# Logistic regression with eigen

I am a new to Eigen, and I implemented a logistic regression model with it. It works but I don't know whether it is implemented in an efficient way.

#include <iostream>
#include <Eigen/Dense>
#include <cmath>

using namespace Eigen;
using namespace Eigen::internal;
using namespace Eigen::Architecture;
using namespace std;

class logistic_regression
{
public:
VectorXd w;
double b;
logistic_regression(int n_in)
{
this->w = VectorXd::Random(n_in);
this->b = 0.0;
}

void train(MatrixXd train_datas, double lr)
{
VectorXd dw(this->w.rows());
double db;
dw = train_datas.rightCols(1) - calc(train_datas.leftCols(this->w.rows()));
db = dw.mean();
MatrixXd tmp = train_datas.leftCols(this->w.rows());
tmp = tmp.array().colwise()*dw.array();
dw = tmp.colwise().mean();
w += lr * dw;
b += lr * db;
}

VectorXd predict(MatrixXd inputs)
{
return ((1 / (1 + ((-inputs*this->w).array() - b).exp())).array() > 0.5).cast<double>();
}

double test_error(MatrixXd datas)
{
VectorXd outputs = predict(datas.leftCols(w.rows()));
/*
cout << outputs << endl << endl << endl;
cout << datas.rightCols(1) << endl << endl << endl;
cout << (outputs - datas.rightCols(1)) << endl;
cout << ((outputs - datas.rightCols(1)).array() > 1e-5).count() << endl;*/

return ((outputs - datas.rightCols(1)).array() > 1e-5).count() / (double)outputs.rows();

}

};

MatrixXd linear_separable_dataset_generator(int n, int dim, VectorXd w, double b)
{
int n_col = w.rows();
MatrixXd datas = MatrixXd::Random(n, n_col + 1);

datas.rightCols(1) = (((datas.leftCols(n_col)*w).array() + b) > 0).cast<double>();
return datas;
}

void test_logistic_regression()
{
logistic_regression lr(2);
VectorXd w(2);
double b = 0.2;
w << 0.3, 0.6;
MatrixXd train_datas = linear_separable_dataset_generator(1000, 2, w, b);
for (int i = 0; i < 500; i++)
{
lr.train(train_datas.topRows(990), 0.1112);
cout << "epoch:" << i << " error:" << lr.test_error(train_datas) << endl;
}
cout << "w:" << lr.w / (lr.b / 0.2) << endl;
cout << "b:" << lr.b << endl;
getchar();
return;
}