# Getting started with the Keras Sequential model

The `Sequential` model is a linear stack of layers.

You can create a `Sequential` model by passing a list of layer instances to the constructor:

    from keras.models import Sequential
    from keras.layers import Dense, Activation

    model = Sequential([
        Dense(32, input_shape=(784,)),
        Activation('relu'),
        Dense(10),
        Activation('softmax'),
    ])

You can also simply add layers via the `.add()` method:

    model = Sequential()
    model.add(Dense(32, input_dim=784))
    model.add(Activation('relu'))

## Specifying the input shape

The model needs to know what input shape it should expect. For this reason, the first layer in a `Sequential` model (and only the first, because following layers can do automatic shape inference) needs to receive information about its input shape. There are several possible ways to do this:

- Pass an `input_shape` argument to the first layer. This is a shape tuple (a tuple of integers or `None` entries, where `None` indicates that any positive integer may be expected). In `input_shape`, the batch dimension is not included.
- Some 2D layers, such as `Dense`, support the specification of their input shape via the argument `input_dim`, and some 3D temporal layers support the arguments `input_dim` and `input_length`.
- If you ever need to specify a fixed batch size for your inputs (this is useful for stateful recurrent networks), you can pass a `batch_size` argument to a layer. If you pass both `batch_size=32` and `input_shape=(6, 8)` to a layer, it will then expect every batch of inputs to have the batch shape `(32, 6, 8)`.

As such, the following snippets are strictly equivalent:

    model = Sequential()
    model.add(Dense(32, input_shape=(784,)))

    model = Sequential()
    model.add(Dense(32, input_dim=784))

## Compilation

Before training a model, you need to configure the learning process, which is done via the `compile` method. It receives three arguments:

- An optimizer. This could be the string identifier of an existing optimizer (such as `rmsprop` or `adagrad`), or an instance of the `Optimizer` class. See: optimizers.
- A loss function. This is the objective that the model will try to minimize. It can be the string identifier of an existing loss function (such as `categorical_crossentropy` or `mse`), or it can be an objective function. See: losses.
- A list of metrics. For any classification problem you will want to set this to `metrics=['accuracy']`. A metric could be the string identifier of an existing metric or a custom metric function.

    # For a multi-class classification problem
    model.compile(optimizer='rmsprop',
                  loss='categorical_crossentropy',
                  metrics=['accuracy'])

    # For a binary classification problem
    model.compile(optimizer='rmsprop',
                  loss='binary_crossentropy',
                  metrics=['accuracy'])

    # For a mean squared error regression problem
    model.compile(optimizer='rmsprop',
                  loss='mse')

    # For custom metrics
    import keras.backend as K

    def mean_pred(y_true, y_pred):
        return K.mean(y_pred)

    model.compile(optimizer='rmsprop',
                  loss='binary_crossentropy',
                  metrics=['accuracy', mean_pred])

## Training

Keras models are trained on Numpy arrays of input data and labels. For training a model, you will typically use the `fit` function. Read its documentation here.

    # For a single-input model with 2 classes (binary classification):

    model = Sequential()
    model.add(Dense(32, activation='relu', input_dim=100))
    model.add(Dense(1, activation='sigmoid'))
    model.compile(optimizer='rmsprop',
                  loss='binary_crossentropy',
                  metrics=['accuracy'])

    # Generate dummy data
    import numpy as np
    data = np.random.random((1000, 100))
    labels = np.random.randint(2, size=(1000, 1))

    # Train the model, iterating on the data in batches of 32 samples
    model.fit(data, labels, epochs=10, batch_size=32)

    # For a single-input model with 10 classes (categorical classification):

    model = Sequential()
    model.add(Dense(32, activation='relu', input_dim=100))
    model.add(Dense(10, activation='softmax'))
    model.compile(optimizer='rmsprop',
                  loss='categorical_crossentropy',
                  metrics=['accuracy'])

    # Generate dummy data
    import numpy as np
    data = np.random.random((1000, 100))
    labels = np.random.randint(10, size=(1000, 1))

    # Convert labels to categorical one-hot encoding
    one_hot_labels = keras.utils.to_categorical(labels, num_classes=10)

    # Train the model, iterating on the data in batches of 32 samples
    model.fit(data, one_hot_labels, epochs=10, batch_size=32)

## Examples

Here are a few examples to get you started!

