bidirectional lstm example added.

examples/imdb_bidirectional_lstm.py

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+from __future__ import absolute_import
+from __future__ import print_function
+import numpy as np
+np.random.seed(1337)  # for reproducibility
+
+from keras.preprocessing import sequence
+from keras.utils.np_utils import accuracy
+from keras.models import Graph
+from keras.layers.core import Dense, Dropout
+from keras.layers.embeddings import Embedding
+from keras.layers.recurrent import LSTM
+from keras.datasets import imdb
+
+'''
+    Train a Bidirectional LSTM on the IMDB sentiment classification task.
+    The dataset is actually too small for bidirectional LSTM to be of any advantage
+    compared to simpler, much faster methods such as TF-IDF+LogReg.
+    Bidirectional LSTM may be not suited for a simple text classification task.
+    Notes:
+    - RNNs are tricky. And in particular Bidirectional RNNs you may experiment
+    with different Recurrent layers and different parameters to find the best configuration
+    for your task.
+
+    GPU command:
+        THEANO_FLAGS=mode=FAST_RUN,device=gpu,floatX=float32 python imdb_bidirectional_lstm.py
+
+    Output after 4 epochs on CPU: ~0.8146 
+'''
+
+max_features = 20000
+maxlen = 100  # cut texts after this number of words (among top max_features most common words)
+batch_size = 32
+
+print("Loading data...")
+(X_train, y_train), (X_test, y_test) = imdb.load_data(nb_words=max_features, test_split=0.2)
+print(len(X_train), 'train sequences')
+print(len(X_test), 'test sequences')
+
+print("Pad sequences (samples x time)")
+X_train = sequence.pad_sequences(X_train, maxlen=maxlen)
+X_test = sequence.pad_sequences(X_test, maxlen=maxlen)
+print('X_train shape:', X_train.shape)
+print('X_test shape:', X_test.shape)
+y_train = np.array(y_train)
+y_test = np.array(y_test)
+
+print('Build model...')
+model = Graph()
+model.add_input(name='input', input_shape=(1,), dtype=int)
+model.add_node(Embedding(max_features, 128, input_length=maxlen), name='embedding', input='input')
+model.add_node(LSTM(64), name='forward', input='embedding' )  # You can change these two layers with GRU
+model.add_node(LSTM(64, go_backwards=True), name='backward', input='embedding' )
+model.add_node(Dropout(0.5), name='dropout', inputs=['forward', 'backward'])
+model.add_node(Dense(1, activation='sigmoid'), name='sigmoid', input='dropout')
+model.add_output(name='output', input='sigmoid')
+
+# try using different optimizers and different optimizer configs
+model.compile('adam',{'output':'binary_crossentropy'})
+
+print("Train...")
+model.fit({'input':X_train, 'output':y_train}, batch_size=batch_size, nb_epoch=4)
+acc = accuracy(y_test, np.round(np.array(model.predict({'input':X_test}, batch_size=batch_size)['output'])))
+print('Test accuracy:', acc)
+