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Does Union[] reliably sort?

• To: mathgroup at smc.vnet.net
• Subject: [mg123654] Does Union[] reliably sort?
• From: Michael Stern <nycstern at gmail.com>
• Date: Wed, 14 Dec 2011 06:02:50 -0500 (EST)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com

I have noticed that if combine two lists of date/data objects using 
Union[], the result is sorted by date.

For example

In[]:= mins={{{2001, 1, 31}, 0.993268}, {{2002, 3, 31}, 1.01395}, 
{{2003, 6, 30},
   1.08647}, {{2005, 11, 30}, 1.14752}, {{2006, 9, 30},
   1.18938}, {{2007, 2, 28}, 1.19658}, {{2008, 1, 31},
   1.20432}, {{2011, 1, 31}, 1.37501}};
In[]:= maxs={{{2000, 10, 31}, 1.01816}, {{2001, 12, 31}, 1.02714}, 
{{2004, 2, 29},
    1.12702}, {{2005, 3, 31}, 1.16986}, {{2010, 10, 31}, 1.39026}};

In[]:= Union[maxs, mins]

Out[]= {{{2000, 10, 31}, 1.01816}, {{2001, 1, 31},
   0.993268}, {{2001, 12, 31}, 1.02714}, {{2002, 3, 31},
   1.01395}, {{2003, 6, 30}, 1.08647}, {{2004, 2, 29},
   1.12702}, {{2005, 3, 31}, 1.16986}, {{2005, 11, 30},
   1.14752}, {{2006, 9, 30}, 1.18938}, {{2007, 2, 28},
   1.19658}, {{2008, 1, 31}, 1.20432}, {{2010, 10, 31},
   1.39026}, {{2011, 1, 31}, 1.37501}}

Is this reliable behavior? Is there any case in which Union would /not/ 
return results that have been properly sorted by date?

Thank you,

-Michael Stern

• Follow-Ups:
  • Re: reliably sort?
    • From: DrMajorBob <btreat1@austin.rr.com>
  • Re: reliably sort?
    • From: Leonid Shifrin <lshifr@gmail.com>