the effect of number of ants n_ant.

  4. Investigate the effect of pheromone influence constant α alpha.

  5. Investigate the effect of evaporation rate ρ rho.