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MathGroup Archive 2002

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Re: Friendly Challenge 2: sort by column

  • To: mathgroup at smc.vnet.net
  • Subject: [mg35020] Re: [mg35005] Friendly Challenge 2: sort by column
  • From: Andrzej Kozlowski <andrzej at bekkoame.ne.jp>
  • Date: Thu, 20 Jun 2002 02:13:17 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Like last time I will make just a single attempt.

In[1]:=
f[a_,k_]:=a[[Ordering[Transpose[a][[k]]]]]

In[2]:=
a = {{77,27,57,53},
      {72,21,40,27},
      {96,76,87,27},
      {61,97,93,71},
      {43,80,21,87}};

In[3]:=
f[a,3]

Out[3]=
{{43,80,21,87},{72,21,40,27},{77,27,57,53},{96,76,87,27},{61,97,93,71}}

I think this ought to be pretty fast, but I shan't give any explanations 
preferring to leave this task in the safe hands of Hartmut ;)

Andrzej Kozlowski
Toyama International University
JAPAN
http://platon.c.u-tokyo.ac.jp/andrzej/


On Wednesday, June 19, 2002, at 06:52  PM, Mr. Wizard wrote:

> 	Once again, I am posting a friendly challenge with the hope of
> provoking thought and encouraging creativity.  This time, the
> challenge is to write the "best" sort-by-column function you can.
>
> 	Once again, have already written several such functions, which I will
> delay posting these until a number of responses are received.  This is
> not because I think I am some kind of Mathematica "guru" will all the
> right answers, but rather to prevent my methods from influencing
> others.
>
> 	Once again, though I am setting guidelines for this challenge, I will
> not be pronouncing some final judgement of a winning algorithm.  I
> would much prefer that this challenge induces a good discussion of the
> merits of various approaches and methods.
>
> Description:
>
> The function shall take two arguments: (1) a rectangular matrix and
> (1) the number of a column in (1).
>
> The function shall sort the rows of (1) according primarily to the
> element in the column specified by (2).
>
> The function shall not effect sub-elements in a three dimensional
> matrix.
>
> Example:
>
> a = {{77,27,57,53},
>      {72,21,40,27},
>      {96,76,87,27},
>      {61,97,93,71},
>      {43,80,21,87}}
>
> In:= f[a, 3]
>
> Out= {{43,80,21,87},
>       {72,21,40,27},
>       {77,27,57,53},
>       {96,76,87,27},
>       {61,97,93,71}}
>
> You may handle duplicates in the sort column as you see fit, but
> please explain it for the rest of us, and be prepared give support for
> this as you would all other behaviors of your function.
>
> Be creative, have fun, and good luck!
>
> Paul
>
>
>
>



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