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MathGroup Archive 1996

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Re: How do I use Select[list,condition]?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg4540] Re: How do I use Select[list,condition]?
  • From: earhart at athena.mit.edu (Elizabeth J Earhart)
  • Date: Wed, 7 Aug 1996 04:17:46 -0400
  • Organization: Massachvsetts Institvte of Technology
  • Sender: owner-wri-mathgroup at wolfram.com

In article <4thatm$j6e at dragonfly.wolfram.com>,
Jason Welter <welter at texoma.com> wrote:
>I have a list that looks like:
>
>data = {{1,1,a,4,5,6},{2,1,a,2,3,4},{3,1,a,7,8,9},
>               {1,2,b,4,5,6},{2,2,b,2,3,4},{3,2,b,7,8,9},
>               {1,3,c,4,5,6},{2,3,c,2,3,4},{3,3,c,7,8,9}}
>
>and I want to pick out the lists that have a 2 as the second element
>of the list.  I tried something like:
>
>data1 = Select[data,data[[#,2]]==2];
>
>but this doesn't work.  How do I do it?

data1 = Select[data,(#[[2]]==2)&];

Don't forget the '&' sign to signify a pure function - that's my
own most common Mathematica typo (just edging out typing '=' instead
of '==').  The test is performed on each element of data, not on the
list as a whole.  You could also use some variant on

data1 = Cases[data, ls_List /; ls[[2]]==2];

I use this form more often because the format is interchangeable with
Count[] and Position[].

-Elizabeth

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