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)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*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