Not able to save the current state of variables in the global array in nested functions in Python (Very confusing title, please read description)

Question

I have made this python program to solve a sudoku and save all its possible valid answers in an array and return it. But it is failing in doing so. The logic is perfect and is being executed perfectly. But the problem is that that in both the nested else in the solver function, I want to save the current situation of the board in allAns array, but am not able to do so. Everytime I tried, it saved the default value (one with the dots) in the allAns array. I tried using nonlocal variables, passing in the allAns array as a parameter and many more, but am unable to do so. Couldn't find anything of help on the web. This place is my last hope

def solveSudoku(board: list[list[str]]) -> None:
    allAns = []
    board1 = board.copy()
    def totalEle(i: int, j: int, allEle: list[int]):
        for b in range(9):
            if board[i][b] != '.':
                allEle[int(board[i][b]) - 1] = False
            if board[b][j] != '.':
                allEle[int(board[b][j]) - 1] = False
            boxI = (i // 3) * 3 + b // 3
            boxJ = (j // 3) * 3 + b % 3
            if board[boxI][boxJ] != '.':
                allEle[int(board[boxI][boxJ]) - 1] = False

    def solver(i: int, j: int):
        if board[i][j] == '.':
            cannotBe = [True for _ in range(9)]
            totalEle(i, j, cannotBe)
            for k in range(1, 10):
                if cannotBe[k-1]:
                    board[i][j] = str(k)
                    if j < 8:
                        solver(i, j + 1)
                    elif i < 8:
                        solver(i + 1, 0)
                    else:
                        for m in range(9):
                            for n in range(9):
                                board1[m][n] = board[m][n]
                        allAns.append(board1)
                    board[i][j] = '.'
        else:
            if j < 8:
                solver(i, j + 1)
            elif i < 8:
                solver(i + 1, 0)
            else:
                for m in range(9):
                    for n in range(9):
                        board1[m][n] = board[m][n]
                allAns.append(board1)
    solver(0, 0)
    print(allAns)
    return allAns

sudoku =   [["5","3",".",".","7",".",".",".","."],
            ["6",".",".","1","9","5",".",".","."],
            [".","9","8",".",".",".",".","6","."],
            ["8",".",".",".","6",".",".",".","3"],
            ["4",".",".","8",".","3",".",".","1"],
            ["7",".",".",".","2",".",".",".","6"],
            [".","6",".",".",".",".","2","8","."],
            [".",".",".","4","1","9",".",".","5"],
            [".",".",".",".","8",".",".","7","9"]]

allAnswers = solveSudoku(sudoku)

Observations: `cannotBe` is always a list of `True` and never changes. `allAns.append(board1)` only runs once. And `allAns.append(board)` never runs. — Dennis Williamson, Nov 11 '22 at 22:03
No man, not true. ```cannotBe``` changes every time according to the current status of the board array. I confirmed it during debugging and as I said, the logic is perfect. If I print the board1 just before I append the board1 into allAns, I can see a completely solved sudoku. — Aayush Gupta, Nov 12 '22 at 07:55
I only see `cannotBe` being assigned a list of `True` and never being modified. Regardless, I believe the answer I posted solves the problem with `allAns`. — Dennis Williamson, Nov 12 '22 at 13:05

score 0 · Accepted Answer · answered Nov 12 '22 at 13:04

0

The issue is due to scope in a recursive function. You can use deepcopy().

from copy import deepcopy
def solveSudoku(board: list[list[str]]) -> None:
    allAns = []
    board1 = deepcopy(board)
    # etc...

See this answer for more.

answered Nov 12 '22 at 13:04

Dennis Williamson

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Thanks mann!!! Deepcopy solved that problem. And thanks for the link. – Aayush Gupta Nov 13 '22 at 20:49
But I don't understand why even after copying every single value of board into board1 (via nested for loops), the values revert back to that of the original board. How can be board1 a reference to the values of board?? – Aayush Gupta Nov 13 '22 at 20:53

Not able to save the current state of variables in the global array in nested functions in Python (Very confusing title, please read description)

1 Answers1