Find elements having a specific row and column numbers in a matrix
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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 个评论
"Of course, I can solve this in a for-loop which is time conssumming"
I doubt that. Why do you think that?
"I wonder if this is the fastest approach?"
Probably a loop. Lets compare against using SUB2IND (which itself is a perfectly reasonable approach):
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 = c; % preallocate
for k = 1:numel(v)
v(k) = A(r(k),c(k));
end
toc
tic
w = A(sub2ind(size(A),r,c));
toc
isequal(v,w)
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
2022-12-21
I suspect sub2ind might just apply the little string of code I devised in my answer, or something very similar to it.
采纳的回答
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))
更多回答（1 个）
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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