How to index a matrix with existing array?

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?

Answer 1

[Stephen23]

Simply use num2cell and sub2ind, then indices are not required:

>> c = num2cell(b);
>> a(sub2ind(size(a),c{:}))
ans =  0.13655

[Xiaohan Du]

true MVP.

Answer 2

[KL]

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)))

[Xiaohan Du]

No, the manual index is what i'm trying to avoid, i.e. there cannot be:

b(:, 1), b(:, 2)

[Xiaohan Du]

Steve's answer is correct, but thanks anyway.