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MathGroup Archive 2001

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Re: Sorting and Selecting in MultiLevel Lists?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg31175] Re: Sorting and Selecting in MultiLevel Lists?
  • From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
  • Date: Tue, 16 Oct 2001 01:18:52 -0400 (EDT)
  • Organization: Universitaet Leipzig
  • References: <9qbi2c$2d7$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Hi,

since you compute x^2+y^2 twice in every comparsion it seems to
be better to compute thist value one for every pair and remove
it when you have sorted the pairs.

Take[#, 2] & /@ Sort[Append[#, Dot[#, #]] & /@ myList, Last[#1] <
Last[#2] &]


Regards
  Jens

aes wrote:
> 
> Suppose I want to Sort, or Select from, a multilevel list, e.g.
> 
>         myList =  { {x1,y1},  {x2,y2},  {x3,y3}, . . . }
> 
> with a Sort or Select criterion that's some function of the xn and yn values
> 
> For example, I can sort the above list on the value of x^2 + y^2 by using
> 
>     Sort[myList, (Take[#1, 1][[1]]^2 + Take[#1, 2][[1]]^2) <
>                         (Take[#2, 1][[1]]^2 + Take[#2, 2][[1]]^2) &]
> 
> Question:  Is there an easier way to get at the "x" and "y" values associated
> with the #1 and #2 arguments in Sort, or with the # argument in Select, than the
> awkward   Take[#,m][[n]]   notation used here?


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