Re: Picking Off Lists That Have No Numbers
- To: mathgroup at smc.vnet.net
- Subject: [mg99548] Re: Picking Off Lists That Have No Numbers
- From: Szabolcs Horvát <szhorvat at gmail.com>
- Date: Thu, 7 May 2009 06:39:41 -0400 (EDT)
- References: <gtrl29$1o5$1@smc.vnet.net>
Gregory Lypny wrote: > Hi everyone, > > In a previous thread, "Select and Cases Give Different Answers," I > discussed a bug, confirmed by others on the list and which Daniel > Lichtblau of Wolfram said has been fixed in the development kernel, > and I wonder whether we have the same problem here. > > I want to know if a list has no numbers in it. Here I get the wrong > answer for the first list. > > FreeQ[#, _Number] & /@ {{"NA", 2.3, 3/8}, {"NA", "NA", "NA"}} > > yields {True, True} > And same here. > > MemberQ[#, _Number] & /@ {{"NA", 2.3, 3/8}, {"NA", "NA", "NA"}} > > {False, False} The pattern _Number matches expressions whose head is Number. Number usually not used as the head of anything in Mathematica. Head /@ {"NA", 2.3, 3/8} {String, Real, Rational} Try _?NumberQ instead. > > If I use Real or Rational for the criterion, I get the right answers, > but that's no good if you have some lists that are mixtures of strings > and reals and others that are mixtures of strings and rationals. > > FreeQ[#, _Real] && FreeQ[#, _Rational] > & /@ {{"NA", 2.3, 3/8}, {"NA", "NA", "NA"}, {"NA", 2.3, "NA"}, {"NA", > 2.3, 9.4}} > > Now this works. > > Count[#, _String] == Length[#] & /@ {{"NA", 2.3, 3/8}, {"NA", "NA", > "NA"}} > > {False, True} > > But this does not! > > Count[#, _Number] == 0 & /@ {{"NA", 2.3, 3/8}, {"NA", "NA", "NA"}} > > {True, True} > > I don't mean to be a pain, but it is important in my sample selection > procedures to be able to make the distinction between strings and > numbers, so I'd be interested in knowing whether I'm misunderstanding > properties such as Number or Numeric (don't really know the > difference) or this is a bug. > Again, _SomeHead matches expressions with head SomeHead. All expression have one unique head, so if something is e.g. Rational it cannot be Real or something else at the same time.