Add softmax function

Author: milancurcic

src/nf/nf_activation.f90

@@ -12,6 +12,7 @@ module nf_activation
   public :: gaussian, gaussian_prime
   public :: relu, relu_prime
   public :: sigmoid, sigmoid_prime
+  public :: softmax, softmax_prime
   public :: softplus, softplus_prime
   public :: step, step_prime
   public :: tanhf, tanh_prime
@@ -103,6 +104,21 @@ pure function sigmoid_prime(x) result(res)
     res = sigmoid(x) * (1 - sigmoid(x))
   end function sigmoid_prime
 
+  pure function softmax(x) result(res)
+    !! Softmax activation function
+    real, intent(in) :: x(:)
+    real :: res(size(x))
+    res = exp(x - maxval(x))
+    res = res / sum(res)
+  end function softmax
+
+  pure function softmax_prime(x) result(res)
+    !! Derivative of the softmax activation function.
+    real, intent(in) :: x(:)
+    real :: res(size(x))
+    res = softmax(x) * (1 - softmax(x))
+  end function softmax_prime
+
   pure function softplus(x) result(res)
     ! Softplus activation function.
     real, intent(in) :: x(:)

src/nf/nf_base_layer_submodule.f90

@@ -6,6 +6,7 @@
                            gaussian, gaussian_prime, &
                            relu, relu_prime, &
                            sigmoid, sigmoid_prime, &
+                           softmax, softmax_prime, &
                            softplus, softplus_prime, &
                            step, step_prime, &
                            tanhf, tanh_prime
@@ -46,6 +47,11 @@ elemental module subroutine set_activation(self, activation)
         self % activation_prime => sigmoid_prime
         self % activation_name = 'sigmoid'
 
+      case('softmax')
+        self % activation => softmax
+        self % activation_prime => softmax_prime
+        self % activation_name = 'softmax'
+
       case('softplus')
         self % activation => softplus
         self % activation_prime => softplus_prime
@@ -64,7 +70,8 @@ elemental module subroutine set_activation(self, activation)
       case default
         error stop 'Activation must be one of: ' // &
           '"elu", "exponential", "gaussian", "relu", ' // &
-          '"sigmoid", "softplus", "step", or "tanh".'
+          '"sigmoid", "softmax", "softplus", "step", ' // &
+          'or "tanh".'
 
     end select