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reading different locations of a matrix specified by indices list
*To*: mathgroup at smc.vnet.net
*Subject*: [mg113866] reading different locations of a matrix specified by indices list
*From*: "Nasser M. Abbasi" <nma at 12000.org>
*Date*: Tue, 16 Nov 2010 05:06:11 -0500 (EST)
*Reply-to*: nma at 12000.org
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?
fyi, in another system, I do the above as follows (just for
comparison)
----------------------
A=[1 2 3;
4 5 6;
7 8 9];
I=[1 3]; % setup the I,J indices
J=[1 3];
A(sub2ind(size(A),I,J))
ans =
1 9
----------------------
I am happy with the Mathematica method, once I figure
out how to do it, the trick for me was to think functional :)
Both ways in both systems are about the same order of
complexity and length.
--Nasser
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