A question about changes in matrix

Hello,

I have a question regarding matrices.
Suppose I have a matrix A and I know all the elements of the matrix.
Now I have a matrix B which has most of the elements same as A but with
a few changed entries.
Is there a way to find out which elements changed in B from A. I know I
can do that by comparing each and every element of A with B. But isnt
that quadratic time. Is there a faster way?


Actually, here is why I want the answer to this. Suppose I know whether
A matrix is positive definite. Is there a near linear time complexity
method of finding out if B is positive definite or not, given that very
few elements differ between A and B.

I can rephrase the question if required.

Thanks,
Dilip