Upsilon/apps/regression/model/logistic_model.cpp

#include "logistic_model.h"
#include <math.h>
#include <assert.h>

namespace Regression {

double LogisticModel::evaluate(double * modelCoefficients, double x) const {
  double a = modelCoefficients[0];
  double b = modelCoefficients[1];
  double c = modelCoefficients[2];
  return c/(1.0+a*exp(-b*x));
}

double LogisticModel::partialDerivate(double * modelCoefficients, int derivateCoefficientIndex, double x) const {
  double a = modelCoefficients[0];
  double b = modelCoefficients[1];
  double c = modelCoefficients[2];
  double denominator = 1.0+a*exp(-b*x);
  if (derivateCoefficientIndex == 0) {
    // Derivate: exp(-b*x)*(-1 * c/(1.0+a*exp(-b*x))^2)
    return -exp(-b*x) * c/(denominator * denominator);
  }
  if (derivateCoefficientIndex == 1) {
    // Derivate: (-x)*a*exp(-b*x)*(-1/(1.0+a*exp(-b*x))^2)
    return x*a*exp(-b*x)*c/(denominator * denominator);
  }
  if (derivateCoefficientIndex == 2) {
    // Derivate: (-x)*a*exp(-b*x)*(-1/(1.0+a*exp(-b*x))^2)
    return 1.0/denominator;
  }
  assert(false);
  return 0.0;
}

}