def __init__(self, 
                 my_data_set = "mnist", 
                 n_hidden_layers = 1, 
                 ay_nodes_layers = [0, 100, 0], # array which should have as much elements as n_hidden + 2
                 n_nodes_layer_out = 10,  # expected number of nodes in output layer 
                                                  
                 my_activation_function = "sigmoid", 
                 my_out_function        = "sigmoid",   
                 my_loss_function       = "LogLoss",   
                 
                 n_size_mini_batch = 50,  # number of data elements in a mini-batch 
                 
                 n_epochs      = 1,
                 n_max_batches = -1,  # number of mini-batches to use during epochs - > 0 only for testing 
                                      # a negative value uses all mini-batches 
                 
                 lambda2_reg = 0.1,     # factor for quadratic regularization term 
                 lambda1_reg = 0.0,     # factor for linear regularization term 
                 
                 vect_mode = 'cols', 
                 
                 learn_rate = 0.001,        # the learning rate (often called epsilon in textbooks) 
                 decrease_const = 0.00001,  # a factor for decreasing the learning rate with epochs
                 mom_rate  
 = 0.0005,       # a factor for momentum learning
                 
                 figs_x1=12.0, figs_x2=8.0, 
                 legend_loc='upper right',
                 
                 b_print_test_data = True
                 
                 ):
        '''
        Initialization of MyANN
        Input: 
            data_set: type of dataset; so far only the "mnist", "mnist_784" datsets are known 
                      We use this information to prepare the input data and learn about the feature dimension. 
                      This info is used in preparing the size of the input layer.     
            n_hidden_layers = number of hidden layers => between input layer 0 and output layer n 
            
            ay_nodes_layers = [0, 100, 0 ] : We set the number of nodes in input layer_0 and the output_layer to zero 
                              Will be set to real number afterwards by infos from the input dataset. 
                              All other numbers are used for the node numbers of the hidden layers.
            n_nodes_out_layer = expected number of nodes in the output layer (is checked); 
                                this number corresponds to the number of categories NC = number of labels to be distinguished
            
            my_activation_function : name of the activation function to use 
            my_out_function : name of the "activation" function of the last layer which produces the output values 
            my_loss_function : name of the "cost" or "loss" function used for optimization 
            
            n_size_mini_batch : Number of elements/samples in a mini-batch of training data 
                                The number of mini-batches will be calculated from this
            
            n_epochs : number of epochs to calculate during training
            n_max_batches : > 0: maximum of mini-batches to use during training 
                            < 0: use all mini-batches  
            
            lambda_reg2:    The factor for the quadartic regularization term 
            lambda_reg1:    The factor for the linear regularization term 
            
            vect_mode: Are 1-dim data arrays (vctors) ordered by columns or rows ?
            
            learn rate :     Learning rate - definies by how much we correct weights in the indicated direction of the gradient on the cost hyperplane.
            decrease_const:  Controls a systematic decrease of the learning rate with epoch number 
            mom_const:       Momentum rate. Controls a mixture of the last with the present weight corrections (momentum learning)
            
            figs_x1=12.0, figs_x2=8.0 : Standard sizing of plots , 
            legend_loc='upper right': Position of legends in the plots
            
            b_print_test_data: Boolean variable to control the print out of some tests data 
             
         '''
        
        # Array (Python list) of known input data sets 
        self._input_data_sets = ["mnist", "mnist_784", "mnist_keras"]  
        self._my_data_set = my_data_set
        
        # X, y, X_train, y_train, X_test, y_test  
            # will be set by analyze_input_data 
            # X: Input array (2D) - at present status of MNIST image data, only.    
            # y: result (=classification data) [digits represent categories in the case of Mnist]
        self._X       = None 
        self._X_train = None 
        self._X_test  = None   
        self._y       = None 
        self._y_train = None 
        self._y_test  = None
        
        # relevant dimensions 
        # from input data information;  will be set in handle_input_data()
        self._dim_sets     = 0  
        self._dim_features = 0  
        self._n_labels     = 0   # number of unique labels - will be extracted from y-data 
        
      
  # Img sizes 
        self._dim_img      = 0 # should be sqrt(dim_features) - we assume square like images  
        self._img_h        = 0 
        self._img_w        = 0 
        
        # Layers
        # ------
        # number of hidden layers 
        self._n_hidden_layers = n_hidden_layers
        # Number of total layers 
        self._n_total_layers = 2 + self._n_hidden_layers  
        # Nodes for hidden layers 
        self._ay_nodes_layers = np.array(ay_nodes_layers)
        # Number of nodes in output layer - will be checked against information from target arrays
        self._n_nodes_layer_out = n_nodes_layer_out
        
        # Weights 
        # --------
        # empty List for all weight-matrices for all layer-connections
        # Numbering : 
        # w[0] contains the weight matrix which connects layer 0 (input layer ) to hidden layer 1 
        # w[1] contains the weight matrix which connects layer 1 (input layer ) to (hidden?) layer 2 
        self._li_w = []
        
        # Arrays for encoded output labels - will be set in _encode_all_mnist_labels()
        # -------------------------------
        self._ay_onehot = None
        self._ay_oneval = None
        
        # Known Randomizer methods ( 0: np.random.randint, 1: np.random.uniform )  
        # ------------------
        self.__ay_known_randomizers = [0, 1]

        # Types of activation functions and output functions 
        # ------------------
        self.__ay_activation_functions = ["sigmoid"] # later also relu 
        self.__ay_output_functions     = ["sigmoid"] # later also softmax 
        
