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