Re: Select and Cases Give Different Answers
- To: mathgroup at smc.vnet.net
- Subject: [mg99185] Re: Select and Cases Give Different Answers
- From: David Bailey <dave at removedbailey.co.uk>
- Date: Wed, 29 Apr 2009 06:37:47 -0400 (EDT)
- References: <gt6fhd$rnu$1@smc.vnet.net>
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
>
Remember that if Mathematica can't evaluate something, it just returns
it unchanged. Unequal only returns True/False if it can do the
calculation - i.e. if both sides are numbers, or both are strings, or a
few obvious cases like xxx=xxx - otherwise it returns unevaluated. One
might have thought it would 'know' a string would never equal a
rational, but it would seem it doesn't! Select only picks those elements
for which the test returns True.
UnsameQ actually compares its arguments to determine if they have the
same structure - i.e. it always returns True or False.
David Bailey
http://www.dbaileyconsultancy.co.uk