part2_neural_network.ipynb

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   "source": [
    "# python notebook for Make Your Own Neural Network\n",
    "# (c) Tariq Rashid, 2016\n",
    "# license is GPLv2"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "import numpy\n",
    "# scipy.special for the sigmoid function expit()\n",
    "import scipy.special"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {},
   "outputs": [],
   "source": [
    "# neural network class definition\n",
    "class neuralNetwork:\n",
    "    \n",
    "    \n",
    "    # initialise the neural network\n",
    "    def __init__(self, inputnodes, hiddennodes, outputnodes, learningrate):\n",
    "        # set number of nodes in each input, hidden, output layer\n",
    "        self.inodes = inputnodes\n",
    "        self.hnodes = hiddennodes\n",
    "        self.onodes = outputnodes\n",
    "        \n",
    "        # link weight matrices, wih and who\n",
    "        # weights inside the arrays are w_i_j, where link is from node i to node j in the next layer\n",
    "        # w11 w21\n",
    "        # w12 w22 etc \n",
    "        self.wih = numpy.random.normal(0.0, pow(self.inodes, -0.5), (self.hnodes, self.inodes))\n",
    "        self.who = numpy.random.normal(0.0, pow(self.hnodes, -0.5), (self.onodes, self.hnodes))\n",
    "\n",
    "        # learning rate\n",
    "        self.lr = learningrate\n",
    "        \n",
    "        # activation function is the sigmoid function\n",
    "        self.activation_function = lambda x: scipy.special.expit(x)\n",
    "        \n",
    "        pass\n",
    "\n",
    "    \n",
    "    # train the neural network\n",
    "    def train(self, inputs_list, targets_list):\n",
    "        # convert inputs list to 2d array\n",
    "        inputs = numpy.array(inputs_list, ndmin=2).T\n",
    "        targets = numpy.array(targets_list, ndmin=2).T\n",
    "        \n",
    "        # calculate signals into hidden layer\n",
    "        hidden_inputs = numpy.dot(self.wih, inputs)\n",
    "        # calculate the signals emerging from hidden layer\n",
    "        hidden_outputs = self.activation_function(hidden_inputs)\n",
    "        \n",
    "        # calculate signals into final output layer\n",
    "        final_inputs = numpy.dot(self.who, hidden_outputs)\n",
    "        # calculate the signals emerging from final output layer\n",
    "        final_outputs = self.activation_function(final_inputs)\n",
    "        \n",
    "        # output layer error is the (target - actual)\n",
    "        output_errors = targets - final_outputs\n",
    "        # hidden layer error is the output_errors, split by weights, recombined at hidden nodes\n",
    "        hidden_errors = numpy.dot(self.who.T, output_errors) \n",
    "        \n",
    "        # update the weights for the links between the hidden and output layers\n",
    "        self.who += self.lr * numpy.dot((output_errors * final_outputs * (1.0 - final_outputs)), numpy.transpose(hidden_outputs))\n",
    "        \n",
    "        # update the weights for the links between the input and hidden layers\n",
    "        self.wih += self.lr * numpy.dot((hidden_errors * hidden_outputs * (1.0 - hidden_outputs)), numpy.transpose(inputs))\n",
    "        \n",
    "        pass\n",
    "\n",
    "    \n",
    "    # query the neural network\n",
    "    def query(self, inputs_list):\n",
    "        # convert inputs list to 2d array\n",
    "        inputs = numpy.array(inputs_list, ndmin=2).T\n",
    "        \n",
    "        # calculate signals into hidden layer\n",
    "        hidden_inputs = numpy.dot(self.wih, inputs)\n",
    "        # calculate the signals emerging from hidden layer\n",
    "        hidden_outputs = self.activation_function(hidden_inputs)\n",
    "        \n",
    "        # calculate signals into final output layer\n",
    "        final_inputs = numpy.dot(self.who, hidden_outputs)\n",
    "        # calculate the signals emerging from final output layer\n",
    "        final_outputs = self.activation_function(final_inputs)\n",
    "        \n",
    "        return final_outputs"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {},
   "outputs": [],
   "source": [
    "# number of input, hidden and output nodes\n",
    "input_nodes = 3\n",
    "hidden_nodes = 3\n",
    "output_nodes = 3\n",
    "\n",
    "# learning rate is 0.3\n",
    "learning_rate = 0.3\n",
    "\n",
    "# create instance of neural network\n",
    "n = neuralNetwork(input_nodes,hidden_nodes,output_nodes, learning_rate)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "array([[ 0.43461026],\n",
       "       [ 0.40331273],\n",
       "       [ 0.56675401]])"
      ]
     },
     "execution_count": 5,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "# test query (doesn't mean anything useful yet)\n",
    "n.query([1.0, 0.5, -1.5])"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
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   "outputs": [],
   "source": []
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