Learn Classes and Objects by Building a Sudoku Solver - Step 73

Tell us what’s happening:

You should assign a list comprehension that iterates over row to the row_str variable.

Your code so far

class Board:
    def __init__(self, board):
        self.board = board

# User Editable Region

    def __str__(self):
        board_str = ''
        for row in self.board:
            row_str = str([[] for i in row])

# User Editable Region

    def find_empty_cell(self):
        for row, contents in enumerate(self.board):
            try:
                col = contents.index(0)
                return row, col
            except ValueError:
                pass
        return None

    def valid_in_row(self, row, num):
        return num not in self.board[row]

    def valid_in_col(self, col, num):
        return all(self.board[row][col] != num for row in range(9))

    def valid_in_square(self, row, col, num):
        row_start = (row // 3) * 3
        col_start = (col // 3) * 3
        for row_no in range(row_start, row_start + 3):
            for col_no in range(col_start, col_start + 3):
                if self.board[row_no][col_no] == num:
                    return False
        return True

    def is_valid(self, empty, num):
        row, col = empty
        valid_in_row = self.valid_in_row(row, num)
        valid_in_col = self.valid_in_col(col, num)
        valid_in_square = self.valid_in_square(row, col, num)
        return all([valid_in_row, valid_in_col, valid_in_square])

    def solver(self):
        if (next_empty := self.find_empty_cell()) is None:
            return True
        for guess in range(1, 10):
            if self.is_valid(next_empty, guess):
                row, col = next_empty
                self.board[row][col] = guess
                if self.solver():
                    return True
                self.board[row][col] = 0
        return False

def solve_sudoku(board):
    gameboard = Board(board)
    print(f'Puzzle to solve:\n{gameboard}')
    if gameboard.solver():
        print(f'Solved puzzle:\n{gameboard}')
    else:
        print('The provided puzzle is unsolvable.')
    return gameboard

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

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Challenge Information:

Learn Classes and Objects by Building a Sudoku Solver - Step 73

declare a variable row_str and assign it a list comprehension that iterates over row and turns each item i in row into a string.

Considering an example list comprehension:

[expression for i in iterable]

You want the expression to be something that turns i into a string. How do you do that using str()?

Right now you are trying to turn a list into a string.

1 Like

row_str = [ for str(i) in row]
It is still showing error.

You are using an empty list as expression and you broke it the iteration part.

The part on the right side of the for keyword is fine

[<expression> for i in row]

You need to write an expression that turns i into a string and put it in place of <expression>.

row_str = [[str(i)] for str(i) in row]

Please help. I am not able to get the answer.

You are getting closer but there’s a logic problem here:

This means create a list in which every element from row is converted into a string and put it into a list.
You end up with a list of lists, with each internal list containing a string:
[[<string>], [<string>], [<string>], ...]
while you want:
[<string>, <string>, <string>, ...]

Actually, I missed a bit. You should keep the for i in row part as it is.
i is the variable that you need to iterate over row. You cannot perform operations on it.

Thanks a lot. I got it.

1 Like