class Problem(object):\n",
+ "\n",
+ " """The abstract class for a formal problem. You should subclass\n",
+ " this and implement the methods actions and result, and possibly\n",
+ " __init__, goal_test, and path_cost. Then you will create instances\n",
+ " of your subclass and solve them with the various search functions."""\n",
+ "\n",
+ " def __init__(self, initial, goal=None):\n",
+ " """The constructor specifies the initial state, and possibly a goal\n",
+ " state, if there is a unique goal. Your subclass's constructor can add\n",
+ " other arguments."""\n",
+ " self.initial = initial\n",
+ " self.goal = goal\n",
+ "\n",
+ " def actions(self, state):\n",
+ " """Return the actions that can be executed in the given\n",
+ " state. The result would typically be a list, but if there are\n",
+ " many actions, consider yielding them one at a time in an\n",
+ " iterator, rather than building them all at once."""\n",
+ " raise NotImplementedError\n",
+ "\n",
+ " def result(self, state, action):\n",
+ " """Return the state that results from executing the given\n",
+ " action in the given state. The action must be one of\n",
+ " self.actions(state)."""\n",
+ " raise NotImplementedError\n",
+ "\n",
+ " def goal_test(self, state):\n",
+ " """Return True if the state is a goal. The default method compares the\n",
+ " state to self.goal or checks for state in self.goal if it is a\n",
+ " list, as specified in the constructor. Override this method if\n",
+ " checking against a single self.goal is not enough."""\n",
+ " if isinstance(self.goal, list):\n",
+ " return is_in(state, self.goal)\n",
+ " else:\n",
+ " return state == self.goal\n",
+ "\n",
+ " def path_cost(self, c, state1, action, state2):\n",
+ " """Return the cost of a solution path that arrives at state2 from\n",
+ " state1 via action, assuming cost c to get up to state1. If the problem\n",
+ " is such that the path doesn't matter, this function will only look at\n",
+ " state2. If the path does matter, it will consider c and maybe state1\n",
+ " and action. The default method costs 1 for every step in the path."""\n",
+ " return c + 1\n",
+ "\n",
+ " def value(self, state):\n",
+ " """For optimization problems, each state has a value. Hill-climbing\n",
+ " and related algorithms try to maximize this value."""\n",
+ " raise NotImplementedError\n",
+ "class Node:\n",
+ "\n",
+ " """A node in a search tree. Contains a pointer to the parent (the node\n",
+ " that this is a successor of) and to the actual state for this node. Note\n",
+ " that if a state is arrived at by two paths, then there are two nodes with\n",
+ " the same state. Also includes the action that got us to this state, and\n",
+ " the total path_cost (also known as g) to reach the node. Other functions\n",
+ " may add an f and h value; see best_first_graph_search and astar_search for\n",
+ " an explanation of how the f and h values are handled. You will not need to\n",
+ " subclass this class."""\n",
+ "\n",
+ " def __init__(self, state, parent=None, action=None, path_cost=0):\n",
+ " """Create a search tree Node, derived from a parent by an action."""\n",
+ " self.state = state\n",
+ " self.parent = parent\n",
+ " self.action = action\n",
+ " self.path_cost = path_cost\n",
+ " self.depth = 0\n",
+ " if parent:\n",
+ " self.depth = parent.depth + 1\n",
+ "\n",
+ " def __repr__(self):\n",
+ " return "<Node {}>".format(self.state)\n",
+ "\n",
+ " def __lt__(self, node):\n",
+ " return self.state < node.state\n",
+ "\n",
+ " def expand(self, problem):\n",
+ " """List the nodes reachable in one step from this node."""\n",
+ " return [self.child_node(problem, action)\n",
+ " for action in problem.actions(self.state)]\n",
+ "\n",
+ " def child_node(self, problem, action):\n",
+ " """[Figure 3.10]"""\n",
+ " next_node = problem.result(self.state, action)\n",
+ " return Node(next_node, self, action,\n",
+ " problem.path_cost(self.path_cost, self.state,\n",
+ " action, next_node))\n",
+ "\n",
+ " def solution(self):\n",
+ " """Return the sequence of actions to go from the root to this node."""\n",
+ " return [node.action for node in self.path()[1:]]\n",
+ "\n",
+ " def path(self):\n",
+ " """Return a list of nodes forming the path from the root to this node."""\n",
