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MathGroup Archive 2007

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RE: Sort with -Infinity fails

  • To: mathgroup at smc.vnet.net
  • Subject: [mg73860] RE: [mg73822] Sort with -Infinity fails
  • From: "Jose Luis Gomez" <jose.luis.gomez at itesm.mx>
  • Date: Fri, 2 Mar 2007 06:31:00 -0500 (EST)
  • Organization: ITESM
  • References: <200703011117.GAA28248@smc.vnet.net>

Dear Tom

I guess the default sort criteria of Sort[] must take into account the =
kind
of expression. -Infinity is Not a number, and Sort[] just moves it after =
the
numbers. Something similar happens in the following example, where all
numbers are placed before all the symbols (assume there are no values
assigned to a, b and c):

In[1]:=
     Sort[{c, 1, 5, 2, b, a}]
Out[1]=
     {1, 2, 5, a, b, c}

Therefore, the default behavior of Sort[] is more "computational" than
"mathematical". However, you can override these behavior if you give the
sort "criteria" as the second parameter of Sort[]:

In[2]:=
     Sort[{-Infinity, -8, 5}, #1 < #2 &]
Out[2]=
     {-Infinity, -8, 5}

Here is another way to write the same:

In[3]:=
     Sort[{-Infinity, -8, 5}, Function[{x1, x2}, x1 < x2]]
Out[3]=
     {-Infinity, -8, 5}     

Regards from Mexico

Jose

http://homepage.cem.itesm.mx/lgomez/

-----Mensaje original----- 
De: Tom Aldenberg [mailto:Tom.Aldenberg at rivm.nl]
Enviado el: Jueves, 01 de Marzo de 2007 05:17 a.m.
Para: mathgroup at smc.vnet.net
Asunto: [mg73822] Sort with -Infinity fails

Dear MathGroup,

Minus Infinity (-Infinity) is smaller than -8, but Sort does not sort it =
as
expected.
Is there a Real minus Infinity?


In[75]:=
      -Infinity < -8
Out[75]=
      True

In[76]:=
      Sort[{-Infinity, -8, 5}]
Out[76]=
      {-8, 5, -=1B$B!g=1B(B}


Regards,

Tom Aldenberg

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