In the examples folder, you will also find example models for real datasets:

- CIFAR10 small images classification: Convolutional Neural Network (CNN) with realtime data augmentation
- IMDB movie review sentiment classification: LSTM over sequences of words
- Reuters newswires topic classification: Multilayer Perceptron (MLP)
- MNIST handwritten digits classification: MLP & CNN
- Character-level text generation with LSTM

...and more.

### Multilayer Perceptron (MLP) for multi-class softmax classification:

    import keras
    from keras.models import Sequential
    from keras.layers import Dense, Dropout, Activation
    from keras.optimizers import SGD

    # Generate dummy data
    import numpy as np
    x_train = np.random.random((1000, 20))
    y_train = keras.utils.to_categorical(np.random.randint(10, size=(1000, 1)), num_classes=10)
    x_test = np.random.random((100, 20))
    y_test = keras.utils.to_categorical(np.random.randint(10, size=(100, 1)), num_classes=10)

    model = Sequential()
    # Dense(64) is a fully-connected layer with 64 hidden units.
    # in the first layer, you must specify the expected input data shape:
    # here, 20-dimensional vectors.
    model.add(Dense(64, activation='relu', input_dim=20))
    model.add(Dropout(0.5))
    model.add(Dense(64, activation='relu'))
    model.add(Dropout(0.5))
    model.add(Dense(10, activation='softmax'))

    sgd = SGD(lr=0.01, decay=1e-6, momentum=0.9, nesterov=True)
    model.compile(loss='categorical_crossentropy',
                  optimizer=sgd,
                  metrics=['accuracy'])

    model.fit(x_train, y_train,
              epochs=20,
              batch_size=128)
    score = model.evaluate(x_test, y_test, batch_size=128)

### MLP for binary classification:

    import numpy as np
    from keras.models import Sequential
    from keras.layers import Dense, Dropout

    # Generate dummy data
    x_train = np.random.random((1000, 20))
    y_train = np.random.randint(2, size=(1000, 1))
    x_test = np.random.random((100, 20))
    y_test = np.random.randint(2, size=(100, 1))

    model = Sequential()
    model.add(Dense(64, input_dim=20, activation='relu'))
    model.add(Dropout(0.5))
    model.add(Dense(64, activation='relu'))
    model.add(Dropout(0.5))
    model.add(Dense(1, activation='sigmoid'))

    model.compile(loss='binary_crossentropy',
                  optimizer='rmsprop',
                  metrics=['accuracy'])

    model.fit(x_train, y_train,
              epochs=20,
              batch_size=128)
    score = model.evaluate(x_test, y_test, batch_size=128)

### VGG-like convnet:

    import numpy as np
    import keras
    from keras.models import Sequential
    from keras.layers import Dense, Dropout, Flatten
    from keras.layers import Conv2D, MaxPooling2D
    from keras.optimizers import SGD

    # Generate dummy data
    x_train = np.random.random((100, 100, 100, 3))
    y_train = keras.utils.to_categorical(np.random.randint(10, size=(100, 1)), num_classes=10)
    x_test = np.random.random((20, 100, 100, 3))
    y_test = keras.utils.to_categorical(np.random.randint(10, size=(20, 1)), num_classes=10)

    model = Sequential()
    # input: 100x100 images with 3 channels -> (100, 100, 3) tensors.
    # this applies 32 convolution filters of size 3x3 each.
    model.add(Conv2D(32, (3, 3), activation='relu', input_shape=(100, 100, 3)))
    model.add(Conv2D(32, (3, 3), activation='relu'))
    model.add(MaxPooling2D(pool_size=(2, 2)))
    model.add(Dropout(0.25))

    model.add(Conv2D(64, (3, 3), activation='relu'))
    model.add(Conv2D(64, (3, 3), activation='relu'))
    model.add(MaxPooling2D(pool_size=(2, 2)))
    model.add(Dropout(0.25))

    model.add(Flatten())
    model.add(Dense(256, activation='relu'))
    model.add(Dropout(0.5))
    model.add(Dense(10, activation='softmax'))

    sgd = SGD(lr=0.01, decay=1e-6, momentum=0.9, nesterov=True)
    model.compile(loss='categorical_crossentropy', optimizer=sgd)

    model.fit(x_train, y_train, batch_size=32, epochs=10)
    score = model.evaluate(x_test, y_test, batch_size=32)

