Tell us what’s happening:
I can’t diagnose the issue with the code. The instruction is to create copy of the paths[current] list.
Instructions
The other bug is subtle. When a shorter distance is found for a neighbor node, paths[current] gets assigned to the neighbor node path, paths[node].
This means both variables point to the same list. Since lists are mutable, when you append the neighbor node to its path, both paths[node] and paths[current] are modified because they are the same list. This results in wrong paths, although the distances are correct.
Fix that bug by assigning a copy of paths[current] to the neighbor node path. Modify the existing assignment inside your if block.
What I modified:
# This
if paths[node] and paths[node][-1] == node:
paths[node] = paths[current]
#Changed to this
if paths[node] and paths[node][-1] == node:
paths[node] = paths[current].copy()
# This is not accepted when submitted
Your code so far
my_graph = {
'A': [('B', 3), ('D', 1)],
'B': [('A', 3), ('C', 4)],
'C': [('B', 4), ('D', 7)],
'D': [('A', 1), ('C', 7)]
}
def shortest_path(graph, start):
unvisited = list(graph)
distances = {node: 0 if node == start else float('inf') for node in graph}
paths = {node: [] for node in graph}
paths[start].append(start)
while unvisited:
current = min(unvisited, key=distances.get)
for node, distance in graph[current]:
if distance + distances[current] < distances[node]:
distances[node] = distance + distances[current]
# User Editable Region
if paths[node] and paths[node][-1] == node:
paths[node] = paths[current].copy()
# User Editable Region
else:
paths[node].extend(paths[current])
paths[node].append(node)
unvisited.remove(current)
print(f'Unvisited: {unvisited}\nDistances: {distances}\nPaths: {paths}')
shortest_path(my_graph, 'A')
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Challenge Information:
Learn Algorithm Design by Building a Shortest Path Algorithm - Step 47