def prim(aGraph, startVertex):
# create a priority queue that uses distance as the value to determine priority and thus its position
# use the distance to the vertex as the priority because while exploring the next vertex, want to explore the vertex that has the smallest distance
# decreaseKey method used when the distance to a vertex that is already in the queue is reduced, and thus moves that vertex toward the front of the queue.
pQueue = PriorityQueue()
# initialize the state of the graph
for vertex in aGraph:
# initialally all vertices values are = infinity (sys.maxint) because we assume the greatest value and then update appropiately
vertex.setDistance(sys.maxsize)
vertex.setPred(None)
# distance represents distance from the startVertex, trivially 0 for the startVertex
startVertex.setDistance(0)
# key value pair
# key is distance and vertex is the value
pQueue.buildHeap([ (vertex.getDistance(), vertex) for vertex in aGraph ])
while not pQueue.isEmpty():
currentVertex = pQueue.delMin()
# iterate over the currentVertex's edges
for nextVertex in currentVertex.getConnections():
# calculate the weight from currentVertex to nextVertex
newCost = currentVertex.getWeight(nextVertex)
# found a shorter path
# node is not considered to be part of the spanning tree until it is removed from the priority queue.
# nextVertex in pQueue means that vertex is not yet in the spanning tree so it is safe to add (ensures an acyclic graph)
if nextVertex in pQueue and newCost< nextVertex.getDistance():
# assign the predecessor appropiately
nextVertex.setPred(currentVertex)
# set a new distance on the nextVertex
nextVertex.setDistance(newCost)
# update the priorityQueue with the correct values
pQueue.decreaseKey(nextVertex, newCost)
def dijkstra(aGraph, startVertex):
# create a priority queue that uses distance as the value to determine priority and thus its position
# use the distance to the vertex as the priority because while exploring the next vertex, want to explore the vertex that has the smallest distance
# decreaseKey method used when the distance to a vertex that is already in the queue is reduced, and thus moves that vertex toward the front of the queue.
pQueue = PriorityQueue()
# distance represents distance from the startVertex, trivially 0 for the startVertex
# initialally all vertices values are = infinity (sys.maxint) because we assume the greatest value and then update appropiately
startVertex.setDistance(0)
# key value pair
# key is distance and vertex is the value
pQueue.buildHeap([ (vertex.getDistance(), vertex) for vertex in aGraph ])
while not pQueue.isEmpty():
currentVertex = pQueue.delMin()
# iterate over the currentVertex's edges
for nextVertex in currentVertex.getConnections():
# distance of current vertex and the weight of it's edges
newDistance = currentVertex.getDistance() + currentVertex.getWeight(nextVertex)
# found a shorter path
if newDistance < nextVertex.getDistance():
# set a new distance on the nextVertex
nextVertex.setDistance(newDistance)
# assign the predecessor appropiately
nextVertex.setPred(currentVertex)
# update the priorityQueue with the correct values
pQueue.decreaseKey(nextVertex, newDistance)
def Dijkstras(graph,start): pq=PriorityQueue() start.setDistance(0) pq.buildHeap([(v.getDistance(),v) for v in graph]) # distance is the key in the priority queue while not pq.isEmpty(): currentvertex=pq.delMin() for newvertex in currentvertex.getConnections(): newDist=currentvertex.getDistance()+currentvertex.getWeight(newvertex) if newDist<newvertex.getDistance(): # at the start the distance of all the vertices is set to maximum. That's why the if statement will be executed newvertex.setDistance(newDist) newvertex.setPredecessor(currentvertex) pq.decreaseKey(newvertex,newDist)
def Dijkstras(graph,start): pq=PriorityQueue() start.setDistance(0) pq.buildHeap([(v.getDistance(),v) for v in graph]) # distance is the key in the priority queue while not pq.isEmpty(): currentvertex=pq.delMin() for newvertex in currentvertex.getConnections(): newDist=currentvertex.getDistance()+currentvertex.getWeight(newvertex) if newDist<newvertex.getDistance(): # at the start the distance of all the vertices is set to maximum. That's why the if statement will be executed newvertex.setDistance(newDist) newvertex.setPredecessor(currentvertex) pq.decreaseKey(newvertex,newDist)
def dijkstra(aGraph, start):
pq = PriorityQueue()
start.setDistance(0)
pq.buildHeap([(v.getDistance(), v) for v in aGraph])
while not pq.isEmpty():
currentVert = pq.delMin()
for nextVert in currentVert.getConnections():
newDist = currentVert.getDistance() \
+ currentVert.getWeight(nextVert)
if newDist < nextVert.getDistance():
nextVert.setDistance(newDist)
nextVert.setPred(currentVert)
pq.decreaseKey(nextVert, newDist)
def prim(graph,start): # it belongs to the family of greedy algorithms pq=PriorityQueue() for v in graph: v.setDistance(sys.maxsize) v.setPredecessor(None) start.setDistance(0) pq.buildHeap([(v.getDistance(),v) for v in graph]) while not pq.isEmpty() currentvertex=pq.delMin() for newvertex in currentvertex.getConnections(): newDist=currentvertex.getWeight(newvertex) if newDist<newvertex.getDistance() and newvertex in pq: newvertex.setDistance(newDist) newvertex.setPredecessor(currentvertex) pq.decreaseKey(newvertex,newDist)
def dijkstra(g, start):
# print('Finding shortest paths to ' + start.payload['city'])
g.reset()
pq = PriorityQueue()
start.setDistance(0)
pq.buildHeap([(v.getDistance(), v) for v in g])
while not pq.isEmpty():
currentVert = pq.delMin()
for nextVert in currentVert.getConnections():
newDist = currentVert.getDistance() \
+ currentVert.getCost(nextVert)
if newDist < nextVert.getDistance():
# print('Setting {0}s predecessor to {1} (distance {2})'.format(nextVert.payload['city'], currentVert.payload['city'], round(newDist)))
nextVert.setDistance(newDist)
nextVert.parent = currentVert
pq.decreaseKey(nextVert, newDist)
def prim(G, start):
pq = PriorityQueue()
for v in G:
v.setDistance(sys.maxsize)
v.setPred(None)
start.setDistance(0)
pq.buildHeap([(v.getDistance(), v) for v in G])
print("\nOrder edges added in Prim's algorithm:")
while not pq.isEmpty():
currentVert = pq.delMin()
# extra print to trace order
if currentVert.getPred() != None:
print("Prim: edge",
currentVert.getPred().getId(), "to", currentVert.getId())
for nextVert in currentVert.getConnections():
newCost = currentVert.getWeight(nextVert)
# Printed textbook has incorrect "+ currentVert.getDistance()" term
if nextVert in pq and newCost < nextVert.getDistance():
nextVert.setPred(currentVert)
nextVert.setDistance(newCost)
pq.decreaseKey(nextVert, newCost)
print("\n")
