Python breadth-first search matrix print path

Question

I have this line of code that tests wether or not there is a path to be found in a labyrinth represented by a matrix. How do I print the path at the end after I\'ve determined w

Answer 1

Here's a fairly simple way to find the path, by tracing it backwards from the exit. You can reverse this by saving the moves to a stack.

To make it easier to see, I've changed your code slightly to put N, S, E, W in the matrix (for North, South, East, West). To decode those direction letters to (row, column) steps I use a dictionary.

from queue import Queue

maze='''\
0 0 0 0 1 0 0 0
0 1 1 0 1 0 1 0
0 1 0 0 1 0 1 0
0 0 0 1 0 0 1 0
0 1 0 1 0 1 1 0
0 0 1 1 0 1 0 0
1 0 0 0 0 1 1 0
0 0 1 1 1 1 0 0
'''

def show(matrix):
    for line in matrix:
        print(*line)
    print()

matrix=maze.splitlines()
matrix=[i.strip() for i in matrix]
matrix=[i.split() for i in matrix]
numrows, numcols = len(matrix), len(matrix[0])

show(matrix)

q=Queue()
row=0
col=0
q.put((row,col))
while not q.empty():
    row, col = q.get()
    if col+1 < numcols and matrix[row][col+1] == "0":
         q.put((row, col+1))
         matrix[row][col+1] = "W"
    if row+1 < numrows and matrix[row+1][col] == "0":
         q.put((row+1, col))
         matrix[row+1][col] = "N"
    if 0 <= col-1 and matrix[row][col-1] == "0":
         q.put((row, col-1))
         matrix[row][col-1] = "E"
    if 0 <= row-1 and matrix[row-1][col] == "0":
         q.put((row-1, col))
         matrix[row-1][col] = "S" 
row,col=numrows-1,numcols-1
var=matrix[row][col]

show(matrix)

if var=="0":
    print ("There is no path.")
    exit()
else:
    print ("There is a path.")

# Trace the path
step = {'N': (-1, 0), 'S': (1, 0), 'E': (0, 1), 'W': (0, -1)}
while True:
    print((row, col), 'go', var)
    if row == 0 and col == 0:
        break
    r, c = step[var]
    row += r
    col += c
    var = matrix[row][col]

print('Home!')

output

0 0 0 0 1 0 0 0
0 1 1 0 1 0 1 0
0 1 0 0 1 0 1 0
0 0 0 1 0 0 1 0
0 1 0 1 0 1 1 0
0 0 1 1 0 1 0 0
1 0 0 0 0 1 1 0
0 0 1 1 1 1 0 0

E W W W 1 S W W
N 1 1 N 1 S 1 N
N 1 E N 1 S 1 N
N W W 1 S W 1 N
N 1 N 1 S 1 1 N
N W 1 1 S 1 E N
1 N W W W 1 1 N
E N 1 1 1 1 E N

There is a path.
(7, 7) go N
(6, 7) go N
(5, 7) go N
(4, 7) go N
(3, 7) go N
(2, 7) go N
(1, 7) go N
(0, 7) go W
(0, 6) go W
(0, 5) go S
(1, 5) go S
(2, 5) go S
(3, 5) go W
(3, 4) go S
(4, 4) go S
(5, 4) go S
(6, 4) go W
(6, 3) go W
(6, 2) go W
(6, 1) go N
(5, 1) go W
(5, 0) go N
(4, 0) go N
(3, 0) go N
(2, 0) go N
(1, 0) go N
(0, 0) go E
Home!