Re: Re: negative pattern matching anyone?

• To: mathgroup at smc.vnet.net
• Subject: [mg43798] Re: [mg43766] Re: negative pattern matching anyone?
• From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
• Date: Mon, 6 Oct 2003 02:07:50 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

```On Friday, October 3, 2003, at 03:28 PM, Paolo Bientinesi wrote:

>>> Then, is there a way to match both -3 and -a with the same pattern?
>>
>> Yes -- use Sign:
>>
>>    h[x_] := Abs[-x] /; Sign[x] == -1
>>    h[x_] := x^2
>>
>> Cheers,
>> Paul
>
>
> Thanks for all the answers, but all of them solve the problem
> with two different patterns (basically using ||).
>
> About the Sign suggestion, it doesn't seem to work, since
> Sign[-a] yields -Sign[a]:
>
> h[x_ /; Sign[x] == -1]:= Abs[x]
> h[x_]:= x^2
>
> h[-3] is 3
>
> but
>
> h[-a] is a^2
> --
> Paolo
>
> pauldj at cs.utexas.edu		        paolo.bientinesi at iit.cnr.it
>
>
>

Actually, you can match both -3 and -a with a single pattern, though it
is a rather "artificial" one:

{-3, -a} /. _?(StringMatchQ[ToString[#], "-*"] &) -> b

{b,b}

Andrzej Kozlowski
Yokohama, Japan
http://www.mimuw.edu.pl/~akoz/
http://platon.c.u-tokyo.ac.jp/andrzej/

```

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