Re: Position of element in a list
- To: mathgroup at smc.vnet.net
- Subject: [mg16412] Re: Position of element in a list
- From: Paul Abbott <paul at physics.uwa.edu.au>
- Date: Sat, 13 Mar 1999 02:21:33 -0500
- Organization: University of Western Australia
- References: <7bt665$ndn@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
Roger Mason wrote:
> I want to find the Position[] of negative elements in a list.
> The obvious methods do not work:
>
> Position[{-1, 3, -2, 1}, -_]
>
> and
>
> Position[{-1, 3, -2, 1}, -x_]
>
> return
>
> {}
The reason is that -1 is an atomic expression (whereas -x is not).
In[1]:= FullForm[{-1,-x}]
Out[1]//FullForm= List[-1, Times[-1, x]]
> So how do I perform this apparently simple task?
Use a test. E.g.,
In[2]:= Position[{-1, 3, -2, 1}, _?Negative]
Out[2]= {{1}, {3}}
Cheers,
Paul
____________________________________________________________________
Paul Abbott Phone: +61-8-9380-2734
Department of Physics Fax: +61-8-9380-1014
The University of Western Australia
Nedlands WA 6907 mailto:paul at physics.uwa.edu.au
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God IS a weakly left-handed dice player
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