Bug in Minimum Spanning Tree using Prim's algorithm

Question

I am trying to implement Prim's algorithm but the output changes every time I run it

{'A': {'C'}, 'B': {'A'}, 'F': {'G'}, 'E': {'B'}, 'D': {'E'}, 'C': {'F'}}

when it is supposed to be

{'D': {'E'}, 'E': {'B'}, 'B': {'A'}, 'A': {'C'}, 'C': {'F'}, 'F': {'G'}}

Not sure what exactly is going on here, I tried debugging to no avail. Does anyone know if I am missing something glaringly obvious?

from collections import defaultdict
import heapq


def create_spanning_tree(graph, starting_vertex):
    mst = defaultdict(set)
    visited = set([starting_vertex])
    edges = [
        (cost, starting_vertex, to)
        for to, cost in graph[starting_vertex].items()
    ]
    heapq.heapify(edges)
    while edges:
        cost, frm, to = heapq.heappop(edges)
        if to not in visited:
            visited.add(to)
            mst[frm].add(to)
            for to_next, cost in graph[to].items():
                if to_next not in visited:
                    heapq.heappush(edges, (cost, to, to_next))
    return mst

example_graph = {
    'A': {'B': 2, 'C': 3},
    'B': {'A': 2, 'C': 12, 'D': 10, 'E': 4},
    'C': {'A': 3, 'B': 12, 'F': 5},
    'D': {'B': 10, 'E': 7},
    'E': {'B': 4, 'D': 7, 'F': 16},
    'F': {'C': 5, 'E': 16, 'G': 9},
    'G': {'F': 9},
}
print(dict(create_spanning_tree(example_graph, 'D')))

paradocslover · Accepted Answer · 2020-05-17 08:22:06Z

1

Looking closely at both the outputs,

{'A': {'C'}, 'B': {'A'}, 'F': {'G'}, 'E': {'B'}, 'D': {'E'}, 'C': {'F'}} and {'D': {'E'}, 'E': {'B'}, 'B': {'A'}, 'A': {'C'}, 'C': {'F'}, 'F': {'G'}} are same.

Since it's a dictionary, (key, value) pairs matter, which is same in both the answers and not the order in which they appear.

edited May 17, 2020 at 8:22

answered Apr 17, 2020 at 7:02

paradocslover

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