        # Types of cost functions 
        # ------------------
        self.__ay_loss_functions = ["LogLoss", "MSE" ] # later also othr types of cost/loss functions  


        # the following dictionaries will be used for indirect function calls 
        self.__d_activation_funcs = {
            'sigmoid': self._sigmoid, 
            'relu':    self._relu
            }
        self.__d_output_funcs = { 
            'sigmoid': self._sigmoid, 
            'softmax': self._softmax
            }  
        self.__d_loss_funcs = { 
            'LogLoss': self._loss_LogLoss, 
            'MSE': self._loss_MSE
            }  
        # Derivative functions 
        self.__d_D_activation_funcs = {
            'sigmoid': self._D_sigmoid, 
            'relu':    self._D_relu
            }
        self.__d_D_output_funcs = { 
            'sigmoid': self._D_sigmoid, 
            'softmax': self._D_softmax
            }  
        self.__d_D_loss_funcs = { 
            'LogLoss': self._D_loss_LogLoss, 
            'MSE': self._D_loss_MSE
            }  
        
        
        # The following variables will later be set by _check_and set_activation_and_out_functions()            
        
        self._my_act_func  = my_activation_function
        self._my_out_func  = my_out_function
        self._my_loss_func = my_loss_function
        self._act_func = None    
        self._out_func = None    
        self._loss_func = None    
        
        # number of data samples in a mini-batch 
        self._n_size_mini_batch = n_size_mini_batch
        self._n_mini_batches = None  # will be determined by _get_number_of_mini_batches()

        # maximum number of epochs - we set this number to an assumed maximum 
        # - as we shall build a backup and reload functionality for training, this should not be a major problem 
        self._n_epochs = n_epochs
        
        # maximum number of batches to handle ( if < 0 => all!) 
        self._n_max_batches = n_max_batches
        # actual number of batches 
        self._n_batches = None

        # regularization parameters
        self._lambda2_reg = lambda2_reg
        self._
lambda1_reg = lambda1_reg
        
        # parameter for momentum learning 
        self._learn_rate = learn_rate
        self._decrease_const = decrease_const
        self._mom_rate   = mom_rate
        self._li_mom = [None] *  self._n_total_layers
        
        # book-keeping for epochs and mini-batches 
        # -------------------------------
        # range for epochs - will be set by _prepare-epochs_and_batches() 
        self._rg_idx_epochs = None
        # range for mini-batches 
        self._rg_idx_batches = None
        # dimension of the numpy arrays for book-keeping - will be set in _prepare_epochs_and_batches() 
        self._shape_epochs_batches = None    # (n_epochs, n_batches, 1) 

        # list for error values at outermost layer for minibatches and epochs during training
        # we use a numpy array here because we can redimension it
        self._ay_theta = None
        # list for cost values of mini-batches during training 
        # The list will later be split into sections for epochs 
        self._ay_costs = None
        
        
        # Data elements for back propagation
        # ----------------------------------
        
        # 2-dim array of partial derivatives of the elements of an additive cost function 
        # The derivative is taken with respect to the output results a_j = ay_ANN_out[j]
        # The array dimensions account for nodes and sampls of a mini_batch. The array will be set in function 
        # self._initiate_bw_propagation()
        self._ay_delta_out_batch = None
        

        # parameter to allow printing of some test data 
        self._b_print_test_data = b_print_test_data

        # Plot handling 
        # --------------
        # Alternatives to resize plots 
        # 1: just resize figure  2: resize plus create subplots() [figure + axes] 
        self._plot_resize_alternative = 1 
        # Plot-sizing
        self._figs_x1 = figs_x1
        self._figs_x2 = figs_x2
        self._fig = None
        self._ax  = None 
        # alternative 2 does resizing and (!) subplots() 
        self.initiate_and_resize_plot(self._plot_resize_alternative)        
        
        
        # ***********
        # operations 
        # ***********
        
        # check and handle input data 
        self._handle_input_data()
        # set the ANN structure 
        self._set_ANN_structure()
        
        # Prepare epoch and batch-handling - sets ranges, limits num of mini-batches and initializes book-keeping arrays
        self._rg_idx_epochs, self._rg_idx_batches = self._prepare_epochs_and_batches()
        
        # perform training 
        start_c = time.perf_counter()
        self._fit(b_print=True, b_measure_batch_time=False)
        end_c = time.perf_counter()
        print('\n\n ------') 
        print('Total training Time_CPU: ', end_c - start_c) 
        print("\nStopping program regularily")
        sys.exit()

    '''-- Method to set ANN structure --''' 
    def _set_ANN_structure(self):
        # check consistency of the node-number list with the number of hidden layers (n_hidden)
        self._check_layer_and_node_numbers()
        # set node numbers for the input layer and the output layer
        self._set_nodes_for_input_output_layers() 
        self._show_node_numbers() 
        
        # create the weight matrix between input and first hidden layer 
        self._create_WM_Input() 
        # create weight matrices between the 
hidden layers and between tha last hidden and the output layer 
        self._create_WM_Hidden() 
        
        # initialize momentum differences
        self._create_momentum_matrices()
        #print("\nLength of li_mom = ", str(len(self._li_mom)))
        
        # check and set activation functions 
        self._check_and_set_activation_and_out_functions()
        self._check_and_set_loss_function()
        
        return None

    '''-- Method to create the weight matrix between L0/L1 --'''
    def _create_WM_Input(self):
        '''
        Method to create the input layer 
        The dimension will be taken from the structure of the input data 
        We need to fill self._w[0] with a matrix for conections of all nodes in L0 with all nodes in L1
        We fill the matrix with random numbers between [-1, 1] 
        '''
        # the num_nodes of layer 0 should already include the bias node 
        num_nodes_layer_0 = self._ay_nodes_layers[0]
        num_nodes_with_bias_layer_0 = num_nodes_layer_0 + 1 
        num_nodes_layer_1 = self._ay_nodes_layers[1] 
        
