[Tutor] reverse diagonal

[Brian van den Broek]

On 1 December 2012 10:40, 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:


Richard,

It is good you copy and pasted everything I snipped. But, you typed in
the line defining M. Better to also copy paste that, as you typed it
in wrong :-)

Here's one way that assumes of M only that it is an n-by-n matrix:

>>> M = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
>>> for i in reversed(range(len(M))):
	M[i][i]

	
9
5
1

Best,

Brian vdB