Re: Changing a list with the information of other

• To: mathgroup at smc.vnet.net
• Subject: [mg131402] Re: Changing a list with the information of other
• From: Bill Rowe <readnews at sbcglobal.net>
• Date: Wed, 3 Jul 2013 05:00:26 -0400 (EDT)
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• Delivered-to: l-mathgroup@wolfram.com
• Delivered-to: mathgroup-outx@smc.vnet.net
• Delivered-to: mathgroup-newsendx@smc.vnet.net

```On 7/2/13 at 12:46 AM, jen at fct.unl.pt (Joaquim Nogueira) wrote:

>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}.

You have indicated your desired output is just the first element
of the first list. You can get this a number of different ways.
Here are two:

In[4]:= a = {{0, 1}, {1, w}, {2, 1 + w}, {3, 1 + 2 w}, {4, 2}, {5,
2 w}, {6, 2 + 2 w}, {7, 2 + w}};

In[5]:= a[[All, 1]]

Out[5]= {0,1,2,3,4,5,6,7}

In[6]:= First /@ a

Out[6]= {0,1,2,3,4,5,6,7}

And if the second list is stored in say b, i.e., b={1, 1 + w, 2
+ w, 2 w, 2, 2 + 2 w, 1 + 2 w, w};

then doing:

b=First/@a; or
b=a[[All,1]];

Replaces what was b with the desired output

```

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