Re: Changing a list with the information of other

• To: mathgroup at smc.vnet.net
• Subject: [mg131391] Re: Changing a list with the information of other
• From: Bob Hanlon <hanlonr357 at gmail.com>
• Date: Wed, 3 Jul 2013 04:56:46 -0400 (EDT)
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• References: <20130702044642.9CA146A4F@smc.vnet.net>

```list1 = {{0, 1}, {1, w}, {2, 1 + w}, {3, 1 + 2 w},
{4, 2}, {5, 2 w}, {6, 2 + 2 w}, {7, 2 + w}};

list2 = {1, 1 + w, 2 + w, 2 w, 2, 2 + 2 w, 1 + 2 w, w};

list1[[Cases[
Position[list1, #, {2}] & /@ list2,
{x_, 2} :> x, 2], 1]]

{0, 2, 7, 5, 4, 6, 3, 1}

Bob Hanlon

On Tue, Jul 2, 2013 at 12:46 AM, Joaquim Nogueira <jen at fct.unl.pt> wrote:

> Good morning
>
> I have the following (possibly very simple) problem.
> I have a list, created using a Table
>
> {{0, 1}, {1, w}, {2, 1 + w}, {3, 1 + 2 w}, {4, 2}, {5, 2 w}, {6, 2 + 2 w},
> {7, 2 + w}}.
>
> Then, using another Table, I created the list
>
> {1, 1 + w, 2 + w, 2 w, 2, 2 + 2 w, 1 + 2 w, w}
>
> Note that all these elements appear always in second place, in the ordered
> pairs of the first list. What I want is an instruction that allows to
> replace all these elements by the ones that appear first in the ordered
> pairs above. That is, I would like to replace this list by the new one
>
> {0, 2, 7, 5, 4, 6, 3, 1}.
>
> As you may guess this is only of interest when the lists are very big
> (with thousands of elements).
>
> Thank you
>
>

```

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