        # fill the matrix with random values 
        #rand_low  = -1.0
        #rand_high = 1.0
        rand_low  = -0.5
        rand_high = 0.5
        rand_size = num_nodes_layer_1 * (num_nodes_with_bias_layer_0) 
        
        randomizer = 1 # method np.random.uniform   
        
        w0 = self._create_vector_with_random_values(rand_low, rand_high, rand_size, randomizer)
        w0 = w0.reshape(num_nodes_layer_1, num_nodes_with_bias_layer_0)
        
        # put the weight matrix into array of matrices 
        self._li_w.append(w0)
        print("\nShape of weight matrix between layers 0 and 1 " + str(self._li_w[0].shape))
        
#
    '''-- Method to create the weight-matrices for hidden layers--''' 
    def _create_WM_Hidden(self):
        '''
        Method to create the weights of the hidden layers, i.e. between [L1, L2] and so on ... [L_n, L_out] 
        We fill the matrix with random numbers between [-1, 1] 
        '''
        
        # The "+1" is required due to range properties ! 
        rg_hidden_layers = range(1, self._n_hidden_layers + 1, 1)

        # for random operation 
        rand_low  = -1.0
        rand_high = 1.0
        
        for i in rg_hidden_layers: 
            print ("Creating weight matrix for layer " + str(i) + " to layer " + str(i+1) )
            
            num_nodes_layer = self._ay_nodes_layers[i] 
            num_nodes_with_bias_layer = num_nodes_layer + 1 
            
            # the number of the next layer is taken without the bias node!
            num_nodes_layer_next = self._ay_nodes_layers[i+1]
            
            # assign random values  
            rand_size = num_nodes_layer_next * num_nodes_with_bias_layer   
            
            randomizer = 1 # np.random.uniform
            
            w_i_next = self._create_vector_with_random_values(rand_low, rand_high, rand_size, randomizer)   
            w_i_next = w_i_next.reshape(num_nodes_layer_next, num_nodes_with_bias_layer)
            
            # put the weight matrix into our array of matrices 
            self._li_w.append(w_i_next)
            print("Shape of weight matrix between layers " + str(i) + " and " + str(i+1) + " = " + str(self._li_w[i].shape))
#
    '''-- Method to create and initialize matrices for momentum learning (differences) '''
    def _create_momentum_matrices(self):
        rg_layers = range(0, self._n_total_layers - 1)
        for i in rg_layers: 
r
            self._li_mom[i] = np.zeros(self._li_w[i].shape)
            #print("shape of li_mom[" + str(i) + "] = ", self._li_mom[i].shape)

    ''' -- Method to set the number of batches based on given batch size -- '''
    def _fit(self, b_print = False, b_measure_batch_time = False):
        
        rg_idx_epochs  = self._rg_idx_epochs 
        rg_idx_batches = self._rg_idx_batches
        if (b_print):    
            print("\nnumber of epochs = " + str(len(rg_idx_epochs)))
            print("max number of batches = " + str(len(rg_idx_batches)))
       
        # loop over epochs
        for idxe in rg_idx_epochs:
            if (b_print):
                print("\n ---------")
                print("\nStarting epoch " + str(idxe+1))
                
                self._learn_rate /= (1.0 + self._decrease_const * idxe)
            
            # loop over mini-batches
            for idxb in rg_idx_batches:
                if (b_print):
                    print("\n ---------")
                    print("\nDealing with mini-batch " + str(idxb+1))
                if b_measure_batch_time: 
                    start_0 = time.perf_counter()
                # deal with a mini-batch
                self._handle_mini_batch(num_batch = idxb, num_epoch=idxe, b_print_y_vals = False, b_print = False)
                if b_measure_batch_time: 
                    end_0 = time.perf_counter()
                    print('Time_CPU for batch ' + str(idxb+1), end_0 - start_0) 
                
                #if idxb == 100: 
                #    sys.exit() 
        
        return None

    ''' -- Method to deal with a batch -- '''
    def _handle_mini_batch(self, num_batch = 0, num_epoch = 0, b_print_y_vals = False, b_print = False, b_keep_bw_matrices = True):
        '''
        For each batch we keep the input data array Z and the output data A (output of activation function!) 
        for all layers in Python lists
        We can use this as input variables in function calls - mutable variables are handled by reference values !
        We receive the A and Z data from propagation functions and proceed them to cost and gradient calculation functions
        
        As an initial step we define the Python lists li_Z_
in_layer and li_A_out_layer 
        and fill in the first input elements for layer L0  
        
        Forward propagation:
        --------------------
        li_Z_in_layer : List of layer-related 2-dim matrices for input values z at each node (rows) and all batch-samples (cols).
        li_A_out_layer: List of layer-related 2-dim matrices for output alues z at each node (rows) and all batch-samples (cols).
                        The output is created by Phi(z), where Phi represents an activation or output function 
        
        Note that the matrices in ay_A_out will be permanently extended by a row (over all samples) 
        to account for a bias node of each inner layer. This happens during FW propagation. 
        
        Note that the matrices ay_Z_in will be temporarily extended by a row (over all samples) 
        to account for a bias node of each inner layer. This happens during BW propagation.
        
