iframe-proxy | Sunbelt Computer Software

73 lines (56 loc) · 2.25 KB

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

Sunbelt Computer Software

PL/B Language Development and Support

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

Sunbelt Computer Software

PL/B Language Development and Support

Uh oh!

FilesExpand file tree

bellman_ford.py

Latest commit

History

bellman_ford.py

File metadata and controls