[Tutor] reverse diagonal

[Jonatán Guadamuz Espinoza]

On Sat, Dec 1, 2012 at 9:40 AM, richard kappler <richkappler at gmail.com> wrote:
> 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
>

The way you are trying to do it would be

>>> revdiag = [M[i][j] for i,j in [(0,2),(1,1),(2,0)]]