decrease the time of calculating

Hi I want to decrease the time of calculating in my algorithm. in this algorithm there is an objective function.in every iteration x is achieved from main loop of algorithm then it is entered to objective function in order to calculate the value of objective function. x is a matrix with 1000 rows and 1000 columns and 4 in third dimension. it is (0-1)matrix. also there is the other matrix (c). this matrix has the same rows, columns and third dimension with values between (1-256).this matrix is constant in every iteration.

this is my code:

 z=sum(sum(sum(c.*x)))

the function is very simple but why it takes many times to calculate in every iteration.

Answer 1

[Jan]

Try:

L = (x == 1);  % Convert it to a logical array outside the loops
...

z = sum(c(L))

To my surprise logical indexing was not implemented efficiently - at least in former Matlab versions. So you can give this a try: FEX: CopyMask

z = sum(CopyMask(c, L));