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