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Re: reading different locations of a matrix specified

  • To: mathgroup at smc.vnet.net
  • Subject: [mg113872] Re: reading different locations of a matrix specified
  • From: "Nasser M. Abbasi" <nma at 12000.org>
  • Date: Tue, 16 Nov 2010 06:14:15 -0500 (EST)

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


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