# list problem...

bruno.desthuilliers at gmail.com bruno.desthuilliers at gmail.com
Wed Sep 29 14:52:32 CEST 2010

```On 29 sep, 14:17, Steven D'Aprano <st... at REMOVE-THIS-
cybersource.com.au> wrote:
> On Tue, 28 Sep 2010 20:11:51 +0100, Rog wrote:
> > On Tue, 28 Sep 2010 11:59:08 -0700, geremy condra wrote:
>
> >> On Tue, Sep 28, 2010 at 11:44 AM, Rog <r... at pynguins.com> wrote:
> >>> Hi all,
> >>> Have been grappling with a list problem for hours... a = [2, 3, 4,
> >>> 5,.....]
> >>> b = [4, 8, 2, 6,.....]
> >>> Basicly I am trying to place a[0], b[0] in a seperate list IF a[2] and
> >>> b[2] is present.
> >>> I have tried sets, zip etc with no success. I am tackling Euler
> >>> projects with Python 3.1, with minimal knowledge, and having to tackle
> >>> the language as I progress. Enjoyable frustration  :)
>
> >> I'm not clear on what your actual problem is, could you restate it?
>
> >> It sounds like you want to copy the ith element out of a and b into
> >> some other list- call it c- when the (i+2)th element meets some
> >> condition. What's the condition?
>
> >> Geremy Condra
>
> > The condition is that the i-th element is inverted, but not equal. eg
> > 4,2 - 2,4 , 34,5 - 5,34 etc.
> > Hope that is clearer.
>
> Clear as mud.
>
> Perhaps you should given an example. Given input
>
> a = [2, 3, 4, 5, 6, 7]
> b = [4, 8, 2, 6, 10, 42]
>
> what output are you expecting,

AFAICT, the OP expects [2, 4] in this case, but it's not clear what
he'd expect for let's say:

a = [2, 3, 21, 4, 5, 6, 7]
b = [4, 8, 22, 2, 6, 10, 42]

(the 'reversed' pair is at i+3, not i+2)

or

a = [0, 2, 3, 4, 5, 6, 7]
b = [3, 4, 8, 2, 6, 10, 42]

(the first pair is at pos 1, not 0)

or

a = [2, 3, 4, 8, 6, 7]
b = [4, 8, 2, 3, 10, 42]

(there's a second 'non-reversed/reversed' match at positions resp. 1 &
3)

```