[Tutor] reverse diagonal

[richard kappler]

I'm working through Mark Lutz's "Python," reviewing the section on lists. I
understand the list comprehension so far, but ran into a snag with the
matrix. I've created the matrix M as follows:

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

then ran through the various comprehension examples, including:

diag = [M[i][i] for i in [0, 1, 2]]

which, of course, gave me [1, 5, 9].

Then I tried creating revdiag, wanting to return [3, 5, 7], tried several
different ways, never quite got what I was looking for, so I'm looking for
guidance as I'm stuck on this idea. Here's the various attempts I made and
the tracebacks:

>>> revdiag = [M[i][i] for i in [2, 1, 0]]
>>> revdiag
[9, 5, 1]
# once I saw the output, this one made sense to me.

>>> revdiag = [M[i][j] for i in [0, 1, 2] and for j in [2, 1, 0]]
  File "<stdin>", line 1
    revdiag = [M[i][j] for i in [0, 1, 2] and for j in [2, 1, 0]]
                                                           ^
SyntaxError: invalid syntax

>>> revdiag = [M[i][j] for i in [0, 1, 2] and j in [2, 1, 0]]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'j' is not defined

>>> revdiag = [M[i][j] for i in [0, 1, 2], for j in [2, 1, 0]]
  File "<stdin>", line 1
    revdiag = [M[i][j] for i in [0, 1, 2], for j in [2, 1, 0]]
                                                      ^
SyntaxError: invalid syntax

>>> revdiag = [M[i][j] for i in [0, 1, 2], and for j in [2, 1, 0]]
  File "<stdin>", line 1
    revdiag = [M[i][j] for i in [0, 1, 2], and for j in [2, 1, 0]]
                                                       ^
SyntaxError: invalid syntax

I see where the errors are occurring, but I'm not sure I fully understand
why they are occurring or how to get what I'm looking for with what I
presently know, which admittedly is not much. Any help would be appreciated.

regards, Richard

-- 

quando omni flunkus moritati
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