Set the particular row and column zeros except the diagonal elements in matlab

Question

Lets say I am given some indices like B = [10 23 32....];

Now lets say I have a matrix A. What I want to do is for each index from B lets say i, I want to set the ith row and ith column of A to 0 except the diagonal element A(i,i)(it remains untouched).

I can do this by looping. But I want some that is based upon some matrix multiplication which is quicker than just looping.

Any ideas guys?

Answer 1

You can store the diagonal elements temporarily somewhere else, index into A with B to set the corresponding rows and columns to zeros and finally plug back in the diagonal elements -

%// rows in A
rows = size(A,1);

%// Store the diagonal elements temporarily somewhere else
tmp_diagA = A(1:rows+1:end);

%// Set the ith rows and cols (obtained from B) to zero
A(B,:)=0;
A(:,B)=0;

%// Plug back in the diagonal elements in place
A(1:rows+1:end) = tmp_diagA;

Function calls are supposed to be expensive in MATLAB and we have almost have no function calls in this code, so I am hoping it to be fast enough.