[mlpack-svn] master: Add implementation of logisitc activation function. (a1f50fc)

[gitdub at big.cc.gt.atl.ga.us]

Repository : https://github.com/mlpack/mlpack

On branch  : master
Link       : https://github.com/mlpack/mlpack/compare/7dad33e2c2fd30e87e125ef826ae7ab652df8cf6...a1f50fc5189e3d29de559c3c7c34011571dfc354

>---------------------------------------------------------------

commit a1f50fc5189e3d29de559c3c7c34011571dfc354
Author: Marcus Edel <marcus.edel at fu-berlin.de>
Date:   Wed Dec 31 21:51:36 2014 +0100

    Add implementation of logisitc activation function.


>---------------------------------------------------------------

a1f50fc5189e3d29de559c3c7c34011571dfc354
 .../ann/activation_functions/logistic_function.hpp | 100 +++++++++++++++++++++
 1 file changed, 100 insertions(+)

diff --git a/src/mlpack/methods/ann/activation_functions/logistic_function.hpp b/src/mlpack/methods/ann/activation_functions/logistic_function.hpp
new file mode 100644
index 0000000..728a2c5
--- /dev/null
+++ b/src/mlpack/methods/ann/activation_functions/logistic_function.hpp
@@ -0,0 +1,100 @@
+/**
+ * @file logistic_function.hpp
+ * @author Marcus Edel
+ *
+ * Definition and implementation of the logistic function.
+ */
+#ifndef __MLPACK_METHODS_ANN_ACTIVATION_FUNCTIONS_LOGISTIC_FUNCTION_HPP
+#define __MLPACK_METHODS_ANN_ACTIVATION_FUNCTIONS_LOGISTIC_FUNCTION_HPP
+
+#include <mlpack/core.hpp>
+
+namespace mlpack {
+namespace ann /** Artificial Neural Network. */ {
+
+/**
+ * The logistic function, defined by
+ *
+ * @f[
+ * f(x) &=& \frac{1}{1 + e^{-x}} \\
+ * f'(x) &=& f(x) * (1 - f(x))
+ * f^{-1}(y) &=& ln(\frac{y}{1-y})
+ * @f]
+ */
+class LogisticFunction
+{
+  public:
+  /**
+   * Computes the logistic function.
+   *
+   * @param x Input data.
+   * @return f(x).
+   */
+  static double fn(const double x)
+  {
+    return 1.0 /  (1.0 + arma::trunc_exp(-x));
+  }
+
+  /**
+   * Computes the logistic function.
+   *
+   * @param x Input data.
+   * @param y The resulting output activation.
+   */
+  template<typename InputVecType, typename OutputVecType>
+  static void fn(const InputVecType& x, OutputVecType& y)
+  {
+    y = 1.0 / (1.0 + arma::trunc_exp(-x));
+  }
+
+  /**
+   * Computes the first derivative of the logistic function.
+   *
+   * @param x Input data.
+   * @return f'(x)
+   */
+  static double deriv(const double y)
+  {
+    return y * (1.0 - y);
+  }
+
+  /**
+   * Computes the first derivatives of the logistic function.
+   *
+   * @param y Input activations.
+   * @param x The resulting derivatives.
+   */
+  template<typename InputVecType, typename OutputVecType>
+  static void deriv(const InputVecType& y, OutputVecType& x)
+  {
+    x = y % (1.0 - y);
+  }
+
+  /**
+   * Computes the inverse of the logistic function.
+   *
+   * @param y Input data.
+   * @return f^{-1}(y)
+   */
+  static double inv(const double y)
+  {
+    return arma::trunc_log(y / (1 - y));
+  }
+
+  /**
+   * Computes the inverse of the logistic function.
+   *
+   * @param y Input data.
+   * @return  x The resulting inverse of the input data.
+   */
+  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