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MathGroup Archive 1996

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Re: Re: Not[OddQ] is not the same as EvenQ (sometimes)

  • To: mathgroup at smc.vnet.net
  • Subject: [mg3957] Re: [mg3899] Re: Not[OddQ] is not the same as EvenQ (sometimes)
  • From: Allan Hayes <hay at haystack.demon.co.uk>
  • Date: Sat, 11 May 1996 23:53:20 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

Dave Wagner<wagner at motel6.cs.colorado.edu>
in [mg3899] Re: Not[OddQ] is not the same as EvenQ (sometimes)
writes

>>Position[{2,3,4,5,6,7}, x_?(!NumberQ[#]&)]
>>{{0}, {}}
>I don't know the cause of the empty set of list braces here.

Dave:

The empty index sequence is the index of the whole expression:

Position[{1,{3}},{1,{3}}]
	{{}}

{1,{3}}[[]]
	{1, {3}}
	
The index gives directions starting from the whole expression --  
with no directions we go nowhere.

Allan Hayes
hay at haystack.demon.co.uk

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