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Re: reading different locations of a matrix specified by indices

  • To: mathgroup at smc.vnet.net
  • Subject: [mg113858] Re: reading different locations of a matrix specified by indices
  • From: "Nasser M. Abbasi" <nma at 12000.org>
  • Date: Tue, 16 Nov 2010 05:04:43 -0500 (EST)
  • Reply-to: nma at 12000.org

On 11/15/2010 3:28 PM, Nasser M. Abbasi wrote:
> Given a matrix A, and list of locations withing the matrix,
> where each location is given by {i,j} entry, I wanted to
> find the most efficient and clear way to obtain the entries
> in the matrix by these locations.
>
> I wanted to ask the experts here what they think of this
> method, and if they can suggest a better way.
>
> Example, given
>
> A={{1,2,3},
>      {4,5,6},
>      {7,8,9}}
>
> and wanted to obtain the entries at say {1,1} and {3,3},
> which will be 1 and 9 in this example. So I first made a
> list of the locations :
>
> pos={ {1,1},{3,3}};
>
> Then typed
>
> Map[ A[[Sequence@@ # ]]&  , pos ]
>
> and got back
>
> {1,9}
>
> So, this seems to work.
>
> Any suggestion of may be of a more 'obvious' or better way?
>

While looking around, I Found one can use ##& in place of Sequence

------------------------------------
A={{1,2,3},{4,5,6},{7,8,9}};
pos={{1,1},{3,3}};

A[[ ##& @@ #]]& /@ pos
---------------------------

{1,9}


--Nasser


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