        Backward propagation:
        -------------------- 
        li_delta_out:  Startup matrix for _out_delta-values at the outermost layer 
        li_grad_layer: List of layer-related matrices with gradient values for the correction of the weights 
        
        Depending on parameter "b_keep_bw_matrices" we keep 
            - a list of layer-related matrices D with values for the derivatives of the act./output functions
            - a list of layer-related matrices for the back propagated delta-values 
        in lists during back propagation. This can support error analysis. 
        
        All matrices in the lists are 2 dimensional with dimensions for nodes (rows) and training samples (cols) 
        All these lists be deleted at the end of the function to accelerate garbadge handling
        
        Input parameters: 
        ----------------
        num_epoch:     Number of present epoch
        num_batch:    Number of present mini-batch 
        '''
        # Layer-related lists to be filled with 2-dim Numpy matrices during FW propagation
        # ********************************************************************************
        li_Z_in_layer  = [None] * self._n_total_layers # List of matrices with z-input values for each layer; filled during FW-propagation
        li_A_out_layer = li_Z_in_layer.copy()          # List of matrices with results of activation/output-functions for each layer; filled during FW-propagation
        li_delta_out   = li_Z_in_layer.copy()          # Matrix with out_delta-values at the outermost layer 
        li_delta_layer = li_Z_in_layer.copy()          # List of the matrices for the BW propagated delta values 
        li_D_layer     = li_Z_in_layer.copy()          # List of the derivative matrices D containing partial derivatives of the activation/ouput functions 
        li_grad_layer  = li_Z_in_layer.copy()          # List of the matrices with gradient values for weight corrections
        
        if b_print: 
            len_lists = len(li_A_out_layer)
            print("\nnum_epoch = ", num_epoch, "  num_batch = ", num_batch )
            print("\nhandle_mini_batch(): length of lists = ", len_lists)
            self._info_point_print("handle_mini_batch: point 1")
        
        # Print some infos
        # ****************
        if b_print:
            self._print_batch_infos()
            self._info_point_print("handle_mini_batch: point 2")
        
        # Major steps for the mini-batch during one epoch iteration 
        # **********************************************************
        
        # Step 0: List of indices for data records in the present mini-batch
        # ******
        ay_idx_batch = self._ay_mini_batches[num_batch]
        
        # Step 1: Special preparation of the Z-input to the MLP's input Layer L0
        # ******
        # Layer L0: Fill in the input vector for the ANN's input layer L0 
        li_
Z_in_layer[0] = self._X_train[ay_idx_batch] # numpy arrays can be indexed by an array of integers
        if b_print:
            print("\nPropagation : Shape of X_in = li_Z_in_layer = ", li_Z_in_layer[0].shape)           
            #print("\nidx, expected y_value of Layer L0-input :")           
            #for idx in self._ay_mini_batches[num_batch]:
            #    print(str(idx) + ', ' + str(self._y_train[idx]) )
            self._info_point_print("handle_mini_batch: point 3")
        
        # Step 2: Layer L0: We need to transpose the data of the input layer 
        # *******
        ay_Z_in_0T       = li_Z_in_layer[0].T
        li_Z_in_layer[0] = ay_Z_in_0T
        if b_print:
            print("\nPropagation : Shape of transposed X_in = li_Z_in_layer = ", li_Z_in_layer[0].shape)           
            self._info_point_print("handle_mini_batch: point 4")
        
        # Step 3: Call forward propagation method for the present mini-batch of training records
        # *******
        # this function will fill the ay_Z_in- and ay_A_out-lists with matrices per layer
        self._fw_propagation(li_Z_in = li_Z_in_layer, li_A_out = li_A_out_layer, b_print = b_print) 
        
        if b_print:
            ilayer = range(0, self._n_total_layers)
            print("\n ---- ")
            print("\nAfter propagation through all " + str(self._n_total_layers) + " layers: ")
            for il in ilayer:
                print("Shape of Z_in of layer L" + str(il) + " = " + str(li_Z_in_layer[il].shape))
                print("Shape of A_out of layer L" + str(il) + " = " + str(li_A_out_layer[il].shape))
                if il < self._n_total_layers-1:
                    print("Shape of W of layer L" + str(il) + " = " + str(self._li_w[il].shape))
                    print("Shape of Mom of layer L" + str(il) + " = " + str(self._li_mom[il].shape))
            self._info_point_print("handle_mini_batch: point 5")
        
        
        # Step 4: Cost calculation for the mini-batch 
        # ********
        ay_y_enc = self._ay_onehot[:, ay_idx_batch]
        ay_ANN_out = li_A_out_layer[self._n_total_layers-1]
        # print("Shape of ay_ANN_out = " + str(ay_ANN_out.shape))
        
        total_costs_batch = self._calculate_loss_for_batch(ay_y_enc, ay_ANN_out, b_print = False)
        # we add the present cost value to the numpy array 
        self._ay_costs[num_epoch, num_batch] = total_costs_batch
        if b_print:
            print("\n total costs of mini_batch = ", self._ay_costs[num_epoch, num_batch])
            self._info_point_print("handle_mini_batch: point 6")
        print("\n total costs of mini_batch = ", self._ay_costs[num_epoch, num_batch])
        
        # Step 5: Avg-error for later plotting 
        # ********
        # mean "error" values - averaged over all nodes at outermost layer and all data sets of a mini-batch 
        ay_theta_out = ay_y_enc - ay_ANN_out
        if (b_print): 
            print("Shape of ay_theta_out = " + str(ay_theta_out.shape))
        ay_theta_avg = np.average(np.abs(ay_theta_out)) 
        self._ay_theta[num_epoch, num_batch] = ay_theta_avg 
        
        if b_print:
            print("\navg total error of mini_batch = ", self._ay_theta[num_epoch, num_batch])
            self._info_point_print("handle_mini_batch: point 7")
        print("avg total error of mini_batch = ", self._ay_theta[num_epoch, num_batch])
        
