The name of this function implies that it returns the number of non-zero elements in the array. The documentation says that, too. In reality, it returns the total number of elements in the array, including any zeros.

To documentation says:
>the number of nonzero coefficients which is in practice the number of stored coefficients.

So this implies that all, including zero coefficients, are counted.

To count the number of numerical non-zeros, you can do either:
- (mat.array() != 0).count()
- mat.sparseView().nonZeros()