Mathematica 9 is now available
Services & Resources / Wolfram Forums / MathGroup Archive
-----

MathGroup Archive 2007

[Date Index] [Thread Index] [Author Index]

Search the Archive

Pattern test and expression sequence

  • To: mathgroup at smc.vnet.net
  • Subject: [mg83122] Pattern test and expression sequence
  • From: Jan Lellmann <lellmann at uni-mannheim.de>
  • Date: Mon, 12 Nov 2007 05:11:00 -0500 (EST)

Hello,

I want to define a pattern that accepts a sequence of expressions (of 
arbitrary length) that matches some condition. So as a minimalist 
example, if I wanted to accept all expressions with f as head and at 
least 2 arguments, I want to write

myQ[x__] := (Print[x]; Length[{x}] > 1)
f[x__?myQ] := works[x];
f[1, 2]

This should print "1 2" and return "works[1,2]". But it doesn't, it just 
prints "1" and returns f[1,2] unevaluated. So apparenty Mathematica does 
not pass the complete expression sequence to myQ, but only the first 
element ("1").

There is a workaround using /; :
f[x__] /; (myQ[x] == True) := works[x];

but it is rather ugly and cumbersome when there is more than one argument.

Does anyone have an idea why Mathematica does this? Is it a bug? Or is 
there a (nice) workaround?

Best regards,
   Jan


  • Prev by Date: Dividers formatting in Grid
  • Next by Date: Re: Optimizing fixed point iteration
  • Previous by thread: Re: Re: Dividers formatting in Grid
  • Next by thread: Re: Pattern test and expression sequence