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MathGroup Archive 2000

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Re: Sorting with Infinity

  • To: mathgroup at smc.vnet.net
  • Subject: [mg23524] Re: [mg23513] Sorting with Infinity
  • From: Andrzej Kozlowski <andrzej at tuins.ac.jp>
  • Date: Tue, 16 May 2000 22:29:56 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Actually the command is not too arcane. Simply use:


In[5]:=
Sort[{7, -2, -Infinity, Infinity, 1}, Less]
Out[5]=
{-Infinity, -2, 1, 7, Infinity}

The point is that Mathematica does "know" things like:

In[7]:=
-Infinity < -2
Out[7]=
True

or 

In[8]:=
23 < Infinity
Out[8]=
True

but unless you specifically tell it to use Less in sorting it sorts Infinity
as a Symbol rather than using Less. This has really nothing to do with
Infinity, for example compare this with:


In[11]:=
Sort[{1, 4, Pi, E}]
Out[11]=
{1, 4, E, Pi}
In[12]:=
Sort[{1, 4, Pi, E}, Less]
Out[12]=
{1, E, Pi, 4}

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


on 5/16/00 3:44 PM, Jack Goldberg at jackgold at math.lsa.umich.edu wrote:

> Hi group;
> 
> I construct lists which may look like this
> 
> {7,-2,-Infinity,Infinity,1}
> 
> When this list is sorted by using Sort, I get
> 
> {-2,1,7,-Infinity,Infinity}
> 
> I would prefer, naturally,
> 
> {-Infinity,-2,1,7,Infinity}
> 
> This is easy to do if one doesn't care about elegance.  Without
> spelling out the obvious details, here are a few kludges:
> 
> (1) Replace Infinity by 10^15, Sort  then switch back to Infinity.
> (2) Use Append and Prepend after Sort and  deleting infinities.
> (3) RotateRight after Sort and switch signs on infinities
> 
> Anyone got a "cute" solution say by using a sorting function or
> perhaps some arcane command unknown to mere mortals?
> 
> Jack
> 
> 
> 
> 





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