Matrix Multiplication for each row

I have a matrix A = 100000*2 and matrix B which is a symmetric square matrix with number of row and columns equal to the number of columns of A.

I want to compute diag (A*B*A') but since A*B*A' is 100000*100000, matlab cannot compute it and then give diagonal of it. Any workaround?

What i am actually looking for is The first row of A multiplied by B and then multiplied by first column of A'. Now, I can perform this 100000 times to get 100000*1 matrix but that is time consuming. Any workaround will be appreciated?

Answer 1

[Chunru]

n = 5; %100000
A = randn(n, 2);  
B = [4 2; 2 3];
if n<100
    d1 = diag(A*B*A')
end

d1 = 5×1

4.3969 45.0577 9.4548 6.3347 1.4279

d2 = sum(A.*(A*B'), 2)

d2 = 5×1

4.3969 45.0577 9.4548 6.3347 1.4279

[Ankush Kumar Mishra]

Thanks, that worked!!!!