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MathGroup Archive 2005

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Re: Position function

  • To: mathgroup at smc.vnet.net
  • Subject: [mg55560] Re: [mg55549] Position function
  • From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
  • Date: Wed, 30 Mar 2005 03:20:55 -0500 (EST)
  • References: <200503290842.DAA07267@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

On 29 Mar 2005, at 10:42, konstantpi at mail15.com wrote:

> suppose i have a List:
> a={{1,3,1,2,1},{5,6,1,1,3,1}}
> how i could know the positions of "1" in every sublist , i could use 
> Position[a
> [[1]],1] and Position[a[[2]],1]
> but i want a one formula to give me {{1,3,5},{3,4,6}}

For example:


In[2]:=
a = {{1, 3, 1, 2, 1}, {5, 6, 1, 1, 3, 1}};

In[3]:=
Split[Position[a, 1], #1[[1]] == #2[[1]] & ] /.
  (l_)?VectorQ :> Last[l]

Out[3]=
{{1, 3, 5}, {3, 4, 6}}

>
> the second question:
> using Position[a,1] will give:
> {{1, 1}, {1, 3}, {1, 5}, {2, 3}, {2, 4}, {2, 6}}
> in general how to extract only the second item from every sublist so 
> the output
> will be;
> {1,3,5,3,4,6}

In[4]:=
Position[a, 1] /.
   (l_)?VectorQ :> Last[l]

Out[4]=
{1,3,5,3,4,6}


Andrzej Kozlowski
Chiba, Japan
http://www.akikoz.net/andrzej/index.html
http://www.mimuw.edu.pl/~akoz/


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