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Re: Select and Cases Give Different Answers

  • To: mathgroup at smc.vnet.net
  • Subject: [mg99202] Re: Select and Cases Give Different Answers
  • From: dh <dh at metrohm.com>
  • Date: Wed, 29 Apr 2009 06:41:19 -0400 (EDT)
  • References: <gt6fhd$rnu$1@smc.vnet.net>


Hi Gregory,

it looks like a bug to me. Consider:

Unequal["NA", -(1/41)  ]

this does not evaluate, it does not return True. But:

Unequal["NA", -(1/41) 1. ]

does return True.

Daniel





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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