使用RNN對文本進行分類實踐電影評論

本教程在IMDB大型影評數據集 上訓練一個循環神經網絡進行情感分類。

 

from __future__ import absolute_import, division, print_function, unicode_literals
# !pip install tensorflow-gpu==2.0.0-alpha0
import tensorflow_datasets as tfds
import tensorflow as tf

導入matplotlib並建立一個輔助函數來繪製圖形

import matplotlib.pyplot as plt
def plot_graphs(history, string):
 plt.plot(history.history[string])
 plt.plot(history.history['val_'+string])
 plt.xlabel("Epochs")
 plt.ylabel(string)
 plt.legend([string, 'val_'+string])
 plt.show()

1. 設置輸入管道

IMDB大型電影影評數據集是一個二元分類數據集，全部評論都有正面或負面的情緒標籤。

使用TFDS下載數據集，數據集附帶一個內置的子字標記器

dataset, info = tfds.load('imdb_reviews/subwords8k', with_info=True,
 as_supervised=True)
train_dataset, test_dataset = dataset['train'], dataset['test']

因爲這是一個子字標記器，它能夠傳遞任何字符串，而且標記器將對其進行標記。

tokenizer = info.features['text'].encoder
print ('Vocabulary size: {}'.format(tokenizer.vocab_size))
 Vocabulary size: 8185
sample_string = 'TensorFlow is cool.'
tokenized_string = tokenizer.encode(sample_string)
print ('Tokenized string is {}'.format(tokenized_string))
original_string = tokenizer.decode(tokenized_string)
print ('The original string: {}'.format(original_string))
assert original_string == sample_string
 Tokenized string is [6307, 2327, 4043, 4265, 9, 2724, 7975]
 The original string: TensorFlow is cool.

若是字符串不在字典中，則標記生成器經過將字符串分解爲子字符串來對字符串進行編碼。

for ts in tokenized_string:
 print ('{} ----> {}'.format(ts, tokenizer.decode([ts])))
 6307 ----> Ten
 2327 ----> sor
 4043 ----> Fl
 4265 ----> ow
 9 ----> is
 2724 ----> cool
 7975 ----> .
BUFFER_SIZE = 10000
BATCH_SIZE = 64
train_dataset = train_dataset.shuffle(BUFFER_SIZE)
train_dataset = train_dataset.padded_batch(BATCH_SIZE, train_dataset.output_shapes)
test_dataset = test_dataset.padded_batch(BATCH_SIZE, test_dataset.output_shapes)

2. 建立模型

構建一個tf.keras.Sequential模型並從嵌入層開始，嵌入層每一個字存儲一個向量，當被調用時，它將單詞索引的序列轉換爲向量序列，這些向量是可訓練的，在訓練以後（在足夠的數據上），具備類似含義的詞一般具備類似的向量。

這種索引查找比經過tf.keras.layers.Dense層傳遞獨熱編碼向量的等效操做更有效。

遞歸神經網絡（RNN）經過迭代元素來處理序列輸入，RNN將輸出從一個時間步傳遞到其輸入端，而後傳遞到下一個時間步。

tf.keras.layers.Bidirectional包裝器也能夠與RNN層一塊兒使用。這經過RNN層向前和向後傳播輸入，而後鏈接輸出。這有助於RNN學習遠程依賴性。

model = tf.keras.Sequential([
 tf.keras.layers.Embedding(tokenizer.vocab_size, 64),
 tf.keras.layers.Bidirectional(tf.keras.layers.LSTM(64)),
 tf.keras.layers.Dense(64, activation='relu'),
 tf.keras.layers.Dense(1, activation='sigmoid')
])
# 編譯Keras模型以配置訓練過程：
model.compile(loss='binary_crossentropy',
 optimizer='adam',
 metrics=['accuracy'])

