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Re: Select and Cases Give Different Answers

  • To: mathgroup at smc.vnet.net
  • Subject: [mg99165] Re: [mg99125] Select and Cases Give Different Answers
  • From: Gregory Lypny <gregory.lypny at videotron.ca>
  • Date: Wed, 29 Apr 2009 03:48:56 -0400 (EDT)
  • References: <200904280842.EAA28447@smc.vnet.net>

Good stuff, Andrzej.  This certainly calls for a bug report.  Maybe  
it's best if the tech people at Wolfram are simply directed to this  
thread.

	Gregory



On Tue, Apr 28, 2009, at 8:38 AM, Andrzej Kozlowski wrote:

>
> On 28 Apr 2009, at 17:42, 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
>>
>
>
> I think that there is indeed something strange here, but it it the  
> opposite of what you seem to find strange. To me the strange thing  
> here is the difference between strings and symbols and between exact  
> and approximate numbers . Replace your strings "NA" by symbols NA  
> and note that now you get:
>
> Select[theList2, #1 != NA & ]
> {}
>
> which is what I would have expected also in your example. In the  
> case of a symbol Unequal and UnsameQ are different, as one would  
> expect:
>
> UnsameQ[x, 2]
> True
>
> Unequal[x, 2]
> x != 2
>
> An undefined symbol x is certainly "not the same as" (i.e. not  
> identical) the number 2, but it could be equal to it (depending on  
> the context) so Unequal[x,2] can't be answered. But when we replace  
> x by the string "x" things become mysterious. First of all we have:
>
> UnsameQ["x", 2]
> True
>
> Unequal["x", 2]
> True
>
> so now there is no difference between UnsameQ and Unequal. One might  
> think that Strings are never equal to numbers (which would be  
> reasonable, since they can't be assigned values) but then:
>
> "x" =!= 2/3
> True
>
> "x" != 2/3
> "x" != 2/3
>
> so when we have a rational on the RHS strings behave the same as  
> symbols! But not so with reals:
>
> "x" =!= N[2/3]
> True
> "x" != N[2/3]
> True
>
> By the way, if we replace Rational[2,3] with Complex[2,3] the  
> behaviour will remain the same. All this is puzzling to me and I  
> can't see any other explanation but that this is just an unintended  
> side-effect of something. Obviously it does not make any  
> mathematical sense to ask the question whether a string is equal to  
> a rational number of a real, so I would not attach any semantic  
> significance to these answers.
>
> Andrzej Kozlowski
>
>
>
>



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