### Sequence classification with LSTM:

    from keras.models import Sequential
    from keras.layers import Dense, Dropout
    from keras.layers import Embedding
    from keras.layers import LSTM

    max_features = 1024

    model = Sequential()
    model.add(Embedding(max_features, output_dim=256))
    model.add(LSTM(128))
    model.add(Dropout(0.5))
    model.add(Dense(1, activation='sigmoid'))

    model.compile(loss='binary_crossentropy',
                  optimizer='rmsprop',
                  metrics=['accuracy'])

    model.fit(x_train, y_train, batch_size=16, epochs=10)
    score = model.evaluate(x_test, y_test, batch_size=16)

### Sequence classification with 1D convolutions:

    from keras.models import Sequential
    from keras.layers import Dense, Dropout
    from keras.layers import Embedding
    from keras.layers import Conv1D, GlobalAveragePooling1D, MaxPooling1D

    seq_length = 64

    model = Sequential()
    model.add(Conv1D(64, 3, activation='relu', input_shape=(seq_length, 100)))
    model.add(Conv1D(64, 3, activation='relu'))
    model.add(MaxPooling1D(3))
    model.add(Conv1D(128, 3, activation='relu'))
    model.add(Conv1D(128, 3, activation='relu'))
    model.add(GlobalAveragePooling1D())
    model.add(Dropout(0.5))
    model.add(Dense(1, activation='sigmoid'))

    model.compile(loss='binary_crossentropy',
                  optimizer='rmsprop',
                  metrics=['accuracy'])

    model.fit(x_train, y_train, batch_size=16, epochs=10)
    score = model.evaluate(x_test, y_test, batch_size=16)

### Stacked LSTM for sequence classification

In this model, we stack 3 LSTM layers on top of each other, making the model capable of learning higher-level temporal representations.

The first two LSTMs return their full output sequences, but the last one only returns the last step in its output sequence, thus dropping the temporal dimension (i.e. converting the input sequence into a single vector).

    from keras.models import Sequential
    from keras.layers import LSTM, Dense
    import numpy as np

    data_dim = 16
    timesteps = 8
    num_classes = 10

    # expected input data shape: (batch_size, timesteps, data_dim)
    model = Sequential()
    model.add(LSTM(32, return_sequences=True,
                   input_shape=(timesteps, data_dim)))  # returns a sequence of vectors of dimension 32
    model.add(LSTM(32, return_sequences=True))  # returns a sequence of vectors of dimension 32
    model.add(LSTM(32))  # return a single vector of dimension 32
    model.add(Dense(10, activation='softmax'))

    model.compile(loss='categorical_crossentropy',
                  optimizer='rmsprop',
                  metrics=['accuracy'])

    # Generate dummy training data
    x_train = np.random.random((1000, timesteps, data_dim))
    y_train = np.random.random((1000, num_classes))

    # Generate dummy validation data
    x_val = np.random.random((100, timesteps, data_dim))
    y_val = np.random.random((100, num_classes))

    model.fit(x_train, y_train,
              batch_size=64, epochs=5,
              validation_data=(x_val, y_val))

### Same stacked LSTM model, rendered "stateful"

A stateful recurrent model is one for which the internal states (memories) obtained after processing a batch of samples are reused as initial states for the samples of the next batch. This allows to process longer sequences while keeping computational complexity manageable.

You can read more about stateful RNNs in the FAQ.

    from keras.models import Sequential
    from keras.layers import LSTM, Dense
    import numpy as np

    data_dim = 16
    timesteps = 8
    num_classes = 10
    batch_size = 32

    # Expected input batch shape: (batch_size, timesteps, data_dim)
    # Note that we have to provide the full batch_input_shape since the network is stateful.
    # the sample of index i in batch k is the follow-up for the sample i in batch k-1.
    model = Sequential()
    model.add(LSTM(32, return_sequences=True, stateful=True,
                   batch_input_shape=(batch_size, timesteps, data_dim)))
    model.add(LSTM(32, return_sequences=True, stateful=True))
    model.add(LSTM(32, stateful=True))
    model.add(Dense(10, activation='softmax'))

    model.compile(loss='categorical_crossentropy',
                  optimizer='rmsprop',
                  metrics=['accuracy'])

    # Generate dummy training data
    x_train = np.random.random((batch_size * 10, timesteps, data_dim))
    y_train = np.random.random((batch_size * 10, num_classes))

    # Generate dummy validation data
    x_val = np.random.random((batch_size * 3, timesteps, data_dim))
    y_val = np.random.random((batch_size * 3, num_classes))

    model.fit(x_train, y_train,
              batch_size=batch_size, epochs=5, shuffle=False,
              validation_data=(x_val, y_val))

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