How to index a matrix with existing array?

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Xiaohan Du
Xiaohan Du 2017-11-3
编辑： Stephen23 2017-11-3
Hi all,
Imagine there is a m by m matrix, I'd like to extract the [p, q] element, while [p, q] is the output of some calculations. For instance:
K>> a = rand(3)
a =
0.88517 0.098712 0.67973
0.91329 0.26187 0.13655
0.79618 0.33536 0.72123
K>> b = [2 3]
b =
2 3
K>> a(b)
ans =
0.91329 0.79618
Here b is the location, which should be the 2nd row, 3rd column. However, here Matlab gives the 2nd and 3rd element by column indexing. What I want is 0.13655.
I know I can use:
K>> a(b(1), b(2))
ans =
0.13655
But this is manual, my matrix may have random dimension, so I prefer something like a(b), any ideas?

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采纳的回答

Stephen23
Stephen23 2017-11-3
编辑：Stephen23 2017-11-3
Simply use num2cell and sub2ind, then indices are not required:
>> c = num2cell(b);
>> a(sub2ind(size(a),c{:}))
ans = 0.13655

更多回答（1 个）

KL
KL 2017-11-3
编辑：KL 2017-11-3
use sub2ind
https://de.mathworks.com/help/matlab/ref/sub2ind.html
[row, col] = sub2ind(size(a),b(:,1),b(:,2))
or directly
a(sub2ind(size(a),b(:,1),b(:,2)))
  2 个评论
Xiaohan Du
Xiaohan Du 2017-11-3
No, the manual index is what i'm trying to avoid, i.e. there cannot be:
b(:, 1), b(:, 2)
Xiaohan Du
Xiaohan Du 2017-11-3
Steve's answer is correct, but thanks anyway.

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