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* Pyupgrade to Python 3.9
* updating DIRECTORY.md
Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
74 lines
2.3 KiB
Python
74 lines
2.3 KiB
Python
from __future__ import annotations
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def print_distance(distance: list[float], src):
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print(f"Vertex\tShortest Distance from vertex {src}")
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for i, d in enumerate(distance):
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print(f"{i}\t\t{d}")
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def check_negative_cycle(
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graph: list[dict[str, int]], distance: list[float], edge_count: int
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):
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for j in range(edge_count):
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u, v, w = (graph[j][k] for k in ["src", "dst", "weight"])
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if distance[u] != float("inf") and distance[u] + w < distance[v]:
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return True
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return False
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def bellman_ford(
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graph: list[dict[str, int]], vertex_count: int, edge_count: int, src: int
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) -> list[float]:
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"""
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Returns shortest paths from a vertex src to all
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other vertices.
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>>> edges = [(2, 1, -10), (3, 2, 3), (0, 3, 5), (0, 1, 4)]
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>>> g = [{"src": s, "dst": d, "weight": w} for s, d, w in edges]
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>>> bellman_ford(g, 4, 4, 0)
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[0.0, -2.0, 8.0, 5.0]
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>>> g = [{"src": s, "dst": d, "weight": w} for s, d, w in edges + [(1, 3, 5)]]
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>>> bellman_ford(g, 4, 5, 0)
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Traceback (most recent call last):
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...
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Exception: Negative cycle found
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"""
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distance = [float("inf")] * vertex_count
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distance[src] = 0.0
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for i in range(vertex_count - 1):
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for j in range(edge_count):
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u, v, w = (graph[j][k] for k in ["src", "dst", "weight"])
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if distance[u] != float("inf") and distance[u] + w < distance[v]:
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distance[v] = distance[u] + w
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negative_cycle_exists = check_negative_cycle(graph, distance, edge_count)
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if negative_cycle_exists:
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raise Exception("Negative cycle found")
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return distance
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if __name__ == "__main__":
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import doctest
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doctest.testmod()
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V = int(input("Enter number of vertices: ").strip())
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E = int(input("Enter number of edges: ").strip())
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graph: list[dict[str, int]] = [dict() for j in range(E)]
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for i in range(E):
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print("Edge ", i + 1)
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src, dest, weight = (
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int(x)
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for x in input("Enter source, destination, weight: ").strip().split(" ")
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)
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graph[i] = {"src": src, "dst": dest, "weight": weight}
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source = int(input("\nEnter shortest path source:").strip())
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shortest_distance = bellman_ford(graph, V, E, source)
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print_distance(shortest_distance, 0)
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