        
        # Step 6: Perform gradient calculation via back propagation of errors
        # ******* 
        self._bw_propagation( ay_y_enc = ay_y_enc, 
                              li_Z_in = li_Z_in_layer, 
                              li_A_out = li_A_out_layer, 
                              li_delta_out = li_delta_out, 
                              li_delta = li_delta_
layer,
                              li_D = li_D_layer, 
                              li_grad = li_grad_layer, 
                              b_print = b_print,
                              b_internal_timing = False 
                              ) 
        
        
        # Step 7: Adjustment of weights  
        # *******
        rg_layer=range(0, self._n_total_layers -1)
        for N in rg_layer:
            delta_w_N = self._learn_rate * li_grad_layer[N]
            self._li_w[N] -= ( delta_w_N + (self._mom_rate * self._li_mom[N]) )
            # save momentum
            self._li_mom[N] = delta_w_N
        
        # try to accelerate garbage handling
        # **************
        if len(li_Z_in_layer) > 0:
            del li_Z_in_layer
        if len(li_A_out_layer) > 0:
            del li_A_out_layer
        if len(li_delta_out) > 0:
            del li_delta_out
        if len(li_delta_layer) > 0:
            del li_delta_layer
        if len(li_D_layer) > 0:
            del li_D_layer
        if len(li_grad_layer) > 0:
            del li_grad_layer
            
        return None

    ''' -- Method to handle FW propagation for a mini-batch --'''
    def _fw_propagation(self, li_Z_in, li_A_out, b_print= False):
        
        b_internal_timing = False
        
        # index range of layers 
        #    Note that we count from 0 (0=>L0) to E L(=>E) / 
        #    Careful: during BW-propgation we may need a correct indexing of lists filled during FW-propagation
        ilayer = range(0, self._n_total_layers-1)

        # propagation loop
        # ***************
        for il in ilayer:
            if b_internal_timing: start_0 = time.perf_counter()
            
            if b_print: 
                print("\nStarting propagation between L" + str(il) + " and L" + str(il+1))
                print("Shape of Z_in of layer L" + str(il) + " (without bias) = " + str(li_Z_in[il].shape))
            
            # Step 1: Take input of last layer and apply activation function 
            # ******
            if il == 0: 
                A_out_il = li_Z_in[il] # L0: activation function is identity 
            else: 
                A_out_il = self._act_func( li_Z_in[il] ) # use real activation function 
            
            # Step 2: Add bias node
            # ****** 
            A_out_il = self._add_bias_neuron_to_layer(A_out_il, 'row')
            # save in array     
            li_A_out[il] = A_out_il
            if b_print: 
                print("Shape of A_out of layer L" + str(il) + " (with bias) = " + str(li_A_out[il].shape))
            
            # Step 3: Propagate by matrix operation
            # ****** 
            Z_in_ilp1 = np.dot(self._li_w[il], A_out_il) 
            li_Z_in[il+1] = Z_in_ilp1
            
            if b_internal_timing: 
                end_0 = time.perf_counter()
                print('Time_CPU for layer propagation L' + str(il) + ' to L' + str(il+1), end_0 - start_0) 
        
        # treatment of the last layer 
        # ***************************
        il = il + 1
        if b_print:
            print("\nShape of Z_in of layer L" + str(il) + " = " + str(li_Z_in[il].shape))
        A_out_il = self._out_func( li_Z_in[il] ) # use the output function 
        li_A_out[il] = A_out_il
        if b_print:
            print("Shape of A_out of last layer L" + str(il) + " = " + str(li_A_out[il].shape))
        
        return None

 
    ''' -- Method to handle error BW propagation for a mini-batch --'''
    def _bw_propagation(self, 
                        ay_y_enc, li_Z_in, li_A_out, 
                        li_delta_out, li_delta, li_D, li_grad, 
                        b_print = True, b_internal_timing = False):
        
        # List initialization: All parameter lists or arrays are filled or to be filled by layers 
        # Note: the lists li_Z_in, li_A_out were already filled by _fw_propagation() for the present batch 
        
        # Initiate BW propagation - provide delta-matrices for outermost layer
        # *********************** 
        # Input Z at outermost layer E  (4 layers -> layer 3)
        ay_Z_E = li_Z_in[self._n_total_layers-1]
        # Output A at outermost layer E (was calculated by output function)
        ay_A_E = li_A_out[self._n_total_layers-1]
        
        # Calculate D-matrix (derivative of output function) at outmost the layer - presently only D_sigmoid 
        ay_D_E = self._calculate_D_E(ay_Z_E=ay_Z_E, b_print=b_print )
        
        # Get the 2 delta matrices for the outermost layer (only layer E has 2 delta-matrices)
        ay_delta_E, ay_delta_out_E = self._calculate_delta_E(ay_y_enc=ay_y_enc, ay_A_E=ay_A_E, ay_D_E=ay_D_E, b_print=b_print) 
        
        # We check the shapes
        shape_theory = (self._n_nodes_layer_out, self._n_size_mini_batch)
        if (b_print and ay_delta_E.shape != shape_theory):
            print("\nError: Shape of ay_delta_E is wrong:")
            print("Shape = ", ay_delta_E.shape, "  ::  should be = ", shape_theory )
        if (b_print and ay_D_E.shape != shape_theory):
            print("\nError: Shape of ay_D_E is wrong:")
            print("Shape = ", ay_D_E.shape, "  ::  should be = ", shape_theory )
        
