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Re: reading different locations of a matrix specified

  • To: mathgroup at smc.vnet.net
  • Subject: [mg113868] Re: reading different locations of a matrix specified
  • From: Oliver Ruebenkoenig <ruebenko at wolfram.com>
  • Date: Tue, 16 Nov 2010 06:13:30 -0500 (EST)

On Tue, 16 Nov 2010, Nasser M. Abbasi 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[A, pos]


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