algorithm for scanning cells in an array in diagonal, may be used for ordering drawing of sprites in an isometric 3d game.

dscan.py

def dscan(w, h):
    """ scans a 2-dimensions array of width 'w', height 'h', in diagonal, starting at the upper-left corner.

        example for w=3,h=3 :

             +---+---+---+
             |000|002|003|
             |001|004|007|
             |005|006|008|
             +---+---+---+

    >>> list(dscan(3,3))
    [(0, 2), (0, 1), (1, 2), (2, 2), (1, 1), (0, 0), (1, 0), (2, 1), (2, 0)]
    >>> list(dscan(2,5))
    [(0, 4), (0, 3), (1, 4), (1, 3), (0, 2), (0, 1), (1, 2), (1, 1), (0, 0), (1, 0)]
    >>> list(dscan(5,2))
    [(0, 1), (0, 0), (1, 1), (2, 1), (1, 0), (2, 0), (3, 1), (4, 1), (3, 0), (4, 0)]
    """
    i, j = 0, h - 1
    di, dj = -1, -1
    for _ in xrange(w * h):
        yield i, j
        i += di
        j += dj
        if i == -1:
            di, dj = 1, 1
            i = 0
            if j == -1:
                j = 0
                i = 1
            continue
        if j == h:
            di, dj = -1, -1
            j = h - 1
            if i == w:
                j = h - 2
                i = w - 1
            continue
        if j == -1:
            di, dj = 1, 1
            j = 0
            i += 2
            continue
        if i == w:
            di, dj = -1, -1
            i = w - 1
            j -= 2
            continue

if __name__ == '__main__':
    import doctest
    doctest.testmod()

    r=''
    mx = 1+max([x for (x,y) in g])
    my = 1+max([y for (x,y) in g])
    size = max([len(str(v)) for k,v in g.iteritems()])
    head = '┌'+('┬'.join(['─'*size for _ in range(mx)]))+'┐\n'
    sep =  '├'+('┼'.join(['─'*size for _ in range(mx)]))+'┤\n'
    foot = '└'+('┴'.join(['─'*size for _ in range(mx)]))+'┘\n'
    r = head
    for y in range(my):
        l=[]
        for x in range(mx):
           if (x,y) in g:
               l.append(str(g[x,y]).center(size))
           else:
               l.append(' '*size)
	r += '│'+('│'.join(l))+'│\n'
        if y <> my-1: r+=sep
    return r+foot