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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