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07_dijkstras_algorithm.py
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07_dijkstras_algorithm.py
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def find_lowest_cost_node(costs):
# Initialize the lowest cost and node
lowest_cost = float("inf")
lowest_cost_node = None
# Go through each node.
for node in costs:
cost = costs[node]
# If it's the lowest cost so far and hasn't been processed yet...
if cost < lowest_cost and node not in processed:
# ... set it as the new lowest-cost node.
lowest_cost = cost
lowest_cost_node = node
# Return the node with the lowest cost
return lowest_cost_node
if __name__ == "__main__":
# Initialize the graph
graph = {}
graph["start"] = {}
graph["start"]["a"] = 6
graph["start"]["b"] = 2
graph["a"] = {}
graph["a"]["fin"] = 1
graph["b"] = {}
graph["b"]["a"] = 3
graph["b"]["fin"] = 5
graph["fin"] = {}
# Initialize the costs table
infinity = float("inf")
costs = {"a": 6, "b": 2, "fin": infinity}
# Initialize the parents table
parents = {"a": "start", "b": "start", "fin": None}
# Initialize the processed nodes list
processed = []
# Find the lowest-cost node that you haven't processed yet.
node = find_lowest_cost_node(costs)
# If you've processed all the nodes, this while loop is done.
while node is not None:
cost = costs[node]
# Go through all the neighbors of this node.
neighbors = graph[node]
for n in neighbors.keys():
new_cost = cost + neighbors[n]
# If it's cheaper to get to this neighbor by going through this node...
if costs[n] > new_cost:
# ... update the cost for this node.
costs[n] = new_cost
# This node becomes the new parent for this neighbor.
parents[n] = node
# Mark the node as processed.
processed.append(node)
# Find the next node to process, and loop.
node = find_lowest_cost_node(costs)
print("Cost from the start to each node:")
print(costs)