Sum of elements in a matrix in OpenCV?

Question

I need to sum all the elements in a matrix. I used the function

  sum(sum(A)); 

in matlab. Where A is a matrix of size 300*360. I want to implement the same function in OpenCV. I used something like this.

  double s=cv::sum(cv::sum(A));

But there is error showing cannot convert scalar to double. How to fix this problem?

Answer 1

Unlike Matlab, in opencv, cv::sum(A) sums along ALL dimensions and returns a single number (scalar) that is equal to Matlab's sum(sum(A)).
So, what you need is

double s = cv::sum(A)[0];

Answer 2

In addition with @Shai you can use;

double sum = cv::sum(A).val(0);