+ " node, path_back = self, []\n",
+ " while node:\n",
+ " path_back.append(node)\n",
+ " node = node.parent\n",
+ " return list(reversed(path_back))\n",
+ "\n",
+ " # We want for a queue of nodes in breadth_first_search or\n",
+ " # astar_search to have no duplicated states, so we treat nodes\n",
+ " # with the same state as equal. [Problem: this may not be what you\n",
+ " # want in other contexts.]\n",
+ "\n",
+ " def __eq__(self, other):\n",
+ " return isinstance(other, Node) and self.state == other.state\n",
+ "\n",
+ " def __hash__(self):\n",
+ " return hash(self.state)\n",
+ "class GraphProblem(Problem):\n",
+ "\n",
+ " """The problem of searching a graph from one node to another."""\n",
+ "\n",
+ " def __init__(self, initial, goal, graph):\n",
+ " Problem.__init__(self, initial, goal)\n",
+ " self.graph = graph\n",
+ "\n",
+ " def actions(self, A):\n",
+ " """The actions at a graph node are just its neighbors."""\n",
+ " return list(self.graph.get(A).keys())\n",
+ "\n",
+ " def result(self, state, action):\n",
+ " """The result of going to a neighbor is just that neighbor."""\n",
+ " return action\n",
+ "\n",
+ " def path_cost(self, cost_so_far, A, action, B):\n",
+ " return cost_so_far + (self.graph.get(A, B) or infinity)\n",
+ "\n",
+ " def find_min_edge(self):\n",
+ " """Find minimum value of edges."""\n",
+ " m = infinity\n",
+ " for d in self.graph.graph_dict.values():\n",
+ " local_min = min(d.values())\n",
+ " m = min(m, local_min)\n",
+ "\n",
+ " return m\n",
+ "\n",
+ " def h(self, node):\n",
+ " """h function is straight-line distance from a node's state to goal."""\n",
+ " locs = getattr(self.graph, 'locations', None)\n",
+ " if locs:\n",
+ " if type(node) is str:\n",
+ " return int(distance(locs[node], locs[self.goal]))\n",
+ "\n",
+ " return int(distance(locs[node.state], locs[self.goal]))\n",
+ " else:\n",
+ " return infinity\n",
+ "class SimpleProblemSolvingAgentProgram:\n",
+ "\n",
+ " """Abstract framework for a problem-solving agent. [Figure 3.1]"""\n",
+ "\n",
+ " def __init__(self, initial_state=None):\n",
+ " """State is an abstract representation of the state\n",
+ " of the world, and seq is the list of actions required\n",
+ " to get to a particular state from the initial state(root)."""\n",
+ " self.state = initial_state\n",
+ " self.seq = []\n",
+ "\n",
+ " def __call__(self, percept):\n",
+ " """[Figure 3.1] Formulate a goal and problem, then\n",
+ " search for a sequence of actions to solve it."""\n",
+ " self.state = self.update_state(self.state, percept)\n",
+ " if not self.seq:\n",
+ " goal = self.formulate_goal(self.state)\n",
+ " problem = self.formulate_problem(self.state, goal)\n",
+ " self.seq = self.search(problem)\n",
+ " if not self.seq:\n",
+ " return None\n",
+ " return self.seq.pop(0)\n",
+ "\n",
+ " def update_state(self, state, percept):\n",
+ " raise NotImplementedError\n",
+ "\n",
+ " def formulate_goal(self, state):\n",
+ " raise NotImplementedError\n",
+ "\n",
+ " def formulate_problem(self, state, goal):\n",
+ " raise NotImplementedError\n",
+ "\n",
+ " def search(self, problem):\n",
+ " raise NotImplementedError\n",
+ "def hill_climbing(problem):\n",
+ " """From the initial node, keep choosing the neighbor with highest value,\n",
+ " stopping when no neighbor is better. [Figure 4.2]"""\n",
+ " current = Node(problem.initial)\n",
+ " while True:\n",
+ " neighbors = current.expand(problem)\n",
+ " if not neighbors:\n",
+ " break\n",
+ " neighbor = argmax_random_tie(neighbors,\n",
+ " key=lambda node: problem.value(node.state))\n",
+ " if problem.value(neighbor.state) <= problem.value(current.state):\n",
+ " break\n",
+ " current = neighbor\n",
+ " return current.state\n",
+ "def genetic_algorithm(population, fitness_fn, gene_pool=[0, 1], f_thres=None, ngen=1000, pmut=0.1):\n",
+ " """[Figure 4.8]"""\n",
+ " for i in range(ngen):\n",
+ " population = [mutate(recombine(*select(2, population, fitness_fn)), gene_pool, pmut)\n",
+ " for i in range(len(population))]\n",
+ "\n",
+ " fittest_individual = fitness_threshold(fitness_fn, f_thres, population)\n",
+ " if fittest_individual:\n",
+ " return fittest_individual\n",
+ "\n",
+ "\n",
