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MathGroup Archive 2007

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Problem with Position applied on 2D list?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg72570] Problem with Position applied on 2D list?
  • From: Heiko Damerau <heiko.damerau.news at cern.ch>
  • Date: Thu, 11 Jan 2007 01:56:11 -0500 (EST)

Dear All,

     There is some strange behaviour of the Position[] function I am 
struggling with:

integerRandomTable = Table[Table[Random[Integer, 10], {10}], {10}];

Find the position of all lists containing the value 5 (just a simple 
example):

Position[integerRandomTable, _?(MemberQ[#, 5] &)]

gives the expected result, e.g. {{1}, {3}, {4}, {5}, {6}, {7}, {8}}.
However, just asking for all lists not containing the value 5

Position[integerRandomTable, _?(Not[MemberQ[#, 5]] &)]

Doesn't give the expected result but some strange thing, e.g. {{0}, 
{1,0}, {1, 1}, {1, 2}, {1, 3}, {1, 4}, ..., {10, 3}, {10, 4}, {10, 5}, 
{10, 6}, {10, 7}, {10, 8}, {10, 9}, {10, 10}, {10}, {}}

     What is the explanation for the different behaviour of MemberQ[] 
and Not[MemberQ[]]?

     Thanks a lot in advance for any help.

Best regards,
Heiko


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