Kumayl Fazal Kumayl Fazal - 2 days ago 8
Python Question

Dijkstra’s Algorithm in python

I am trying to implement Dijkstra’s algorithm on my python code but I can't really get the algorithm right. The algorithm I am using is from this youtube link: https://www.youtube.com/watch?v=pVfj6mxhdMw

So basically my class has these 3 variables:

self.nodes = [] #a,b,c
self.neighbours = {} # a:[b,c], b:[c], c:[a]
self.weights = {} #[a,b] = 2, [a,c] = 5


Here is how I partially implemented my shortest path function using the algorithm provided in the video:

def dijkstra(self, start, end):

nodes = {}

for n in self.nodes:
if n == start:
nodes[n] = 0
else:
nodes[n] = float('inf')

unvisited = self.neighbours
visited = []
current_node = start
current_distance = 0

while unvisited:
for n in unvisited[current_node]:
print(n)
#calc_weight = nodes[n] + self.weights[n, current_node]
#if (unvisited[n] is None or calc_weight > nodes[n]):
#nodes[n] = calc_weight
visited.append(current_node)
del unvisited[current_node]

if not unvisited: break


I havent really completed because I know I missing something out somewhere. Can someone please help me with this. Thank you

Answer
def dijkstra(self, start, end):

    nodes = self.neighbours #{'A': {'B':2}, 'B': {'C':4}, ... }

    unvisited = {n: 1000 for n in self.nodes} #unvisited node & distance
    unvisited[start] = 0 #set start vertex to 0
    visited = {} #list of all visited nodes
    parent = {} #predecessors

    while unvisited:
        min_node = min(unvisited, key=unvisited.get) #get smallest distance

        for neighbour in nodes[min_node].items():
            if neighbour not in visited:
                new_distance = unvisited[min_node] + nodes[min_node][neighbour]
                if new_distance < unvisited[neighbour]:
                    unvisited[neighbour] = new_distance
                    parent[neighbour] = min_node

        visited[min_node] = unvisited[min_node]
        unvisited.pop(min_node)

    print(parent, visited)
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