Named patterns forbidden in Except?
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From: cca on 18 Jun 2010 01:25 Hi, Consider this: In[1]:= Cases[{-0.5, 1, -3, 2/5, I}, Except[x_Integer]] During evaluation of In[1]:= Except::named: Named pattern variables are not allowed in the first argument of Except[x_Integer]. >> Out[1]= {} Why are named patterns not allowed in Except? Recently, during a course of mine (on Mathematica), I needed to illustrate the concept of function as a set of ordered pairs. I had the following definiton(inter alia): FunctionQ[f_] := MatchQ[f, {{_, _} ...}] && ! MatchQ[f, {___, {x_, _}, ___, {x_, _}, ___}] Then it occurred to me to use Except with two arguments just to give it a chance to shine in the course: (*doesn=B4t work!*) FunctionQ[f_] := MatchQ[f, Except[{___, {x_, _}, ___, {x_, _}, ___}, {{_, _} ...}]] Wouldn't this be simpler and natural? I would much appreciate any explanation... Atenciosamente, Carlos C=E9sar de Ara=FAjo Gregos & Troianos Educacional www.gregosetroianos.mat.br Belo Horizonte, MG, Brasil (31) 3283-1122
From: David Bailey on 19 Jun 2010 07:47
On 18/06/10 06:25, cca(a)gregosetroianos.mat.br wrote: > Hi, > > Consider this: > > In[1]:= Cases[{-0.5, 1, -3, 2/5, I}, Except[x_Integer]] > > During evaluation of In[1]:= Except::named: Named pattern variables are not > allowed in the first argument of Except[x_Integer].>> > > Out[1]= {} > > Why are named patterns not allowed in Except? > I guess the point is that you could never use the name - because in your example, if x took a value, Except would fail to match by definition! David Bailey http://www.dbaileyconsultancy.co.uk |