Method error

src/regression.rs:126–131

Computes the error of the hypothesis for the examples in the design matrix `x` and the target values `y`. $$\frac{1}{m}(X\theta-y)\^T(X\theta-y)$$

(&self, x: &Matrix<f64>, y: &[f64])

    ///
    /// $$\frac{1}{m}(X\theta-y)\^T(X\theta-y)$$
    pub fn error(&self, x: &Matrix<f64>, y: &[f64]) -> f64 {

        x.mul_vec_minus_vec(&self.thetas, y)
            .iter()
            .fold(0.0, |acc, &x| acc + x*x) / 2.0 / (x.rows() as f64)
    }

    pub fn derivatives(&self, x: &Matrix<f64>, y: &[f64]) -> Vec<f64> {