[Tutor] reverse diagonal

[eryksun]

On Sat, Dec 1, 2012 at 11:31 AM, Dave Angel <d at davea.name> wrote:
>
> revdiag = [M[i][len(M)-1-i] for i  in range(len(M)) ]

You might sometimes see this using the bitwise invert operator ~ (i.e.
__invert__, operator.invert):

    >>> M = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

    >>> [M[i][~i] for i in xrange(len(M))]
    [3, 5, 7]

~i returns the value (-i - 1):

    >>> [~i for i in range(4)]
    [-1, -2, -3, -4]

If a sequence index is negative, Python normalizes it by adding the
sequence length. For example, seq[-1] == seq[3], where len(seq) == 4.
You can think of the sequence index on a ring:

          2
     3         1

  -4             0

    -3        -1
         -2


The corresponding negative index of the sequence is 180 degrees (pi
radians) around the ring. So the bitwise complement of index i
traverses the sequence in reverse order.