+ " return argmax(population, key=fitness_fn)\n",
+ "def recombine(x, y):\n",
+ " n = len(x)\n",
+ " c = random.randrange(0, n)\n",
+ " return x[:c] + y[c:]\n",
+ "def mutate(x, gene_pool, pmut):\n",
+ " if random.uniform(0, 1) >= pmut:\n",
+ " return x\n",
+ "\n",
+ " n = len(x)\n",
+ " g = len(gene_pool)\n",
+ " c = random.randrange(0, n)\n",
+ " r = random.randrange(0, g)\n",
+ "\n",
+ " new_gene = gene_pool[r]\n",
+ " return x[:c] + [new_gene] + x[c+1:]\n",
+ "def init_population(pop_number, gene_pool, state_length):\n",
+ " """Initializes population for genetic algorithm\n",
+ " pop_number : Number of individuals in population\n",
+ " gene_pool : List of possible values for individuals\n",
+ " state_length: The length of each individual"""\n",
+ " g = len(gene_pool)\n",
+ " population = []\n",
+ " for i in range(pop_number):\n",
+ " new_individual = [gene_pool[random.randrange(0, g)] for j in range(state_length)]\n",
+ " population.append(new_individual)\n",
+ "\n",
+ " return population\n",
+ "def genetic_algorithm(population, fitness_fn, gene_pool=[0, 1], f_thres=None, ngen=1000, pmut=0.1):\n",
+ " """[Figure 4.8]"""\n",
+ " for i in range(ngen):\n",
+ " population = [mutate(recombine(*select(2, population, fitness_fn)), gene_pool, pmut)\n",
+ " for i in range(len(population))]\n",
+ "\n",
+ " fittest_individual = fitness_threshold(fitness_fn, f_thres, population)\n",
+ " if fittest_individual:\n",
+ " return fittest_individual\n",
+ "\n",
+ "\n",
+ " return argmax(population, key=fitness_fn)\n",
+ "class NQueensProblem(Problem):\n",
+ "\n",
+ " """The problem of placing N queens on an NxN board with none attacking\n",
+ " each other. A state is represented as an N-element array, where\n",
+ " a value of r in the c-th entry means there is a queen at column c,\n",
+ " row r, and a value of -1 means that the c-th column has not been\n",
+ " filled in yet. We fill in columns left to right.\n",
+ " >>> depth_first_tree_search(NQueensProblem(8))\n",
+ " <Node (7, 3, 0, 2, 5, 1, 6, 4)>\n",
+ " """\n",
+ "\n",
+ " def __init__(self, N):\n",
+ " self.N = N\n",
+ " self.initial = tuple([-1] * N)\n",
+ " Problem.__init__(self, self.initial)\n",
+ "\n",
+ " def actions(self, state):\n",
+ " """In the leftmost empty column, try all non-conflicting rows."""\n",
+ " if state[-1] is not -1:\n",
+ " return [] # All columns filled; no successors\n",
+ " else:\n",
+ " col = state.index(-1)\n",
+ " return [row for row in range(self.N)\n",
+ " if not self.conflicted(state, row, col)]\n",
+ "\n",
+ " def result(self, state, row):\n",
+ " """Place the next queen at the given row."""\n",
+ " col = state.index(-1)\n",
+ " new = list(state[:])\n",
+ " new[col] = row\n",
+ " return tuple(new)\n",
+ "\n",
+ " def conflicted(self, state, row, col):\n",
+ " """Would placing a queen at (row, col) conflict with anything?"""\n",
+ " return any(self.conflict(row, col, state[c], c)\n",
+ " for c in range(col))\n",
+ "\n",
+ " def conflict(self, row1, col1, row2, col2):\n",
+ " """Would putting two queens in (row1, col1) and (row2, col2) conflict?"""\n",
+ " return (row1 == row2 or # same row\n",
+ " col1 == col2 or # same column\n",
+ " row1 - col1 == row2 - col2 or # same \\ diagonal\n",
+ " row1 + col1 == row2 + col2) # same / diagonal\n",
+ "\n",
+ " def goal_test(self, state):\n",
+ " """Check if all columns filled, no conflicts."""\n",
+ " if state[-1] is -1:\n",
+ " return False\n",
+ " return not any(self.conflicted(state, state[col], col)\n",
+ " for col in range(len(state)))\n",
+ "\n",
+ " def h(self, node):\n",
+ " """Return number of conflicting queens for a given node"""\n",
+ " num_conflicts = 0\n",
+ " for (r1, c1) in enumerate(node.state):\n",
+ " for (r2, c2) in enumerate(node.state):\n",
+ " if (r1, c1) != (r2, c2):\n",
+ " num_conflicts += self.conflict(r1, c1, r2, c2)\n",
+ "\n",
+ " return num_conflicts\n",
+ " def h(self, node):\n",
+ " """Return number of conflicting queens for a given node"""\n",
+ " num_conflicts = 0\n",
+ " for (r1, c1) in enumerate(node.state):\n",
+ " for (r2, c2) in enumerate(node.state):\n",
+ " if (r1, c1) != (r2, c2):\n",
+ " num_conflicts += self.conflict(r1, c1, r2, c2)\n",
+ "\n",
+ " return num_conflicts\n",
+ "