logistic_function.hpp

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#ifndef MLPACK_METHODS_ANN_ACTIVATION_FUNCTIONS_LOGISTIC_FUNCTION_HPP
#define MLPACK_METHODS_ANN_ACTIVATION_FUNCTIONS_LOGISTIC_FUNCTION_HPP

#include <mlpack/prereqs.hpp>

namespace mlpack {
namespace ann  {

class LogisticFunction
{
  public:
  template<typename eT>
  static double fn(const eT x)
  {
    if (x < arma::Datum<eT>::log_max)
    {
      if (x > -arma::Datum<eT>::log_max)
        return 1.0 /  (1.0 + std::exp(-x));

      return 0.0;
    }

    return 1.0;
  }

  template<typename InputVecType, typename OutputVecType>
  static void fn(const InputVecType& x, OutputVecType& y)
  {
    y = (1.0 / (1 + arma::exp(-x)));
  }

  static double deriv(const double y)
  {
    return y * (1.0 - y);
  }

  template<typename InputVecType, typename OutputVecType>
  static void deriv(const InputVecType& y, OutputVecType& x)
  {
    x = y % (1.0 - y);
  }

  static double inv(const double y)
  {
    return arma::trunc_log(y / (1 - y));
  }

  template<typename InputVecType, typename OutputVecType>
  static void inv(const InputVecType& y, OutputVecType& x)
  {
    x = arma::trunc_log(y / (1 - y));
  }
}; // class LogisticFunction

} // namespace ann
} // namespace mlpack

#endif