def dijkstra(g, start):
# Distance is the priority in PQ
pq = PriorityQueue()
visited = set() # Redundant as weighs are always positive
distance = {} # Store distance to nodes from start
previous = {} # Store the previous node to reach a node
# prepare the start node
distance[start] = 0 # start to start have zero distance
previous[start] = None # No previous for start
# Add start to priority queue
pq.add_with_priority(start, 0)
while pq.length > 0: # PQ will have unvisited nodes
node_id = pq.extract_min()
node = g.vert_list[node_id]
for neigh in node.get_neighs():
if neigh not in visited:
n_dist = node.get_weight(neigh)
# if distance to neigh is less through, node_id, update.
if neigh not in distance or distance[node_id] + n_dist < distance[neigh]:
distance[neigh] = distance[node_id] + n_dist
previous[neigh] = node_id
# Update/add neigh in PQ, with new distance
# Here, decrease_priority will add as well as update.
# If such is not available, we will have to prepare PQ with
# all nodes with float('inf') distance, except start
pq.decrease_priority(neigh, distance[neigh])
# Done processing the node. Mark it as visited
visited.add(node_id)
print(distance)
print(previous)
def dijkstra(g, start):
# Distance is the priority in PQ
pq = PriorityQueue()
visited = set() # Redundant as weighs are always positive
distance = {} # Store distance to nodes from start
previous = {} # Store the previous node to reach a node
# Set all the weight to infinity
for node_id in g.get_vertices():
distance[node_id] = float('inf') # infinity
distance[start] = 0 # start to start have zero distance
previous[start] = None # No previous for start
# Add start to priority queue
pq.add_with_priority(start, 0)
while pq.length > 0: # PQ will have unvisited nodes
node_id = pq.extract_min()
node = g.vert_list[node_id]
for neigh in node.get_neighs():
if neigh not in visited:
n_dist = node.get_weight(neigh)
# if distance to neigh is less through, node_id, update.
if distance[node_id] + n_dist < distance[neigh]:
distance[neigh] = distance[node_id] + n_dist
previous[neigh] = node_id
# Update/add neigh in PQ, with new distance
pq.decrease_priority(neigh, distance[neigh])
# Done processing the node. Mark it as visited
visited.add(node_id)
print(distance)
print(previous)
def prims_mst(g):
visited = set() # visited set, already in MST
distance = {} # edge-distance to add the node to MST
previous = {}
pq = PriorityQueue()
start = 5 # Arbitrarily choose a start node.
for node in g.get_vertices():
distance[node] = float('inf')
previous[node] = None
distance[start] = 0
pq.add_with_priority(start, 0)
while pq.length > 0:
node_id = pq.extract_min()
visited.add(node_id) # Add node to MST
node = g.vert_list[node_id]
# Check all the neighbours and add them to PQ if needed.
for neigh in node.get_neighs():
if neigh not in visited:
# A node may be neigh of many nodes which are in MST
if node.get_weight(neigh) < distance[neigh]:
# Unlike dijkstra, just replace the distance with new edge.
distance[neigh] = node.get_weight(neigh)
previous[neigh] = node_id
pq.decrease_priority(neigh, distance[neigh])
print(previous)
print(distance)
