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Re: Select and Cases Give Different Answers

  • To: mathgroup at smc.vnet.net
  • Subject: [mg99163] Re: Select and Cases Give Different Answers
  • From: Gregory Lypny <gregory.lypny at videotron.ca>
  • Date: Wed, 29 Apr 2009 03:48:33 -0400 (EDT)
  • References: <gt6fhd$rnu$1@smc.vnet.net> <49F6DC77.20202@metrohm.com>

Thanks for confirming, Daniel.

I think I'll send in a bug report.

	Gregory

On Tue, Apr 28, 2009, at 6:37 AM, dh wrote:

> 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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