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MathGroup Archive 2006

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Re: MemberQ

  • To: mathgroup at smc.vnet.net
  • Subject: [mg68655] Re: MemberQ
  • From: Peter Pein <petsie at dordos.net>
  • Date: Mon, 14 Aug 2006 06:44:41 -0400 (EDT)
  • References: <0533005404FE2E45B90AF737DE5A991A015312F3@de01exm67.ds.mot.com> <ebmt7i$6cj$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

leigh pascoe schrieb:
...
> While we are talking about MemberQ, what about the following behavior?
> In[15]:=Range[0,1,1/10]
>              MemberQ[Range[0,1,1/10],2/10]
> 
> Out[15]=\!\({0, 1\/10, 1\/5, 3\/10, 2\/5, 1\/2, 3\/5, 7\/10, 4\/5, 
> 9\/10, 1}\)
> 
> Out[16]=True
> 
> Is the above result correct given that 2/10 doesn't appear in the list 
> produced by the range statement??

Mathematica automagically cancels 2/10.

> On the other hand 0.5 can be
> represented exactly as a binary number, but
> In[29]:=MemberQ[Range[0,1,1/10],.5]
> 
> Out[29]=False
> 

1/2 has head Rational, 0.5 has got head _Real. So they must not be the same.

> LP
> 

I wrote a (very) small package:

http://people.freenet.de/Peter_Berlin/Mathe/NMemberQ/NMemberQ.m
and
http://people.freenet.de/Peter_Berlin/Mathe/NMemberQ/NMemberQ.nb

Feel free to add the function NMemberQ to your own collection of utilities.

Cheers,
   Peter


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