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Re: Select and Cases Give Different Answers

  • To: mathgroup at smc.vnet.net
  • Subject: [mg99201] Re: Select and Cases Give Different Answers
  • From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
  • Date: Wed, 29 Apr 2009 06:41:08 -0400 (EDT)
  • Organization: Uni Leipzig
  • References: <gt6fhd$rnu$1@smc.vnet.net>
  • Reply-to: kuska at informatik.uni-leipzig.de

Hi,

and

Select[theList, # =!= "NA" &]

gives what ?

And what is the difference between Equal[] (==) and SameQ[] (===) and
Unequal[] (!=) and UnsameQ[] (=!=) ..

Regards
   Jens


Gregory Lypny wrote:
> Hello everyone,
> 
> Suppose I have the following list that is a mixture of the string "NA" 
> and five numeric quantities.
> 
> 	theList = {"NA", "NA", -0.01315, 0.0120957, -1/41, 0.00625, "NA", 
> "NA", 5/8,  "NA"}
> 
> I want to weed out the NAs, and all of the following
> 
> 	Cases[theList, _?NumericQ]
> 
> 	DeleteCases[theList, _?StringQ]
> 
> 	Select[theList, # =E2=88=88 Reals &]
> 
> 	Select[theList, # =!= "NA" &]
> 
> work fine because each returns {-0.01315, 0.0120957, -(1/41), 0.00625, 
> 5/8}.
> 
> But I don't understand why Unequal, when used as the criterion for 
> Select as in
> 
> 	Select[theList, # != "NA" &],
> 
> fails to return the rationals, and I get only three elements 
> {-0.01315, 0.0120957, 0.00625}.  I'd appreciate any clarification on 
> the difference between UnsameQ and Unequal because my inclination in 
> most circumstances is to use an equal sign or an unequal sign to make 
> simple comparisons yet it doesn't give me the answer I expect, and I'd 
> hate to make a costly mistake somewhere down the road!
> 
> Regards,
> 
> 	Gregory
> 


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