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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