        # add the matrices to their lists ; li_delta_out gets only one element 
        idxE = self._n_total_layers - 1
        li_delta_out[idxE] = ay_delta_out_E # this happens only once
        li_delta[idxE]     = ay_delta_E
        li_D[idxE]         = ay_D_E
        li_grad[idxE]      = None    # On the outermost layer there is no gradient ! 
        
        if b_print:
            print("bw: Shape delta_E = ", li_delta[idxE].shape)
            print("bw: Shape D_E = ", ay_D_E.shape)
            self._info_point_print("bw_propagation: point bw_1")
        
        
        # Loop over all layers in reverse direction 
        # ******************************************
        # index range of target layers N in BW direction (starting with E-1 => 4 layers -> layer 2))
        if b_print:
            range_N_bw_layer_test = reversed(range(0,
 self._n_total_layers-1))   # must be -1 as the last element is not taken 
            rg_list = list(range_N_bw_layer_test) # Note this exhausts the range-object
            print("range_N_bw_layer = ", rg_list)
        
        range_N_bw_layer = reversed(range(0, self._n_total_layers-1))   # must be -1 as the last element is not taken 
        
        # loop over layers 
        for N in range_N_bw_layer:
            if b_print:
                print("\n N (layer) = " + str(N) +"\n")
            # start timer 
            if b_internal_timing: start_0 = time.perf_counter()
            
            # Back Propagation operations between layers N+1 and N 
            # *******************************************************
            # this method handles the special treatment of bias nodes in Z_in, too
            ay_delta_N, ay_D_N, ay_grad_N = self._bw_prop_Np1_to_N( N=N, li_Z_in=li_Z_in, li_A_out=li_A_out, li_delta=li_delta, b_print=False )
            
            if b_internal_timing: 
                end_0 = time.perf_counter()
                print('Time_CPU for BW layer operations ', end_0 - start_0) 
            
            # add matrices to their lists 
            li_delta[N] = ay_delta_N
            li_D[N]     = ay_D_N
            li_grad[N]= ay_grad_N
            #sys.exit()
        
        return

    
    ''' -- Method to calculate the matrix with the derivative values of the output function at outermost layer '''
    def _calculate_D_E(self, ay_Z_E, b_print= True):
        '''
        This method calculates and returns the D-matrix for the outermost layer
        The D matrix contains derivatives of the output function with respect to local input "z_j" at outermost nodes. 
        
        Returns
        ------
        ay_D_E:    Matrix with derivative values of the output function 
                   with respect to local z_j valus at the nodes of the outermost layer E
        Note: This is a 2-dim matrix over layer nodes and training samples of the mini-batch
        '''
        if self._my_out_func == 'sigmoid':
            ay_D_E = self._D_sigmoid(ay_Z=ay_Z_E)
        
        else:
            print("The derivative for output function " + self._my_out_func + " is not known yet!" )
            sys.exit()
        
        return ay_D_E

    ''' -- Method to calculate the delta_E matrix as a starting point of the backward propagation '''
    def _calculate_delta_E(self, ay_y_enc, ay_A_E, ay_D_E, b_print= False):
        '''
        This method calculates and returns the 2 delta-matrices for the outermost layer 
        
        Returns
        ------
        delta_E:     delta_matrix of the outermost layer (indicated by E)
        delta_out:   delta_out matrix => elements are local derivative values of the cost function 
                     with respect to the output "a_j" at an outermost node  
                     !!! delta_out will only be returned if calculable !!!
        
        Note: these are 2-dim matrices over layer nodes and training samples of the mini-batch
        '''
        
        if self._my_loss_func == 'LogLoss':
            # Calculate delta_S_E directly to avoid problems with zero denominators
            ay_delta_E = ay_A_E - ay_y_enc
            # delta_out is fetched but may be None 
            ay_delta_out, ay_D_
numerator, ay_D_denominator = self._D_loss_LogLoss(ay_y_enc, ay_A_E, b_print = False)
            
            # To be done: Analyze critical values in D_denominator 
            
            # Release variables explicitly 
            del ay_D_numerator
            del ay_D_denominator
            
        
        if self._my_loss_func == 'MSE':
            # First calculate delta_out and then the delta_E
            delta_out = self._D_loss_MSE(ay_y_enc, ay_A_E, b_print=False)
            # calculate delta_E via matrix multiplication 
            ay_delta_E = delta_out * ay_D_E
                    
        return ay_delta_E, ay_delta_out

    ''' method to calculate the logistic regression loss function '''
    def _loss_LogLoss(self, ay_y_enc, ay_ANN_out, b_print = False):
        '''
        Method which calculates LogReg loss function in a vectorized form on multidimensional Numpy arrays 
        '''
        b_test = False

        if b_print:
            print("From LogLoss: shape of ay_y_enc =  " + str(ay_y_enc.shape))
            print("From LogLoss: shape of ay_ANN_out =  " + str(ay_ANN_out.shape))
            print("LogLoss: ay_y_enc = ", ay_y_enc) 
            print("LogLoss: ANN_out = \n", ay_ANN_out) 
            print("LogLoss: log(ay_ANN_out) =  \n", np.log(ay_ANN_out) )

        # The following means an element-wise (!) operation between matrices of the same shape!
        Log1 = -ay_y_enc * (np.log(ay_ANN_out))
        # The following means an element-wise (!) operation between matrices of the same shape!
        Log2 = (1 - ay_y_enc) * np.log(1 - ay_ANN_out)
        
