Question

I am having an error that I don’t understand in my Dijkstra Algorithm code – here is the error message:

Traceback (most recent call last):
  File "C:\Documents and Settings\Harvey\Desktop\algorithm.py", line 52, in <module>
    tentativeDistance(currentNode,populateNodeTable())
  File "C:\Documents and Settings\Harvey\Desktop\algorithm.py", line 29, in tentativeDistance
    currentDistance = nodeTable[currentNode].distFromSource + network[currentNode][nearestNeighbour] #gets current distance from source
TypeError: object cannot be interpreted as an index

Here is my code:

infinity = 1000000
invalid_node = -1
startNode = 0

class Node:
     distFromSource = infinity
     previous = invalid_node
     visited = False

def populateNodeTable(): 
    nodeTable = []
    index =0
    f = open('route.txt', 'r')
    for line in f: 
      node = map(int, line.split(',')) 
      nodeTable.append(Node()) 
      print nodeTable[index].previous 
      print nodeTable[index].distFromSource 
      index +=1
    nodeTable[startNode].distFromSource = 0 
    #currentNode = nodeTable[startNode] 

    return nodeTable

def tentativeDistance(currentNode, nodeTable):
    nearestNeighbour = []
    #j = nodeTable[startNode]
    for currentNode in nodeTable:
      currentDistance = nodeTable[currentNode].distFromSource + network[currentNode][nearestNeighbour] #gets current distance from source
      if currentDistance != 0 & NodeTable[currentNode].distFromSource < Node[currentNode].distFromSource:
         nodeTable[currentNode].previous = currentNode
         nodeTable[currentNode].length = currentDistance
         nodeTable[currentNode].visited = True
         nodeTable[currentNode] +=1
         nearestNeighbour.append(currentNode)
      print nearestNeighbour

    return nearestNeighbour

currentNode = startNode

if __name__ == "__main__":
    populateNodeTable()
    tentativeDistance(currentNode,populateNodeTable())

My first function performs correctly, and my logic is correct for my second function though searching online for the solution has proved fruitless

Answer 1

Given the way for loops work in Python, you don’t have to write

for currentNode in nodeTable:
    currentDistance = nodeTable[currentNode].distFromSource + network[currentNode][nearestNeighbour] #gets current distance from source

You should instead write:

for currentNode in nodeTable:
    currentDistance = currentNode.distFromSource + network[currentNode][nearestNeighbour]

Assuming network is a dictionary with nodes for keys, that will work fine.