# Find elements having a specific row and column numbers in a matrix

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Mohammad Shojaei Arani
on 21 Dec 2022

Commented: Davide Masiello
on 21 Dec 2022

Hello,

I have a simple question but could not find a single command which can do the job. I explain via an example. Consider the following matrix

A = [ 85 45 15 28 11 4 59

79 81 87 32 94 10 85

90 79 64 62 93 25 62

29 41 82 66 100 16 72

37 33 72 59 22 41 98];

Assume that r = [2 5 2 2 5 2 1], c = [3 7 7 1 2 1 6]; What I want, are 7 elements of A whose first component comes from vector r but whose second component comes from vector c. I mean, A(2,3), A(5,7), A(2,7), A(2,1),A(5,2), A(2,1), A(1,6). Of course, I can solve this in a for-loop which is time conssumming. In Matlab, if I write A(r,c) then I get something else which I do not want. The answer to my question is actually the diagonal elements of this matric, that is to say,

diag(A(r,c))

but, I wonder if this is the fastest approach?

Do you know a better way?

Thanks in advance!

Babak

##### 3 Comments

Torsten
on 21 Dec 2022

A = [85,45,15,28,11,4,59;79,81,87,32,94,10,85;90,79,64,62,93,25,62;29,41,82,66,100,16,72;37,33,72,59,22,41,98];

r = [2,5,2,2,5,2,1];

c = [3,7,7,1,2,1,6];

tic

v = A(size(A,1)*(c-1)+r);

toc

Davide Masiello
on 21 Dec 2022

### Accepted Answer

Torsten
on 21 Dec 2022

A = [ 85 45 15 28 11 4 59

79 81 87 32 94 10 85

90 79 64 62 93 25 62

29 41 82 66 100 16 72

37 33 72 59 22 41 98];

r = [2 5 2 2 5 2 1];

c = [3 7 7 1 2 1 6];

A(sub2ind(size(A),r,c))

### More Answers (1)

Davide Masiello
on 21 Dec 2022

If the answer is the diagonal, then I'd use the diag function.

However, if you are still interested in generalizing the code, you could do

A = [ 85 45 15 28 11 4 59

79 81 87 32 94 10 85

90 79 64 62 93 25 62

29 41 82 66 100 16 72

37 33 72 59 22 41 98];

r = [2 5 2 2 5 2 1];

c = [3 7 7 1 2 1 6];

A(size(A,1)*(c-1)+r)

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