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Re: reading different locations of a matrix specified

  • To: mathgroup at smc.vnet.net
  • Subject: [mg113877] Re: reading different locations of a matrix specified
  • From: Sebastian Schmitt <sschmitt at physi.uni-heidelberg.de>
  • Date: Wed, 17 Nov 2010 05:27:00 -0500 (EST)

Hi Nasser!

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?

I use Extract:

In[6]:= A = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}}

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

In[8]:= pos = {{1, 1}, {3, 3}};

In[9]:= Extract[A, pos]

Out[9]= {1, 9}

Cheers,

Sebastian


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