Getting Started with Keras

Preparing the Data

The MNIST dataset is included with Keras and can be accessed using the dataset_mnist() function. Here we load the dataset then create variables for our test and training data:

library(keras3)
mnist <- dataset_mnist()
x_train <- mnist$train$x
y_train <- mnist$train$y
x_test <- mnist$test$x
y_test <- mnist$test$y

The x data is a 3-d array (images, width, height) of grayscale values. To prepare the data for training we convert the 3-d arrays into matrices by reshaping width and height into a single dimension (28x28 images are flattened into length 784 vectors). Then, we convert the grayscale values from integers ranging between 0 to 255 into floating point values ranging between 0 and 1:

# reshape
x_train <- array_reshape(x_train, c(nrow(x_train), 784))
x_test <- array_reshape(x_test, c(nrow(x_test), 784))
# rescale
x_train <- x_train / 255
x_test <- x_test / 255

Note that we use the array_reshape() function rather than the dim<-() function to reshape the array. This is so that the data is re-interpreted using row-major semantics (as opposed to R’s default column-major semantics), which is in turn compatible with the way that the numerical libraries called by Keras interpret array dimensions.

The y data is an integer vector with values ranging from 0 to 9. To prepare this data for training we one-hot encode the vectors into binary class matrices using the Keras to_categorical() function:

y_train <- to_categorical(y_train, 10)
y_test <- to_categorical(y_test, 10)

Defining the Model

The core data structure of Keras is a model, a way to organize layers. The simplest type of model is the Sequential model, a linear stack of layers.

We begin by creating a sequential model and then adding layers using the pipe (|>) operator:

model <- keras_model_sequential(input_shape = c(784))
model |>
  layer_dense(units = 256, activation = 'relu') |>
  layer_dropout(rate = 0.4) |>
  layer_dense(units = 128, activation = 'relu') |>
  layer_dropout(rate = 0.3) |>
  layer_dense(units = 10, activation = 'softmax')

The input_shape argument to the first layer specifies the shape of the input data (a length 784 numeric vector representing a grayscale image). The final layer outputs a length 10 numeric vector (probabilities for each digit) using a softmax activation function.

Use the summary() function to print the details of the model:

summary(model)

## Model: "sequential"
## ┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━┓
## ┃ Layer (type)                    ┃ Output Shape           ┃       Param # ┃
## ┡━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━┩
## │ dense (Dense)                   │ (None, 256)            │       200,960 │
## ├─────────────────────────────────┼────────────────────────┼───────────────┤
## │ dropout (Dropout)               │ (None, 256)            │             0 │
## ├─────────────────────────────────┼────────────────────────┼───────────────┤
## │ dense_1 (Dense)                 │ (None, 128)            │        32,896 │
## ├─────────────────────────────────┼────────────────────────┼───────────────┤
## │ dropout_1 (Dropout)             │ (None, 128)            │             0 │
## ├─────────────────────────────────┼────────────────────────┼───────────────┤
## │ dense_2 (Dense)                 │ (None, 10)             │         1,290 │
## └─────────────────────────────────┴────────────────────────┴───────────────┘
##  Total params: 235,146 (918.54 KB)
##  Trainable params: 235,146 (918.54 KB)
##  Non-trainable params: 0 (0.00 B)

plot(model)

Next, compile the model with appropriate loss function, optimizer, and metrics:

model |> compile(
  loss = 'categorical_crossentropy',
  optimizer = optimizer_rmsprop(),
  metrics = c('accuracy')
)

Training and Evaluation

Use the fit() function to train the model for 30 epochs using batches of 128 images:

history <- model |> fit(
  x_train, y_train,
  epochs = 30, batch_size = 128,
  validation_split = 0.2
)

The history object returned by fit() includes loss and accuracy metrics which we can plot:

plot(history)

plot of chunk unnamed-chunk-12

Evaluate the model’s performance on the test data:

model |> evaluate(x_test, y_test)

## 313/313 - 1s - 2ms/step - accuracy: 0.9816 - loss: 0.0847

## $accuracy
## [1] 0.9816
##
## $loss
## [1] 0.08465149

Generate predictions on new data:

probs <- model |> predict(x_test)

## 313/313 - 0s - 1ms/step

max.col(probs) - 1L

##   [1] 7 2 1 0 4 1 4 9 5 9 0 6 9 0 1 5 9 7 3 4 9 6 6 5 4 0 7 4 0 1 3 1 3 4 7
##  [36] 2 7 1 2 1 1 7 4 2 3 5 1 2 4 4 6 3 5 5 6 0 4 1 9 5 7 8 9 3 7 4 6 4 3 0
##  [71] 7 0 2 9 1 7 3 2 9 7 7 6 2 7 8 4 7 3 6 1 3 6 9 3 1 4 1 7 6 9
##  [ reached getOption("max.print") -- omitted 9900 entries ]

Keras provides a vocabulary for building deep learning models that is simple, elegant, and intuitive. Building a question answering system, an image classification model, a neural Turing machine, or any other model is just as straightforward.

Deep Learning with R Book

If you want a more comprehensive introduction to both Keras and the concepts and practice of deep learning, we recommend the Deep Learning with R, 2nd Edition book from Manning. This book is a collaboration between François Chollet, the creator of (Python) Keras, J.J. Allaire, who wrote the original R interface to Keras, and Tomasz Kalinowski, the maintainer of the R interface to Keras.

The book presumes no significant knowledge of machine learning and deep learning, and goes all the way from basic theory to advanced practical applications, all using the R interface to Keras.