        # the next operation calculates the sum over all matrix elements 
        # - thus getting the total costs for all mini-batch elements 
        cost = np.sum(Log1 - Log2)
        
        #if b_print and b_test:
            # Log1_x = -ay_y_enc.dot((np.log(ay_ANN_out)).T)
            # print("From LogLoss: L1 =   " + str(L1))
            # print("From LogLoss: L1X =  " + str(L1X))
        
        if b_print: 
            print("From LogLoss: cost =  " + str(cost))
        
        # The total costs is just a number (scalar)
        return cost
#
    ''' method to calculate the derivative of the logistic regression loss function 
        with respect to the output values '''
    def _D_loss_LogLoss(self, ay_y_enc, ay_ANN_out, b_print = False):
        '''
        This function returns the out_delta_S-matrix which is required to initialize the 
        BW propagation (EBP) 
        Note ANN_out is the A_out-list element ( a 2-dim matrix) for the outermost layer 
        In this case we have to take care of denominators = 0 
        '''
        D_numerator = ay_ANN_out - ay_y_enc
        D_denominator = -(ay_ANN_out - 1.0) * ay_ANN_out
        n_critical = np.count_nonzero(D_denominator < 1.0e-8)
        if n_critical > 0:
            delta_s_out = None
        else:
            delta_s_out = np.divide(D_numerator, D_denominator)
        return delta_s_out, D_numerator, D_denominator
#
    ''' method to calculate the MSE loss function '''
    def _loss_MSE(self, ay_y_enc, ay_ANN_out, b_print = False):
        '''
        Method which calculates LogReg loss function in a vectorized form on multidimensional Numpy arrays 
        '''
        if b_print:
            print("From loss_MSE: shape of ay_y_enc =  " + str(ay_y_enc.shape))
            print("From loss_MSE: shape of ay_ANN_out =  " + str(ay_ANN_out.shape))
            #print("LogReg: ay_y_enc = ", ay_y_enc) 
            #print("LogReg: ANN_out = \n", ay_
ANN_out) 
            #print("LogReg: log(ay_ANN_out) =  \n", np.log(ay_ANN_out) )
        
        cost = 0.5 * np.sum( np.square( ay_y_enc - ay_ANN_out ) )

        if b_print: 
            print("From loss_MSE: cost =  " + str(cost))
        
        return cost
#
    ''' method to calculate the derivative of the MSE loss function 
        with respect to the output values '''
    def _D_loss_MSE(self, ay_y_enc, ay_ANN_out, b_print = False):
        '''
        This function returns the out_delta_S - matrix which is required to initialize the 
        BW propagation (EBP) 
        Note ANN_out is the A_out-list element ( a 2-dim matrix) for the outermost layer
        In this case the output is harmless (no critical denominator) 
        '''
        delta_s_out = ay_ANN_out - ay_y_enc
        return delta_s_out

    
    ''' -- Method to calculate the BW-propagated delta-matrix and the gradient matrix to/for layer N '''
    def _bw_prop_Np1_to_N(self, N, li_Z_in, li_A_out, li_delta, b_print=False):
        '''
        BW-error-propagation bewtween layer N+1 and N 
        Inputs: 
            li_Z_in:  List of input Z-matrices on all layers - values were calculated during FW-propagation
            li_A_out: List of output A-matrices - values were calculated during FW-propagation
            li_delta: List of delta-matrices - values for outermost ölayer E to layer N+1 should exist 
        
        Returns: 
            ay_delta_N - delta-matrix of layer N (required in subsequent steps)
            ay_D_N     - derivative matrix for the activation function on layer N 
            ay_grad_N  - matrix with gradient elements of the cost fnction with respect to the weights on layer N 
        '''
        
        if b_print:
            print("Inside _bw_prop_Np1_to_N: N = " + str(N) )
        
        # Prepare required quantities - and add bias neuron to ay_Z_in 
        # ****************************
        
        # Weight matrix meddling betwen layer N and N+1 
        ay_W_N = self._li_w[N]
        shape_W_N   = ay_W_N.shape # due to bias node first dim is 1 bigger than Z-matrix 
        if b_print:
            print("shape of W_N = ", shape_W_N )

        # delta-matrix of layer N+1
        ay_delta_Np1 = li_delta[N+1]
        shape_delta_Np1 = ay_delta_Np1.shape 

        # !!! Add intermediate row (for bias) to Z_N !!!
        ay_Z_N = li_Z_in[N]
        shape_Z_N_orig = ay_Z_N.shape
        ay_Z_N = self._add_bias_neuron_to_layer(ay_Z_N, 'row')
        shape_Z_N = ay_Z_N.shape # dimensions should fit now with W- and A-matrix 
        
        # Derivative matrix for the activation function (with extra bias node row)
        #    can only be calculated now as we need the z-values
        ay_D_N = self._calculate_D_N(ay_Z_N)
        shape_D_N = ay_D_N.shape 
        
        ay_A_N = li_A_out[N]
        shape_A_N = ay_A_N.shape
        
        # print shapes 
        if b_print:
            print("shape of W_N = ", shape_W_N)
            print("
shape of delta_(N+1) = ", shape_delta_Np1)
            print("shape of Z_N_orig = ", shape_Z_N_orig)
            print("shape of Z_N = ", shape_Z_N)
            print("shape of D_N = ", shape_D_N)
            print("shape of A_N = ", shape_A_N)
        
        
        # Propagate delta
        # **************
        if li_delta[N+1] is None:
            print("BW-Prop-error:\n No delta-matrix found for layer " + str(N+1) ) 
            sys.exit()
            
