[Tutor] reverse diagonal

[Dave Angel]

On 12/01/2012 10:40 AM, richard kappler 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]]
There's an obvious typo in that line.  You really need to copy/paste
into a message, since some retyping errors could seriously mislead us.
> 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
>
There's no such syntax.  If you want to assign two variables, then use
tuple-unpacking, like:

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

Or even better:

revdiag = [M[i][2-i] for i  in [0, 1, 2]]
   Notice I compute the j value, since it's very dependent on i

or even

revdiag = [M[i][len(M)-1-i] for i  in range(len(M)) ]

which would still work for other sizes of M

(All my code untested, as I have just run out of time)

-- 

DaveA