After debugging for more than an hour, I found the problem in my code. I do not understand why it updates board[y][x] but not board itself. Here is the code:

puzzle = [
    [0, 0, 7, 0, 0, 6, 2, 0, 0],
    [1, 0, 0, 8, 9, 0, 7, 6, 0],
    [8, 0, 2, 0, 7, 3, 0, 5, 0],
    [0, 0, 0, 1, 0, 9, 5, 0, 6],
    [0, 1, 0, 0, 0, 0, 0, 8, 0],
    [7, 0, 3, 6, 0, 8, 0, 0, 0],
    [0, 7, 0, 2, 4, 0, 6, 0, 3],
    [0, 4, 1, 0, 6, 7, 0, 0, 5],
    [0, 0, 9, 3, 0, 0, 1, 0, 0]
];

function find_empty(board) {
    for (let y = 0; y < 9;   y) {
        for (let x = 0; x < 9;   x) {
            if (board[y][x] === 0) {
                return [y, x];
            }
        }
    }
    return null;
}

function is_valid(board, num, pos) {
    let y = pos[0];
    let x = pos[1];

    // Horizontal checking...
    for (let i = 0; i < 9;   i) {
        if (board[y][i] === num && x !== i) {
            return false;
        }
    }

    // Vertical checking
    for (let i = 0; i < 9;   i) {
        if (board[i][x] === num && y !== i) {
            return false;
        }
    }

    // 3*3 box checking
    box_start_y = (Math.floor(y / 3)) * 3;
    box_start_x = (Math.floor(x / 3)) * 3;

    for (let i = box_start_y; i < box_start_y   3;   i) {
        for (let j = box_start_x; j < box_start_x   3;   j) {
            if (board[i][j] === num && y !== i && x !== j) {
                return false;
            }
        }
    }

    return true;
}

function solve_board(board) {
    console.log("start", board);
    empty_pos = find_empty(board);
    console.log("empty_pos", empty_pos)
    if (empty_pos === null) {
        return board;
    }
        
    let y = empty_pos[0];
    let x = empty_pos[1];

    for (let num = 1; num < 10;   num) {
        console.log(board)
        if (is_valid(board, num, empty_pos)) {
            console.log(true, num)
            console.log("Before assignment", board[y][x]);
            board[y][x] = num;
            console.log("y", y, ", x", x);
            console.log("After true", board[y][x])
            console.log("After true", board);
            let solved_board = solve_board(board)
            console.log("solved", solved_board);
            if (solved_board !== false) {
                return solved_board;
            }
            console.log(num, "did not work!!")
            board[y][x] = 0;
        }
        console.log(false, num)
    }
    return false;
}

console.log("result", solve_board(puzzle))

The problem is on line 72 in solve_board function. It updates board[y][x] as seen in the console.logs, but does not affect board. Is it some reference issue or pass by value problem?

If anyone can help me solve this issue, I would be really grateful.

CodePudding user response：

you are assigning the new value correctly. Arrays are passed to the function by reference, so there should be no problem. You can verify it with this code snippet:

const arr = [[0, 0, 0], [0, 0, 0], [0, 0, 0]]

function foo(x) {
  x[0][0] = 2
}

foo(arr)
console.log(arr[0][0]) // 2

There probably is some issue with the game logic itself, the conditions in the is_valid don't look right. Consider taking a look on this article explaining how to solve sudoku programmatically with logic and without brute force.

Alternatively, I suggest you look at the backtracking algorithm, which tries all possible options, which should be straightforward to implement.