3. 訓練模型

history = model.fit(train_dataset, epochs=10,
 validation_data=test_dataset)
 ...
 Epoch 10/10
 391/391 [==============================] - 70s 180ms/step - loss: 0.3074 - accuracy: 0.8692 - val_loss: 0.5533 - val_accuracy: 0.7873
test_loss, test_acc = model.evaluate(test_dataset)
print('Test Loss: {}'.format(test_loss))
print('Test Accuracy: {}'.format(test_acc))
 391/Unknown - 19s 47ms/step - loss: 0.5533 - accuracy: 0.7873Test Loss: 0.553319326714
 Test Accuracy: 0.787320017815

上面的模型沒有屏蔽應用於序列的填充。若是咱們對填充序列進行訓練，並對未填充序列進行測試，就會致使偏斜。理想狀況下，模型應該學會忽略填充，可是正如您在下面看到的，它對輸出的影響確實很小。

若是預測 >=0.5，則爲正，不然爲負。

def pad_to_size(vec, size):
 zeros = [0] * (size - len(vec))
 vec.extend(zeros)
 return vec
def sample_predict(sentence, pad):
 tokenized_sample_pred_text = tokenizer.encode(sample_pred_text)
 if pad:
 tokenized_sample_pred_text = pad_to_size(tokenized_sample_pred_text, 64)
 predictions = model.predict(tf.expand_dims(tokenized_sample_pred_text, 0))
 return (predictions)
# 對不帶填充的示例文本進行預測 
sample_pred_text = ('The movie was cool. The animation and the graphics '
 'were out of this world. I would recommend this movie.')
predictions = sample_predict(sample_pred_text, pad=False)
print (predictions)
 [[ 0.68914342]]
# 對帶填充的示例文本進行預測 
sample_pred_text = ('The movie was cool. The animation and the graphics '
 'were out of this world. I would recommend this movie.')
predictions = sample_predict(sample_pred_text, pad=True)
print (predictions)
 [[ 0.68634349]]
plot_graphs(history, 'accuracy')

 

plot_graphs(history, 'loss')

 

4. 堆疊兩個或更多LSTM層

Keras遞歸層有兩種能夠用的模式，由return_sequences構造函數參數控制：

• 返回每一個時間步的連續輸出的完整序列（3D張量形狀 (batch_size, timesteps, output_features)）。
• 僅返回每一個輸入序列的最後一個輸出（2D張量形狀 (batch_size, output_features)）。

model = tf.keras.Sequential([
 tf.keras.layers.Embedding(tokenizer.vocab_size, 64),
 tf.keras.layers.Bidirectional(tf.keras.layers.LSTM(
 64, return_sequences=True)),
 tf.keras.layers.Bidirectional(tf.keras.layers.LSTM(32)),
 tf.keras.layers.Dense(64, activation='relu'),
 tf.keras.layers.Dense(1, activation='sigmoid')
])
model.compile(loss='binary_crossentropy',
 optimizer='adam',
 metrics=['accuracy'])
history = model.fit(train_dataset, epochs=10,
 validation_data=test_dataset)
 ...
 Epoch 10/10
 391/391 [==============================] - 154s 394ms/step - loss: 0.1120 - accuracy: 0.9643 - val_loss: 0.5646 - val_accuracy: 0.8070
test_loss, test_acc = model.evaluate(test_dataset)
print('Test Loss: {}'.format(test_loss))
print('Test Accuracy: {}'.format(test_acc))
 391/Unknown - 45s 115ms/step - loss: 0.5646 - accuracy: 0.8070Test Loss: 0.564571284348
 Test Accuracy: 0.80703997612
# 在沒有填充的狀況下預測示例文本
sample_pred_text = ('The movie was not good. The animation and the graphics '
 'were terrible. I would not recommend this movie.')
predictions = sample_predict(sample_pred_text, pad=False)
print (predictions)
 [[ 0.00393916]]
# 在有填充的狀況下預測示例文本
sample_pred_text = ('The movie was not good. The animation and the graphics '
 'were terrible. I would not recommend this movie.')
predictions = sample_predict(sample_pred_text, pad=True)
print (predictions)
 [[ 0.01098633]]
plot_graphs(history, 'accuracy')

 

plot_graphs(history, 'loss')

 

你能夠查看其它現有的遞歸層，例如GRU層。