A* Algorithm for TSP
def aStarAlgo(start_node, stop_node):
open_set = set(start_node)
closed_set = set()
g = {} #store distance from starting node
parents = {}# parents contains an adjacency map of all nodes
#ditance of starting node from itself is zero
g[start_node] = 0
#start_node is root node i.e it has no parent nodes
#so start_node is set to its own parent node
parents[start_node] = start_node
while len(open_set) > 0:
n = None
#node with lowest f() is found
for v in open_set:
if n == None or g[v] + heuristic(v) < g[n] + heuristic(n):
n = v
if n == stop_node or Graph_nodes[n] == None:
pass
else:
for (m, weight) in get_neighbors(n):
#nodes 'm' not in first and last set are added to first
#n is set its parent
if m not in open_set and m not in closed_set:
open_set.add(m)
parents[m] = n
g[m] = g[n] + weight
#for each node m,compare its distance from start i.e g(m) to the
#from start through n node
else:
if g[m] > g[n] + weight:
#update g(m)
g[m] = g[n] + weight
#change parent of m to n
parents[m] = n
#if m in closed set,remove and add to open
if m in closed_set:
closed_set.remove(m)
open_set.add(m)
if n == None:
print('Path does not exist!')
return None
# if the current node is the stop_node
# then we begin reconstructin the path from it to the start_node
if n == stop_node:
path = []
while parents[n] != n:
path.append(n)
n = parents[n]
path.append(start_node)
path.reverse()
print('Path found: {}'.format(path))
return path
# remove n from the open_list, and add it to closed_list
# because all of his neighbors were inspected
open_set.remove(n)
closed_set.add(n)
print('Path does not exist!')
return None
#define fuction to return neighbor and its distance
#from the passed node
def get_neighbors(v):
if v in Graph_nodes:
return Graph_nodes[v]
else:
return None
#for simplicity we ll consider heuristic distances given
#and this function returns heuristic distance for all nodes
def heuristic(n):
H_dist = {
'A': 11,
'B': 6,
'C': 99,
'D': 1,
'E': 7,
'G': 0,
}
return H_dist[n]
#Describe your graph here
Graph_nodes = {
'A': [('B', 2), ('E', 3)],
'B': [('C', 1),('G', 9)],
'C': None,
'E': [('D', 6)],
'D': [('G', 1)],
}
aStarAlgo('A', 'G')