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Re: Sort on vector component

  • To: mathgroup at smc.vnet.net
  • Subject: [mg122024] Re: Sort on vector component
  • From: Bill Rowe <readnews at sbcglobal.net>
  • Date: Sun, 9 Oct 2011 03:53:28 -0400 (EDT)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com

On 10/8/11 at 5:36 AM, weh at snafu.de (Dr. Wolfgang Hintze) wrote:

>Given a list

>u = {{2, 12, 7, 21}, {2, 48, 47, 97}, {4, 42, 41, 87}, {6, 8, 1,
>15}, {6, 12, 9, 27}}

>How can I sort it with respect to a given index position i=(1, ...,
>4)?

One way to do this that should be fairly fast using index 3 as an example is:

In[2]:= u[[Ordering[u[[All, 3]]]]]

Out[2]= {{6, 8, 1, 15}, {2, 12, 7, 21}, {6, 12, 9, 27}, {4, 42, 41,
  87},
   {2, 48, 47, 97}}


Another way would be to use a pure function as the ordering function with Sort. That is:

In[5]:= Sort[u, #1[[3]] < #2[[3]] &]

Out[5]= {{6, 8, 1, 15}, {2, 12, 7, 21}, {6, 12, 9, 27}, {4, 42, 41,
  87},
   {2, 48, 47, 97}}




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