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MathGroup Archive 2004

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Re: Sorting a list of pairs on the second elements

  • To: mathgroup at smc.vnet.net
  • Subject: [mg51372] Re: Sorting a list of pairs on the second elements
  • From: "Peltio" <peltio at twilight.zone>
  • Date: Fri, 15 Oct 2004 02:47:12 -0400 (EDT)
  • References: <cklmi7$f1o$1@smc.vnet.net>
  • Reply-to: "Peltio" <peltioNOSP at Mdespammed.com.invalid>
  • Sender: owner-wri-mathgroup at wolfram.com

"János" wrote

>    Map[Reverse,Sort[Map[Reverse,collectedDnaBin]]]

>    Sort[collectedDnaBin,#1[[2]] < #2[[2]] &]
> but that does not want to do the trick for me.

I wonder why. It should work, albeit inefficently. Mmmmm, have you tried
with parenthesis wrapped around the pure function?
    Sort[collectedDnaBin,(#1[[2]] < #2[[2]])&]
Have you tried it in a fresh kernel?

>In general, if I have a list whose elements are m-member lists, like
>{{a1,...,am},...,{z1,...,zm}}, then my question is what is the
>best/fastest way to sort it on the k-th elements of the members where
>1<=k<=m and ak,...,zk are arbitrary objects - in my case Strings - and
>use as little memory for it as possible.

Dunno for the general case, but this -very inelegant - approach to sort
according to the last element seems to be pretty fast (lots of procedures
but applied to the data as a whole, without timeconsuming mapping on every
single element)

    Transpose@Reverse@Transpose@Sort@Transpose@Reverse@Transpose[
collectedDnaBin ]

( The first and last three procs just implement a swap function
    swap[list_]:=Transpose@Reverse@Transpose[list]
)

The group's gurus will certainly find a proc at least ten times faster than
this, I'm sure.

cheers,
Peltio
Invalid address in reply-to. Crafty demunging required to mail me.




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