Groups > comp.soft-sys.math.mathematica > #2786

Re: Problem combining patterns

From	David Bailey <dave@removedbailey.co.uk>
Newsgroups	comp.soft-sys.math.mathematica
Subject	Re: Problem combining patterns
Date	2011-05-29 11:36 +0000
Organization	Steven M. Christensen and Associates, Inc and MathTensor, Inc.
Message-ID	<irtb4o$pjk$1@smc.vnet.net> (permalink)
References	<irqlng$e7i$1@smc.vnet.net>

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On 28/05/2011 12:19, Leonid Shifrin wrote:
> You can do
>
> ClassPattern[] := _?(Length[#] === 1&);
>
> or
>
> ClassPattern[] := _ [ _ ]
>
> Regards,
> Leonid
>
>
> On Fri, May 27, 2011 at 2:12 PM, Rui<rui.rojo@gmail.com>  wrote:
>
>> Hey people.
>> I want to have an expression that evaluates to a pattern. Imagine
>> something simple
>>
>> ClassPattern[]:=HoldPattern[i_/;Length[i]===1];
>> So far so good. But then I'd like to use it in compound expressions
>> like
>>
>> MatchQ[{b[3], c[5]}, {ClassPattern[], ClassPattern[]}]
>>
>> When I do so, the label "i" inside the pattern is treated as the same
>> in both ClassPatterns, so I get false. But I wanna get True in cases
>> like the one before
>>
>> Any ideas?
>>
>>
I think that ClassPattern may be a cut down example, and naming the 
pattern variables may be useful in the real problem, so how about:

ClassPattern[] :=Module[{pp}, HoldPattern[i_ /; Length[i] === 1] /. i -> pp]

David Bailey
http://www.dbaileyconsultancy.co.uk

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Re: Problem combining patterns Leonid Shifrin <lshifr@gmail.com> - 2011-05-28 11:19 +0000
  Re: Problem combining patterns David Bailey <dave@removedbailey.co.uk> - 2011-05-29 11:36 +0000

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