       AW: Sorting with Infinity

• To: mathgroup at smc.vnet.net
• Subject: [mg23518] AW: [mg23513] Sorting with Infinity
• From: Wolf Hartmut <hwolf at debis.com>
• Date: Tue, 16 May 2000 22:29:51 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

```
-----Ursprüngliche Nachricht-----
Von:	Jack Goldberg [SMTP:jackgold at math.lsa.umich.edu]
Gesendet am:	Dienstag, 16. Mai 2000 08:45
An:	mathgroup at smc.vnet.net
Betreff:	[mg23513] Sorting with Infinity

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?

Hello Jack,

Sort uses canonical order, and such obviously just recognizes Infinity as a
Symbol, see:

Sort[{7, -2, Infamity, -Infinity, Infinity, 1, -Infamity}]

{-2, 1, 7, -Infamity, Infamity, -Infinity, Infinity}

This is the same as explicitly using the ordering function OrderedQ:

Sort[{7, -2, -Infinity, Infinity, 1}, OrderedQ[{#1, #2}] &]

{-2, 1, 7, -Infinity, Infinity}

But choosing numerical ordering helps:

Sort[{7, -2, -Infinity, Infinity, 1}, Less]

{-Infinity, -2, 1, 7, Infinity}

Hartmut

```

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