Python 迷宫最短路径上的 BFS 搜索无法解决，我正在尝试执行广度优先搜索，以查找在网格中具有 4 次移动的代理

Question

I am trying to perform a breadth first search for an agent that has 4 moves across the grid that is read in through a text file. I cannot get it to run for some reason. although I follow the algorithm as in my textbook. I am new to coding and any help would be appreciated. This is my first Stack Overflow post, its nice to join you folks:) Please don't roast me:( edit** the text is in a file and is something like this: 3 0 0 0 0 1 0 0 3 3 3 0 3 0 0 0 0 0 3 3 3 0 3 0 0 0 0 0 3 3 3 0 3 0 0 0 0 0 3 3 3 0 3 0 0 0 0 0 3 3 3 0 3 0 0 0 0 2 3 3 3 0 3 0 0 0 0 3 3 3 3 0 3 0 0 0 0 3 3 3 3 0 3 2 2 0 0 3 3 3 3 3 3 3 3 2 0 3 3 3. edit**It is presently crashing and I do not understand how to fix the errors as every error I fix causes more errors to appear, I simply want to get it to display the output of moves in a sequence of 'Move Up', 'Move Down' etc.

import collections

    def read_user_input():
        file_name = input('Enter the name of your file :\n')
        return file_name
    
    
    def read_to_grid():
        file_name = read_user_input()
        for nums in open(file_name):
            line = list(nums.split())
            result = []
            for _ in range(0, len(line), 10):
                result.append(line[_:_ + 10])
            print(result)
            return result
        file_name.close()
    
    
    def grid_start_position():
        grid = read_to_grid()
        for x, pos in enumerate(grid[0]):
            if pos == "O":
                start = x
                return start
    
    
    def bfs():
        grid = read_to_grid()
        start = grid_start_position()
        queue = collections.deque([[start]])
        seen = set([start])
        while queue:
            path = queue.popleft()
            path_moves = ''
            x, y = path[-1]
            if grid[y][x] == '2':
                return path
            for x2, y2 in (x + 1, y):
                if 0 <= x2 < 10 and 0 <= y2 < 10 and grid[x2][y2] != '3' and (x2, y2) not in seen:
                    queue.append(path + [(x2, y2)])
                    path_moves = path_moves + 'Move Up '
                    seen.add((x2, y2))
            for x2, y2 in (x - 1, y):
                if 0 <= x2 < 10 and 0 <= y2 < 10 and grid[x2][y2] != '3' and (x2, y2) not in seen:
                    queue.append(path + [(x2, y2)])
                    path_moves = path_moves + 'Move Down '
                    seen.add((x2, y2))
            for x2, y2 in (x, y + 1):
                if 0 <= x2 < 10 and 0 <= y2 < 10 and grid[x2][y2] != '3' and (x2, y2) not in seen:
                    queue.append(path + [(x2, y2)])
                    path_moves = path_moves + 'Move Right'
                    seen.add((x2, y2))
            for x2, y2 in (x, y - 1):
                if 0 <= x2 < 10 and 0 <= y2 < 10 and grid[x2][y2] != '3' and (x2, y2) not in seen:
                    queue.append(path + [(x2, y2)])
                    path_moves = path_moves + 'Move Left'
                    seen.add((x2, y2))
    
            print(path_moves)
    
    
    bfs()

Answer 1

Some issues:

• grid_start_position only returns an x-coordinate, and you put that start in the queue, yet in the loop you expect the queue to have coordinate- pairs (x and y). You should better update grid_start_position to return an x, y pair. NB: it may need an extra loop on the y-coordinate, as you can not be sure that the start is always on the first line?

• There is a if pos == "O" condition there, that is never true, as the input you provided only has numeric data, so no letter "O". This should probably be a "0" (zero).

• You call read_to_grid() twice: once in bfs and then again in grid_start_position . Instead you should pass the grid in the call to grid_start_position .

• for x2, y2 in (x + 1, y) will not work. You could say x2, y2 = (x + 1, y) , but your in operator will try to unpack x + 1 into two values. In order to avoid the code repetition you have, you should probably make a loop like this:

   for move, x2, y2 in (("Right", x + 1, y), ("Left", x - 1, y), ("Up", x, y - 1), ("Down", x, y + 1)):

• With path_moves = path_moves + "some string" you keep adding all nodes you are visiting to that path, which makes no sense: this is not a path at all. You need a different technique, where you keep back-references ("where I came from") for each of the nodes that you visit. Then when you arrive at the target, you should (only then.) reconstruct the path from that information.

I will not provide the corrected code, as this has been solved many times before. You can for instance take inspiration from How do I find shortest path in maze with BFS? , where the OP had a similar issue with building the path.