在python中使用BFS的Unweighted-Single-Source-Shortest-Path

Question

Im new at python. I try to get the Unweighted-Single-Source-Shortest-Path using BFS.

from queue import *


def ussp(graph, s):
    len_graph = len(graph)
    prev = [[]*len_graph for i in range(len_graph)]
    visited = [False for i in range(len_graph)]
    queue2 = Queue
    dist = [[float('inf')] for i in range(len_graph)]
    queue2.put_nowait(graph[s], 0)  # starting with s
    visited[s] = True
    dist[s] = 0

    # modified BFS alg.
    while queue2.empty() == False:
        h = queue2.get_nowait()
        for i in len(graph[h]):
            if visited[graph[h][i]] == False:
                visited[graph[h][i]] = True
                dist[graph[h][i]] = dist[h] + 1
                prev[graph[h][i]] = h
                queue2.put_nowait(graph[h][i], 0)
    print(dist)


graph2 = {1: [2, 3, 5], 2: [4, 6, 1], 3: [5, 1], 4: [6], 5: [2], 6: [1, 7], 7: [2]}
ussp(graph2, 1)

Thats what I got for now. I was pretty sure it should work, but it does not at all. It doesn't even get compiled. Im pretty new with lists, arrays and queues in python, too. Would be kind if you could help me. Thanks in advance

Answer 1

First, I added in a destination parameter to your function signature. Assuming you wanted to find the shortest path from node 1 to node 7, the program below works. I also added some python boilerplate since you said you are new to python.

import sys
from queue import Queue as Q

def ussp(graph, s, d):
    len_graph = len(graph)

    prev = [ -1 for i in range(len_graph)]
    visited = [False for i in range(len_graph)]
    q = Q()
    dist = [sys.maxsize for i in range(len_graph)]
    q.put(s, False)
    visited[s-1] = True
    dist[s-1] = 0

    # modified BFS alg.
    while q.empty() == False:
        h = q.get_nowait()
        for i in range(len(graph[h])):
            if visited[graph[h][i]-1] == False:
                visited[graph[h][i]-1] = True
                dist[graph[h][i]-1] = dist[h-1] + 1
                prev[graph[h][i]-1] = h
                q.put_nowait(graph[h][i])

    path = []
    crawl = d # destination
    path.append(crawl)
    while (prev[crawl-1] != -1):
        path.append(prev[crawl-1])
        crawl = prev[crawl-1]

    print(list(reversed(path)))


def main():
    graph2 = {1: [2, 3, 5], 2: [4, 6, 1], 3: [5, 1], 4: [6], 5: [2], 6: [1, 7], 7: [2]}
    ussp(graph2, 1, 7)

if __name__ == '__main__':
    main()

Answer 2

This line is the cause of your error:

queue2 = Queue

You're setting queue2 equal to the Queue class, instead of an instance of the Queue class.

Change that line to this:

queue2 = Queue()