        # Check shapes for np.dot()-operation - here for element [0] of both shapes - as we operate with W.T !
        if ( shape_W_N[0] != shape_delta_Np1[0]): 
            print("BW-Prop-error:\n shape of W_N [", shape_W_N, "]) does not fit shape of delta_N+1 [", shape_delta_Np1, "]" )
            sys.exit() 
        
        # intermediate delta 
        # ~~~~~~~~~~~~~~~~~~
        ay_delta_w_N = ay_W_N.T.dot(ay_delta_Np1)
        shape_delta_w_N = ay_delta_w_N.shape
        
        # Check shapes for element wise *-operation !
        if ( shape_delta_w_N != shape_D_N ): 
            print("BW-Prop-error:\n shape of delta_w_N [", shape_delta_w_N, "]) does not fit shape of D_N [", shape_D_N, "]" )
            sys.exit() 
        
        # final delta 
        # ~~~~~~~~~~~
        ay_delta_N = ay_delta_w_N * ay_D_N
        # reduce dimension again 
        ay_delta_N = ay_delta_N[1:, :]
        shape_delta_N = ay_delta_N.shape
        
        # Check dimensions again - ay_delta_N.shape should fit shape_Z_in_orig
        if shape_delta_N != shape_Z_N_orig: 
            print("BW-Prop-error:\n shape of delta_N [", shape_delta_N, "]) does not fit original shape Z_in_N [", shape_Z_N_orig, "]" )
            sys.exit() 
        
        if N > 0:
            shape_W_Nm1 = self._li_w[N-1].shape 
            if shape_delta_N[0] != shape_W_Nm1[0] : 
                print("BW-Prop-error:\n shape of delta_N [", shape_delta_N, "]) does not fit shape of W_Nm1 [", shape_W_Nm1, "]" )
                sysexit() 
        
        
        # Calculate gradient
        # ********************
        #     required for all layers down to 0 
        # check shapes 
        if shape_delta_Np1[1] != shape_A_N[1]:
            print("BW-Prop-error:\n shape of delta_Np1 [", shape_delta_Np1, "]) does not fit shape of A_N [", shape_A_N, "] for matrix multiplication" )
            sys.exit() 
        
        # calculate gradient             
        ay_grad_N = np.dot(ay_delta_Np1, ay_A_N.T)
        
        # regularize gradient (!!!! without adding bias nodes in the L1, L2 sums) 
        ay_grad_N[:, 1:] += (self._li_w[N][:, 1:] * self._lambda2_reg + np.sign(self._li_w[N][:, 1:]) * self._lambda1_reg) 
        
        #
        # Check shape 
        shape_grad_N = ay_grad_N.shape
        if shape_grad_N != shape_W_N:
            print("BW-Prop-error:\n shape of grad_N [", shape_grad_N, "]) does not fit shape of W_N [", shape_W_N, "]" )
            sys.exit() 
        
        # print shapes 
        if b_print:
            print("shape of delta_N = ", shape_delta_N)
            print("shape of grad_N = ", shape_grad_N)
            
            print(ay_grad_N)
        
        return ay_delta_N, ay_D_N, ay_grad_N

   
#
    ''' -- Method to calculate the matrix with the derivative values of the output function at outermost layer '''

    def _calculate_D_N(self, ay_Z_N, b_print= False):
        '''
        This method calculates and returns the D-matrix for the outermost layer
        The D matrix contains derivatives of the output function with respect to local input "z_j" at outermost nodes. 
        
        Returns
        ------
        ay_D_E:    Matrix with derivative values of the output function 
                   with respect to local z_j valus at the nodes of the outermost layer E
        Note: This is a 2-dim matrix over layer nodes and training samples of the mini-batch
        '''
        if self._my_out_func == 'sigmoid':
            ay_D_E = self._D_sigmoid(ay_Z = ay_Z_N)
        
        else:
            print("The derivative for output function " + self._my_out_func + " is not known yet!" )
            sys.exit()
        
        return ay_D_E
 

   
#
    ''' method do calculate the quadratic regularization term for the loss function '''
    def _regularize_by_L2(self, b_print=False): 
        '''
        The L2 regularization term sums up all quadratic weights (without the weight for the bias) 
        over the input and all hidden layers (but not the output layer)
        The weight for the bias is in the first column (index 0) of the weight matrix - 
        as the bias node's output is in the first row of the output vector of the layer 
        '''
        ilayer = range(0, self._n_total_layers-1) # this excludes the last layer 
        L2 = 0.0
        for idx in ilayer:
            L2 += (np.sum( np.square(self._li_w[idx][:, 1:])) ) 
        L2 *= 0.5 * self._lambda2_reg
        if b_print: 
            print("\nL2: total L2 = " + str(L2) )
        return L2 
#
    ''' method do calculate the linear regularization term for the loss function '''
    def _regularize_by_L1(self, b_print=False): 
        '''
        The L1 regularization term sums up all weights (without the weight for the bias) 
        over the input and all hidden layers (but not the output layer
        The weight for the bias is in the first column (index 0) of the weight matrix - 
        as the bias node's output is in the first row of the output vector of the layer 
        '''
        ilayer = range(0, self._n_total_layers-1) # this excludes the last layer 
        L1 = 0.0
        for idx in ilayer:
            L1 += np.sum(np.abs( self._li_w[idx][:, 1:]))
        L1 *= 0.5 * self._lambda1_reg
        if b_print:
            print("\nL1: total L1 = " + str(L1))
        return L1