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Re: reading different locations of a matrix specified

  • To: mathgroup at smc.vnet.net
  • Subject: [mg113875] Re: reading different locations of a matrix specified
  • From: Oliver Ruebenkoenig <ruebenko at wolfram.com>
  • Date: Tue, 16 Nov 2010 06:14:49 -0500 (EST)

On Tue, 16 Nov 2010, Nasser M. Abbasi wrote:

> On 11/16/2010 2:30 AM, Oliver Ruebenkoenig wrote:
>
>>> ------------------------------------
>>> A={{1,2,3},{4,5,6},{7,8,9}};
>>> pos={{1,1},{3,3}};
>>> 
>>> A[[ ##&  @@ #]]&  /@ pos
>>> ---------------------------
>>> 
>>> {1,9}
>>> 
>
>
>> 
>> Extract[A, pos]
>> 
>
> I thought I should add something more as to why I ended up with the command
>
> A[[ Sequence @@ #]]&  /@ pos
>
> It started like this:
>
> pos={1,1}
>
> Typing A[[ pos ]] does not work as I wanted (to get element A[[ 1,1 ]])
> And this what started the whole investigation.
>
> I needed a way to remove the '{ }' from around the {1,1} so
> it will be A[[ 1,1 ]]
>
> Then I remembered Sequence does that, so I typed
>
> A[[Sequence@@pos]]  and that is how I got to the command above.
>
> Now, I see that
>
> Extract[A,pos]  to be equivalent to A[[ Sequence@@pos ]]
> without the need to have to worry about '{ }' there.
>
> So, basically Extract[A,{1,1}] works as meant to be, but
> A[[ {1,1} ]] did not.
>
> So, my choice will be now to use Extract[A,pos]
>
> so many commands, so little time :)
>
> --Nasser
>

Maybe a small addition, Extract works well with commands like Position, 
which return data like you have.


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