annotate code_tutoriel/mlp.py @ 148:72a2d431d047

Rajout d'un seed random et d'une fonction get_seed
author SylvainPL <sylvain.pannetier.lebeuf@umontreal.ca>
date Wed, 24 Feb 2010 13:14:02 -0500
parents 827de2cc34f8
children 4bc5eeec6394
rev   line source
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1 """
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2 This tutorial introduces the multilayer perceptron using Theano.
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3
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4 A multilayer perceptron is a logistic regressor where
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5 instead of feeding the input to the logistic regression you insert a
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6 intermidiate layer, called the hidden layer, that has a nonlinear
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7 activation function (usually tanh or sigmoid) . One can use many such
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8 hidden layers making the architecture deep. The tutorial will also tackle
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9 the problem of MNIST digit classification.
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10
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11 .. math::
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12
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13 f(x) = G( b^{(2)} + W^{(2)}( s( b^{(1)} + W^{(1)} x))),
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14
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15 References:
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16
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17 - textbooks: "Pattern Recognition and Machine Learning" -
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18 Christopher M. Bishop, section 5
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19
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20 TODO: recommended preprocessing, lr ranges, regularization ranges (explain
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21 to do lr first, then add regularization)
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22
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23 """
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24 __docformat__ = 'restructedtext en'
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25
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26
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27 import numpy, cPickle, gzip
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28
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29
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30 import theano
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31 import theano.tensor as T
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32
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33 import time
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34
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35 import theano.tensor.nnet
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36
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37 class MLP(object):
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38 """Multi-Layer Perceptron Class
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39
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40 A multilayer perceptron is a feedforward artificial neural network model
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41 that has one layer or more of hidden units and nonlinear activations.
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42 Intermidiate layers usually have as activation function thanh or the
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43 sigmoid function while the top layer is a softamx layer.
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44 """
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45
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46
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47
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48 def __init__(self, input, n_in, n_hidden, n_out):
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49 """Initialize the parameters for the multilayer perceptron
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50
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51 :param input: symbolic variable that describes the input of the
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52 architecture (one minibatch)
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53
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54 :param n_in: number of input units, the dimension of the space in
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55 which the datapoints lie
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56
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57 :param n_hidden: number of hidden units
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58
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59 :param n_out: number of output units, the dimension of the space in
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60 which the labels lie
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61
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62 """
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63
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64 # initialize the parameters theta = (W1,b1,W2,b2) ; note that this
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65 # example contains only one hidden layer, but one can have as many
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66 # layers as he/she wishes, making the network deeper. The only
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67 # problem making the network deep this way is during learning,
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68 # backpropagation being unable to move the network from the starting
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69 # point towards; this is where pre-training helps, giving a good
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70 # starting point for backpropagation, but more about this in the
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71 # other tutorials
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72
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73 # `W1` is initialized with `W1_values` which is uniformely sampled
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74 # from -6./sqrt(n_in+n_hidden) and 6./sqrt(n_in+n_hidden)
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75 # the output of uniform if converted using asarray to dtype
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76 # theano.config.floatX so that the code is runable on GPU
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77 W1_values = numpy.asarray( numpy.random.uniform( \
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78 low = -numpy.sqrt(6./(n_in+n_hidden)), \
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79 high = numpy.sqrt(6./(n_in+n_hidden)), \
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80 size = (n_in, n_hidden)), dtype = theano.config.floatX)
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81 # `W2` is initialized with `W2_values` which is uniformely sampled
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82 # from -6./sqrt(n_hidden+n_out) and 6./sqrt(n_hidden+n_out)
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83 # the output of uniform if converted using asarray to dtype
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84 # theano.config.floatX so that the code is runable on GPU
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85 W2_values = numpy.asarray( numpy.random.uniform(
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86 low = -numpy.sqrt(6./(n_hidden+n_out)), \
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87 high= numpy.sqrt(6./(n_hidden+n_out)),\
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88 size= (n_hidden, n_out)), dtype = theano.config.floatX)
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89
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90 self.W1 = theano.shared( value = W1_values )
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91 self.b1 = theano.shared( value = numpy.zeros((n_hidden,),
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92 dtype= theano.config.floatX))
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93 self.W2 = theano.shared( value = W2_values )
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94 self.b2 = theano.shared( value = numpy.zeros((n_out,),
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95 dtype= theano.config.floatX))
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96
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97 # symbolic expression computing the values of the hidden layer
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98 self.hidden = T.tanh(T.dot(input, self.W1)+ self.b1)
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99
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100 # symbolic expression computing the values of the top layer
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101 self.p_y_given_x= T.nnet.softmax(T.dot(self.hidden, self.W2)+self.b2)
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102
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103 # compute prediction as class whose probability is maximal in
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104 # symbolic form
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105 self.y_pred = T.argmax( self.p_y_given_x, axis =1)
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106
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107 # L1 norm ; one regularization option is to enforce L1 norm to
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108 # be small
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109 self.L1 = abs(self.W1).sum() + abs(self.W2).sum()
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110
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111 # square of L2 norm ; one regularization option is to enforce
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112 # square of L2 norm to be small
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113 self.L2_sqr = (self.W1**2).sum() + (self.W2**2).sum()
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114
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115
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116
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117 def negative_log_likelihood(self, y):
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118 """Return the mean of the negative log-likelihood of the prediction
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119 of this model under a given target distribution.
