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Re: confused about == vs === in this equality

  • To: mathgroup at smc.vnet.net
  • Subject: [mg103852] Re: confused about == vs === in this equality
  • From: Albert Retey <awnl at gmx-topmail.de>
  • Date: Fri, 9 Oct 2009 07:16:57 -0400 (EDT)
  • References: <20091003104738.LCJ3I.416659.imail@eastrmwml34> <200910040935.FAA07794@smc.vnet.net> <hacmoa$sko$1@smc.vnet.net> <hakjmt$d8r$1@smc.vnet.net>

Drago Ganic wrote:
> Hi Andrzej,
> there is one function (Missing) and one symbol (Null) which >>should<< 
> behave the same as Indetereminate and ComplexInfinity but unfortunatly does 
> not.
> 
> In[1]:= Null == Null
> Out[1]= True
> 
> In[2]:= Missing[] == Missing[]
> Out[2]= True
> 
> Null is Mathematica legacy which has basically the same meaning as Missing[] 
> (or maybe Missing["Nonexistent"]). Missing incorporates Indetereminate via 
> Missing["Indeterminate"].

Why would you call Null "Mathematica legacy"? AFAIK Missing is by no
means meant to "replace" Null. From the documentation Missing is
something that is returned by Datafunctions only while Null has much
wider applications in the Mathematica language. Have you information
that is beyond the documentation?

> All of those (Indetereminate & ComplexInfinity for numeric data and 
> Null/Missing for any kind of data), are so called "null values" in database 
> systems and for them the Equal and other logical connectivities (And, Or, 
> Not, etc.) are overloaded. Unfortunatly this is not the case in Mathematica.

While I think (but not yet am convinced) that this might make sense for
Missing, I think Null is not that easily compared to a database null
value and thus I would expect lots of problems and side effects if Null
== Null would return False...

just wondering...

albert


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