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Dijkstra's Algorithm

Python

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  • main.py
main.py
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#graph = {'a':{'b':4,'c':2},'b':{'d':5,'c':1},'c':{'d':8,'e':10},'d':{'e':2,'z':6},'e':{'z':5}}




graph = {'a':{'b':10, 'c':3}, 'b':{'c':1, 'd':2}, 'c':{'b':4,'d':8,'e':2},'d':{'e':7},'e':{'d':9}}
#dictionary for the graph (a links to b weight 4 and c weight 2)
#This means that node ‘a’ is connected to node ‘b’ with a weight of 10 and ‘a’ is also connected to node ‘c’ with a weight of 2.

def dijkstra(graph, start, goal):
  shortest_distance = {}
  predecessor = {}
  unseenNodes = graph
  infinity = 99999
  path = []

  #set all final distances to infinity except the start node
  for node in unseenNodes: #set all nodes apart from the starting node to infinity
    shortest_distance[node] = infinity
  shortest_distance[start] = 0

  #loop through the unseenNodes until there are none left so we need to check if the dictionary is empty
  while unseenNodes:
    minNode = None
    for node in unseenNodes:
      if minNode is None:
        minNode = node
      elif shortest_distance[node]< shortest_distance[minNode]:
         minNode = node
      
    #now the minimumNode has been identified, we need to check in all it's child nodes and their weights
    for childNode, weight in graph[minNode].items():
      if weight + shortest_distance[minNode] < shortest_distance[childNode]:
        shortest_distance[childNode] = weight + shortest_distance[minNode]
        predecessor[childNode] = minNode
    unseenNodes.pop(minNode)

  #we have found the distance, but now we need ot find the path by running the path backward and recording every step taken from the start to the goal node
  currentNode = goal
  while currentNode != start:
    try:
      path.insert(0,currentNode)
      currentNode = predecessor[currentNode]
    except KeyError:
      print("error")
      break
  path.insert(0,start)
  if shortest_distance[goal] != infinity:
    print("Shortest Distance: ",shortest_distance[goal])
    print("Path: ", path)

strt = str(input("Start: "))
gl = str(input("Goal: "))
dijkstra(graph,strt,gl)
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