Right way to compute cosine similarity between two arrays?

The correct definition of cosine similarity is :

Your code does not compute the denominator, hence the values are wrong.

double cosine_similarity(double *A, double *B, unsigned int Vector_Length)
{
    double dot = 0.0, denom_a = 0.0, denom_b = 0.0 ;
     for(unsigned int i = 0u; i < Vector_Length; ++i) {
        dot += A[i] * B[i] ;
        denom_a += A[i] * A[i] ;
        denom_b += B[i] * B[i] ;
    }
    return dot / (sqrt(denom_a) * sqrt(denom_b)) ;
}