Create a black and white chessboard in a two dimensional array

Question

Is there a better (and shorter) way of how to create chessboard like array. Requirements for the board are:

• board can be different size (in my example it's 3x3)
• bottom left square of the board should always be black
• black square is presented by "B", white square is presented by "W"

Code that I have:

def isEven(number):
    return number % 2 == 0

board = [["B" for x in range(3)] for x in range(3)]
if isEven(len(board)):
    for rowIndex, row in enumerate(board):
        if isEven(rowIndex + 1):
            for squareIndex, square in enumerate(row):
                if isEven(squareIndex + 1):
                    board[rowIndex][squareIndex] = "W"
        else:
            for squareIndex, square in enumerate(row):
                if not isEven(squareIndex + 1):
                    board[rowIndex][squareIndex] = "W"
else:
    for rowIndex, row in enumerate(board):
        if not isEven(rowIndex + 1):
            for squareIndex, square in enumerate(row):
                if isEven(squareIndex + 1):
                    board[rowIndex][squareIndex] = "W"
        else:
            for squareIndex, square in enumerate(row):
                if not isEven(squareIndex + 1):
                    board[rowIndex][squareIndex] = "W"

for row in board:
    print row

Output:

['B', 'W', 'B']
['W', 'B', 'W']
['B', 'W', 'B']

Answer 1

How about:

>>> n = 3
>>> board = [["BW"[(i+j+n%2+1) % 2] for i in range(n)] for j in range(n)]
>>> print board
[['B', 'W', 'B'], ['W', 'B', 'W'], ['B', 'W', 'B']]
>>> n = 4
>>> board = [["BW"[(i+j+n%2+1) % 2] for i in range(n)] for j in range(n)]
>>> print board
[['W', 'B', 'W', 'B'], ['B', 'W', 'B', 'W'], ['W', 'B', 'W', 'B'], ['B', 'W', 'B', 'W']]

Answer 2

Here is an itertools solution:

from itertools import cycle
N = 4

colors = cycle(["W","B"])
row_A  = [colors.next() for _ in xrange(N)]
if not N%2: colors.next()
row_B  = [colors.next() for _ in xrange(N)]

rows = cycle([row_A, row_B])
board = [rows.next() for _ in xrange(N)]

For N=4 this gives

['W', 'B', 'W', 'B']
['B', 'W', 'B', 'W']
['W', 'B', 'W', 'B']
['B', 'W', 'B', 'W']

This should be extensible to multiple colors (say a board that goes "B", "W", "G") if you make sure to add each new row and cycle to the list of rows.

Answer 3

Kind of a hack but

print [["B" if abs(n - row) % 2 == 0 else "W" for n in xrange(3)] for row in xrange(3)][::-1]

This seems like requirements creep or something =)

def make_board(n):
    ''' returns an empty list for n <= 0 '''
    return [["B" if abs(c - r) % 2 == 0 else "W" for c in xrange(n)] for r in xrange(n)][::-1]