How do I index into a 2D matrix using two equal length R and C vectors?

I have a 2D matrix rho, and two vectors of indices R and C. How do I use R and C to index into rho?

>> rho=d

rho =

      16     2     3    13
       5    11    10     8
       9     7     6    12
       4    14    15     1

>> R=[1 2]; C=[ 2 3];

i.e I want to obtain rho(R(1), C(1) ..etc

Answer 1

[Ashish Uthama]

You can use a combination of linear indexing and the function sub2ind to index into rho:

>> rho=d

rho =

      16     2     3    13
       5    11    10     8
       9     7     6    12
       4    14    15     1

>> R=[1 2]; C=[ 2 3];
>> ind = sub2ind(size(rho), R, C);
>> rho(ind)

ans =

       2    10

Answer 2

[Sean de Wolski]

Or:

rho = magic(4);
R=[1 2]; C=[ 2 3];
diag(rho(R,C))

runs much faster for smaller R,C but much longer for big ones...

[Ashish Uthama]

+1 for an interesting answer.