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Re: Changing a list with the information of other

  • To: mathgroup at smc.vnet.net
  • Subject: [mg131411] Re: Changing a list with the information of other
  • From: Alexei Boulbitch <Alexei.Boulbitch at iee.lu>
  • Date: Wed, 3 Jul 2013 05:03:26 -0400 (EDT)
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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

Hi,
Try this:

lst1 = {{0, 1}, {1, w}, {2, 1 + w}, {3, 1 + 2 w}, {4, 2}, {5,
    2 w}, {6, 2 + 2 w}, {7, 2 + w}};
lst2 = {1, 1 + w, 2 + w, 2 w, 2, 2 + 2 w, 1 + 2 w, w};


f[a_] := Select[lst1, #[[2]] == a &][[1, 1]];
Map[f, lst2]

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

Have fun, Alexei


Alexei BOULBITCH, Dr., habil.
IEE S.A.
ZAE Weiergewan,
11, rue Edmond Reuter,
L-5326 Contern, LUXEMBOURG

Office phone :  +352-2454-2566
Office fax:       +352-2454-3566
mobile phone:  +49 151 52 40 66 44

e-mail: alexei.boulbitch at iee.lu






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