TActivationSigmoid.cxx

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// @(#)root/tmva $Id$
// Author: Matt Jachowski
 
/**********************************************************************************
 * Project: TMVA - a Root-integrated toolkit for multivariate data analysis       *
 * Package: TMVA                                                                  *
 * Class  : TActivationSigmoid                                                    *
 *                                             *
 *                                                                                *
 * Description:                                                                   *
 *      Sigmoid activation function for TNeuron                                   *
 *                                                                                *
 * Authors (alphabetical):                                                        *
 *      Matt Jachowski  <jachowski@stanford.edu> - Stanford University, USA       *
 *                                                                                *
 * Copyright (c) 2005:                                                            *
 *      CERN, Switzerland                                                         *
 *                                                                                *
 * Redistribution and use in source and binary forms, with or without             *
 * modification, are permitted according to the terms listed in LICENSE           *
 * (see tmva/doc/LICENSE)                                          *
 **********************************************************************************/
 
/*! \class TMVA::TActivationSigmoid
\ingroup TMVA
Sigmoid activation function for TNeuron.
*/
 
#include "TMVA/TActivationSigmoid.h"
 
#include "TMVA/TActivation.h"
 
#include "TMath.h"
#include "TString.h"
 
#include <iostream>
 
 
 
////////////////////////////////////////////////////////////////////////////////
/// evaluate the sigmoid
 
Double_t TMVA::TActivationSigmoid::Eval(Double_t arg)
{
   return 1.0 / (1.0 + TMath::Exp(-arg));
}
 
////////////////////////////////////////////////////////////////////////////////
/// evaluate the derivative of the sigmoid
 
Double_t TMVA::TActivationSigmoid::EvalDerivative(Double_t arg)
{
   Double_t tmp = (1.0 + TMath::Exp(-arg));
   return TMath::Exp(-arg) / (tmp * tmp);
}
 
////////////////////////////////////////////////////////////////////////////////
/// get expressions for the sigmoid and its derivatives
 
TString TMVA::TActivationSigmoid::GetExpression()
{
   TString expr = "1.0/(1.0+TMath::Exp(-x))\t\tTMath::Exp(-x)/(1.0+TMath::Exp(-x))^2";
   return expr;
}
 
////////////////////////////////////////////////////////////////////////////////
/// writes the sigmoid activation function source code
 
void TMVA::TActivationSigmoid::MakeFunction( std::ostream& fout, const TString& fncName )
{
   fout << "double " << fncName << "(double x) const {" << std::endl;
   fout << "   // sigmoid" << std::endl;
   fout << "   return 1.0/(1.0+exp(-x));" << std::endl;
   fout << "}" << std::endl;
}