Feedforward Neural Network For Binary Classification
Preliminaries
/* Load libraries */
import numpy as np
from keras.datasets import imdb
from keras.preprocessing.text import Tokenizer
from keras import models
from keras import layers
/* Set random seed */
np.random.seed(0)
Using TensorFlow backend.
Load Movie Review Data
/* Set the number of features we want */
number_of_features = 1000
/* Load data and target vector from movie review data */
(train_data, train_target), (test_data, test_target) = imdb.load_data(num_words=number_of_features)
/* Convert movie review data to one-hot encoded feature matrix */
tokenizer = Tokenizer(num_words=number_of_features)
train_features = tokenizer.sequences_to_matrix(train_data, mode='binary')
test_features = tokenizer.sequences_to_matrix(test_data, mode='binary')
Construct Neural Network Architecture
Because this is a binary classification problem, one common choice is to use the sigmoid activation function in a one-unit output layer.
/* Start neural network */
network = models.Sequential()
/* Add fully connected layer with a ReLU activation function */
network.add(layers.Dense(units=16, activation='relu', input_shape=(number_of_features,)))
/* Add fully connected layer with a ReLU activation function */
network.add(layers.Dense(units=16, activation='relu'))
/* Add fully connected layer with a sigmoid activation function */
network.add(layers.Dense(units=1, activation='sigmoid'))
Compile Feedforward Neural Network
/* Compile neural network */
network.compile(loss='binary_crossentropy', # Cross-entropy
optimizer='rmsprop', # Root Mean Square Propagation
metrics=['accuracy']) # Accuracy performance metric
Train Feedforward Neural Network
In Keras, we train our neural network using the fit
method. There are six significant parameters to define. The first two parameters are the features and target vector of the training data.
The epochs
parameter defines how many epochs to use when training the data. verbose
determines how much information is outputted during the training process, with 0
being no out, 1
outputting a progress bar, and 2
one log line per epoch. batch_size
sets the number of observations to propagate through the network before updating the parameters.
Finally, we held out a test set of data to use to evaluate the model. These test features and test target vector can be arguments of the validation_data
, which will use them for evaluation. Alternatively, we could have used validation_split
to define what fraction of the training data we want to hold out for evaluation.
In scikit-learn fit
method returned a trained model, however in Keras the fit
method returns a History
object containing the loss values and performance metrics at each epoch.
/* Train neural network */
history = network.fit(train_features, # Features
train_target, # Target vector
epochs=3, # Number of epochs
verbose=1, # Print description after each epoch
batch_size=100, # Number of observations per batch
validation_data=(test_features, test_target)) # Data for evaluation
Train on 25000 samples, validate on 25000 samples
Epoch 1/3
25000/25000 [==============================] - 2s - loss: 0.4215 - acc: 0.8102 - val_loss: 0.3385 - val_acc: 0.8558
Epoch 2/3
25000/25000 [==============================] - 1s - loss: 0.3241 - acc: 0.8646 - val_loss: 0.3261 - val_acc: 0.8626
Epoch 3/3
25000/25000 [==============================] - 2s - loss: 0.3120 - acc: 0.8700 - val_loss: 0.3268 - val_acc: 0.8593