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120
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121 .. math::
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122
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123 \frac{1}{|\mathcal{D}|}\mathcal{L} (\theta=\{W,b\}, \mathcal{D}) =
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124 \frac{1}{|\mathcal{D}|}\sum_{i=0}^{|\mathcal{D}|} \log(P(Y=y^{(i)}|x^{(i)}, W,b)) \\
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125 \ell (\theta=\{W,b\}, \mathcal{D})
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126
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127
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128 :param y: corresponds to a vector that gives for each example the
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129 :correct label
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130 """
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131 return -T.mean(T.log(self.p_y_given_x)[T.arange(y.shape[0]),y])
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132
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133
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134
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135
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136 def errors(self, y):
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137 """Return a float representing the number of errors in the minibatch
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138 over the total number of examples of the minibatch
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139 """
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140
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141 # check if y has same dimension of y_pred
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142 if y.ndim != self.y_pred.ndim:
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143 raise TypeError('y should have the same shape as self.y_pred',
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144 ('y', target.type, 'y_pred', self.y_pred.type))
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145 # check if y is of the correct datatype
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146 if y.dtype.startswith('int'):
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147 # the T.neq operator returns a vector of 0s and 1s, where 1
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148 # represents a mistake in prediction
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149 return T.mean(T.neq(self.y_pred, y))
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150 else:
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151 raise NotImplementedError()
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152
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153
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154
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155 def sgd_optimization_mnist( learning_rate=0.01, L1_reg = 0.00, \
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156 L2_reg = 0.0001, n_iter=100):
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157 """
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158 Demonstrate stochastic gradient descent optimization for a multilayer
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159 perceptron
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160
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161 This is demonstrated on MNIST.
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162
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163 :param learning_rate: learning rate used (factor for the stochastic
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164 gradient
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165
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166 :param L1_reg: L1-norm's weight when added to the cost (see
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167 regularization)
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168
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169 :param L2_reg: L2-norm's weight when added to the cost (see
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170 regularization)
2
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171
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172 :param n_iter: maximal number of iterations ot run the optimizer
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173
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174 """
0
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175
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176 # Load the dataset
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177 f = gzip.open('mnist.pkl.gz','rb')
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178 train_set, valid_set, test_set = cPickle.load(f)
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179 f.close()
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180
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181 # make minibatches of size 20
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182 batch_size = 20 # sized of the minibatch
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183
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184 # Dealing with the training set
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185 # get the list of training images (x) and their labels (y)
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186 (train_set_x, train_set_y) = train_set
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187 # initialize the list of training minibatches with empty list
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188 train_batches = []
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189 for i in xrange(0, len(train_set_x), batch_size):
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190 # add to the list of minibatches the minibatch starting at
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191 # position i, ending at position i+batch_size
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192 # a minibatch is a pair ; the first element of the pair is a list
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193 # of datapoints, the second element is the list of corresponding
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194 # labels
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195 train_batches = train_batches + \
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196 [(train_set_x[i:i+batch_size], train_set_y[i:i+batch_size])]
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197
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198 # Dealing with the validation set
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199 (valid_set_x, valid_set_y) = valid_set
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200 # initialize the list of validation minibatches
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201 valid_batches = []
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202 for i in xrange(0, len(valid_set_x), batch_size):
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203 valid_batches = valid_batches + \
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204 [(valid_set_x[i:i+batch_size], valid_set_y[i:i+batch_size])]
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205
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206 # Dealing with the testing set
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207 (test_set_x, test_set_y) = test_set
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208 # initialize the list of testing minibatches
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209 test_batches = []
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210 for i in xrange(0, len(test_set_x), batch_size):
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211 test_batches = test_batches + \
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212 [(test_set_x[i:i+batch_size], test_set_y[i:i+batch_size])]
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213
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214
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215 ishape = (28,28) # this is the size of MNIST images
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216
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217 # allocate symbolic variables for the data
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218 x = T.fmatrix() # the data is presented as rasterized images
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219 y = T.lvector() # the labels are presented as 1D vector of
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220 # [long int] labels
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221
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222 # construct the logistic regression class
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223 classifier = MLP( input=x.reshape((batch_size,28*28)),\
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224 n_in=28*28, n_hidden = 500, n_out=10)
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225
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226 # the cost we minimize during training is the negative log likelihood of
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227 # the model plus the regularization terms (L1 and L2); cost is expressed
