MATLAB: Question about assigning value in a matrix.

Question

Hi, all, again, I want to do the work quickly: I have a large matrix A, and I want to assign value in some entry in each row.

For example:
A=zeros(4);
a=[1,2;
 2,3;
 3,2;
 4,1];

I want (1,2), (2,3),(3,2), (4,1) of matrix A be 1. How shall I make it work in no more than 3 commands?

What I am doing now is using loops:

for i=1:4
A(a(i,1),a(i,2)=1;
end

However, as i is very large like 3^14, the program is very slow. So I wonder is there a way to speed the programming?

Thanks.

Answer 1

idx = sub2ind(size(A), [1 2 3 4], [2 3 2 1]);
A(idx) = 1;