Sudoku Checker in Python

Question

I am trying to create a sudoku checker in python:

ill_formed = [[5,3,4,6,7,8,9,1,2],
              [6,7,2,1,9,5,3,4,8],
              [1,9,8,3,4,2,5,6,7],
              [8,5,9,7,6,1,4,2,3],
              [4,2,6,8,5,3,7,9],  # <---
              [7,1,3,9,2,4,8,5,6],
              [9,6,1,5,3,7,2,8,4],
              [2,8,7,4,1,9,6,3,5],
              [3,4,5,2,8,6,1,7,9]]
easy = [[2,9,0,0,0,0,0,7,0],
       [3,0,6,0,0,8,4,0,0],
       [8,0,0,0,4,0,0,0,2],
       [0,2,0,0,3,1,0,0,7],
       [0,0,0,0,8,0,0,0,0],
       [1,0,0,9,5,0,0,6,0],
       [7,0,0,0,9,0,0,0,1],
       [0,0,1,2,0,0,3,0,6],
       [0,3,0,0,0,0,0,5,9]]

I am expecting input like that- a list of 9 lists. The zeros represent number that have not been filled in by the user. They can appear multiple times in a row, column or 3x3.

def check_sudoku(grid):
if len(grid) == 9:
    numsinrow = 0
    for i in range(9):
        if len(grid[i]) == 9:
            numsinrow += 1
    if numsinrow == 9:
        for i in range(9):
            rowoccurence = [0,0,0,0,0,0,0,0,0,0]
            for j in range(9):
                rowoccurence[grid[i][j]] += 1
                temprow = rowoccurence[1:10]
                if temprow == [1,1,1,1,1,1,1,1,1]:
                    return True
                else:
                    return False
    else:
        return False
else:
    return False

I obviously need to check that there is a 9x9 list of lists (grid), and that there are no duplicates in each row, column and 3x3 small square. In the code, I first check to see if there are a proper number of rows (There should be 9). Then I check that each row has 9 elements in it (with the ill_formed example you see that this is not the case). I then attempt to check duplicates in each row but I am having some trouble doing so. I thought that I could loop over each row and loop over each element in that row, and add 1 to a list of ints (rowoccurence). For example, if the first number is a 2, then rowoccurence[2] should be equal to 1. The zeros are in rowoccurence[0] and are not checked(I have a temporary list which should take everything except that first element- the zeros- because there could be more than 1 zero in a row and the grid could still be legit). I try to check the temp list (basically rowoccurence) against a reference list of correct values but it does not seem to be working. Could you help me check the rows for duplicates in this sudoku checker? Thank you so much in advance!

Answer 1

Remember, you're not searching for duplicates -- merely nonzero duplicates. Summing a set works for this. You can also check the legality of the row/column at the same time:

def sudoku_ok(line):
    return (len(line) == 9 and sum(line) == sum(set(line)))

def check_sudoku(grid):
    bad_rows = [row for row in grid if not sudoku_ok(row)]
    grid = list(zip(*grid))
    bad_cols = [col for col in grid if not sudoku_ok(col)]
    squares = []
    for i in range(9, step=3):
        for j in range(9, step=3):
          square = list(itertools.chain(row[j:j+3] for row in grid[i:i+3]))
          squares.append(square)
    bad_squares = [square for square in squares if not sudoku_ok(square)]
    return not (bad_rows or bad_cols or bad_squares)