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228 # here symbolically
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229 cost = classifier.negative_log_likelihood(y) \
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230 + L1_reg * classifier.L1 \
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231 + L2_reg * classifier.L2_sqr
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232
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233 # compiling a theano function that computes the mistakes that are made by
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234 # the model on a minibatch
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235 test_model = theano.function([x,y], classifier.errors(y))
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236
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237 # compute the gradient of cost with respect to theta = (W1, b1, W2, b2)
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238 g_W1 = T.grad(cost, classifier.W1)
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239 g_b1 = T.grad(cost, classifier.b1)
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240 g_W2 = T.grad(cost, classifier.W2)
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241 g_b2 = T.grad(cost, classifier.b2)
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242
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243 # specify how to update the parameters of the model as a dictionary
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244 updates = \
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245 { classifier.W1: classifier.W1 - learning_rate*g_W1 \
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246 , classifier.b1: classifier.b1 - learning_rate*g_b1 \
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247 , classifier.W2: classifier.W2 - learning_rate*g_W2 \
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248 , classifier.b2: classifier.b2 - learning_rate*g_b2 }
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249
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250 # compiling a theano function `train_model` that returns the cost, but in
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251 # the same time updates the parameter of the model based on the rules
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252 # defined in `updates`
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253 train_model = theano.function([x, y], cost, updates = updates )
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254 n_minibatches = len(train_batches)
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255
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256 # early-stopping parameters
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257 patience = 10000 # look as this many examples regardless
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258 patience_increase = 2 # wait this much longer when a new best is
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259 # found
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260 improvement_threshold = 0.995 # a relative improvement of this much is
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261 # considered significant
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262 validation_frequency = n_minibatches # go through this many
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263 # minibatche before checking the network
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264 # on the validation set; in this case we
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265 # check every epoch
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266
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267
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268 best_params = None
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269 best_validation_loss = float('inf')
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270 best_iter = 0
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271 test_score = 0.
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272 start_time = time.clock()
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273 # have a maximum of `n_iter` iterations through the entire dataset
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274 for iter in xrange(n_iter* n_minibatches):
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275
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276 # get epoch and minibatch index
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277 epoch = iter / n_minibatches
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278 minibatch_index = iter % n_minibatches
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279
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280 # get the minibatches corresponding to `iter` modulo
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281 # `len(train_batches)`
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282 x,y = train_batches[ minibatch_index ]
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283 cost_ij = train_model(x,y)
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284
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285 if (iter+1) % validation_frequency == 0:
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286 # compute zero-one loss on validation set
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287 this_validation_loss = 0.
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288 for x,y in valid_batches:
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289 # sum up the errors for each minibatch
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290 this_validation_loss += test_model(x,y)
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291 # get the average by dividing with the number of minibatches
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292 this_validation_loss /= len(valid_batches)
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293
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294 print('epoch %i, minibatch %i/%i, validation error %f %%' % \
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295 (epoch, minibatch_index+1, n_minibatches, \
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296 this_validation_loss*100.))
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297
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298
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299 # if we got the best validation score until now
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300 if this_validation_loss < best_validation_loss:
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301
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302 #improve patience if loss improvement is good enough
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303 if this_validation_loss < best_validation_loss * \
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304 improvement_threshold :
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305 patience = max(patience, iter * patience_increase)
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306
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307 # save best validation score and iteration number
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308 best_validation_loss = this_validation_loss
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309 best_iter = iter
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310
0
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311 # test it on the test set
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312 test_score = 0.
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313 for x,y in test_batches:
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314 test_score += test_model(x,y)
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315 test_score /= len(test_batches)
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316 print((' epoch %i, minibatch %i/%i, test error of best '
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317 'model %f %%') %
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318 (epoch, minibatch_index+1, n_minibatches,
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319 test_score*100.))
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320
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321 if patience <= iter :
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322 break
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323
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324 end_time = time.clock()
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325 print(('Optimization complete. Best validation score of %f %% '
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326 'obtained at iteration %i, with test performance %f %%') %
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327 (best_validation_loss * 100., best_iter, test_score*100.))
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328 print ('The code ran for %f minutes' % ((end_time-start_time)/60.))
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329
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330
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331 if __name__ == '__main__':
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332 sgd_optimization_mnist()
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333