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Re: reading different locations of a matrix specified by indices

  • To: mathgroup at smc.vnet.net
  • Subject: [mg113871] Re: reading different locations of a matrix specified by indices
  • From: Raffy <adraffy at gmail.com>
  • Date: Tue, 16 Nov 2010 06:14:03 -0500 (EST)
  • References: <ibtkvu$5io$1@smc.vnet.net>

On Nov 16, 2:04 am, "Nasser M. Abbasi" <n... at 12000.org> wrote:
> 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

Extract[{{1, 2, 3}, {4, 5, 6}, {7, 8, 9}}, {{1, 1}, {3, 3}}] === {1, =
9}


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