Feature columns

This document is an adaptation of the official TensorFlow Feature Columns guide.

This document details feature columns and how they can be used as inputs to neural networks using TensorFlow. Feature columns are very rich, enabling you to transform a diverse range of raw data into formats that neural networks can use, allowing easy experimentation.

What kind of data can a deep neural network operate on? The answer is, of course, numbers (for example, tf$float32). After all, every neuron in a neural network performs multiplication and addition operations on weights and input data. Real-life input data, however, often contains non-numerical (categorical) data. For example, consider a product_class feature that can contain the following three non-numerical values:

ML models generally represent categorical values as simple vectors in which a 1 represents the presence of a value and a 0 represents the absence of a value. For example, when product_class is set to sports, an ML model would usually represent product_class as [0, 0, 1], meaning:

So, although raw data can be numerical or categorical, an ML model represents all features as numbers.

This document explains nine of the feature columns available in tfdatasets. As the following figure shows, all nine functions return either a categorical_column or a dense_column object, except bucketized_column, which inherits from both classes:

Feature column methods fall into two main categories and one hybrid category.

Let’s look at these functions in more detail.

Feature spec interface

We are going to use the feature_spec interface in the examples. The feature_spec interface is an abstraction that makes it easier to define feature columns in R.

You can initialize a feature_spec with:

hearts_dataset <- tensor_slices_dataset(hearts)
spec <- feature_spec(hearts_dataset, target ~ .)

We then add steps in order to define feature_columns. Read the feature_spec vignette for more information.

Numeric column

We use step_numeric_column to add numeric columns to our spec.

spec %>% 

By default, a numeric column creates a single value (scalar). Use the shape argument to specify another shape. For example:

# Represent a 10-element vector in which each cell contains a tf$float32.
spec %>% 
  step_numeric_column(bowling, shape = 10)

# Represent a 10x5 matrix in which each cell contains a tf$float32.
spec %>% 
  step_numeric_column(my_matrix, shape = c(10, 5))

A nice feature of step_numeric_columnis that you can also specify normalizer functions. When using a scaler, the scaling constants will be learned from data when fitting the feature_spec.

# use a function that defines tensorflow ops.
spec %>% 
  step_numeric_column(age, normalizer_fn = function(x) (x-10)/5)

# use a scaler
spec %>% 
  step_numeric_column(age, normalizer_fn = scaler_standard())

Bucketized column

Often, you don’t want to feed a number directly into the model, but instead split its value into different categories based on numerical ranges. To do so, create a bucketized_column. For example, consider raw data that represents the year a house was built. Instead of representing that year as a scalar numeric column, we could split the year into the following four buckets:

Dividing year data into four buckets.

Why would you want to split a number — a perfectly valid input to your model — into a categorical value? Well, notice that the categorization splits a single input number into a four-element vector. Therefore, the model now can learn four individual weights rather than just one; four weights creates a richer model than one weight. More importantly, bucketizing enables the model to clearly distinguish between different year categories since only one of the elements is set (1) and the other three elements are cleared (0). For example, when we just use a single number (a year) as input, a linear model can only learn a linear relationship. So, bucketing provides the model with additional flexibility that the model can use to learn.

The following code demonstrates how to create a bucketized feature:

# First, convert the raw input to a numeric column.
spec <- spec %>% 

# Then, bucketize the numeric column.
spec <-  spec %>% 
  step_bucketized_column(age, boundaries = c(30, 50, 70))

Note that specifying a three-element boundaries vector creates a four-element bucketized vector.

Categorical identity column

Categorical identity columns are to tfdatasets what factors are to R. Put differently, they can be seen as a special case of bucketized columns. In traditional bucketized columns, each bucket represents a range of values (for example, from 1960 to 1979). In a categorical identity column, each bucket represents a single, unique integer. For example, let’s say you want to represent the integer range [0, 4). That is, you want to represent the integers 0, 1, 2, or 3. In this case, the categorical identity mapping looks like this:

A categorical identity column mapping. Note that this is a one-hot encoding, not a binary numerical encoding.

As with bucketized columns, a model can learn a separate weight for each class in a categorical identity column. For example, instead of using a string to represent the product_class, let’s represent each class with a unique integer value. That is:

Call step_categorical_column_with_identity to add a categorical identity column to the feature_spec. For example:

# Create categorical output for an integer feature named "my_feature_b",
# The values of my_feature_b must be >= 0 and < num_buckets
spec <- spec %>% 
  step_categorical_column_with_identity(my_feature_b, num_buckets = 4)

Categorical vocabulary column

We cannot input strings directly to a model. Instead, we must first map strings to numeric or categorical values. Categorical vocabulary columns provide a good way to represent strings as a one-hot vector. For example:

Mapping string values to vocabulary columns.

As you can see, categorical vocabulary columns are kind of an enum version of categorical identity columns. tfdatasets provides two different functions to create categorical vocabulary columns:

categorical_column_with_vocabulary_list maps each string to an integer based on an explicit vocabulary list. For example:

spec <- spec %>% 
    vocabulary_list = c("fixed", "normal", "reversible")

Note that the vocabulary_list argument is optional in R and the vocabulary will be discovered when fitting the featture_spec which saves us a lot of typing.

You can also place the vocabulary in a separate file that should contain one line for each vocabulary element. You can then use:

spec <- spec %>% 
  step_categorical_column_with_vocabulary_file(thal, vocabulary_file = "thal.txt")

Hashed Column

So far, we’ve worked with a naively small number of categories. For example, our product_class example has only 3 categories. Often though, the number of categories can be so big that it’s not possible to have individual categories for each vocabulary word or integer because that would consume too much memory. For these cases, we can instead turn the question around and ask, “How many categories am I willing to have for my input?” In fact, the step_categorical_column_with_hash_bucket function enables you to specify the number of categories. For this type of feature column the model calculates a hash value of the input, then puts it into one of the hash_bucket_size categories using the modulo operator, as in the following pseudocode:

# pseudocode
feature_id = hash(raw_feature) % hash_bucket_size

The code to add the feature_column to the feature_spec might look something like this:

spec <- spec %>% 
  step_categorical_column_with_hash_bucket(thal, hash_bucket_size = 100)

At this point, you might rightfully think: “This is crazy!” After all, we are forcing the different input values to a smaller set of categories. This means that two probably unrelated inputs will be mapped to the same category, and consequently mean the same thing to the neural network. The following figure illustrates this dilemma, showing that kitchenware and sports both get assigned to category (hash bucket) 12:

Representing data with hash buckets.

As with many counterintuitive phenomena in machine learning, it turns out that hashing often works well in practice. That’s because hash categories provide the model with some separation. The model can use additional features to further separate kitchenware from sports.

Crossed column

Combining features into a single feature, better known as feature crosses, enables the model to learn separate weights for each combination of features.

More concretely, suppose we want our model to calculate real estate prices in Atlanta, GA. Real-estate prices within this city vary greatly depending on location. Representing latitude and longitude as separate features isn’t very useful in identifying real-estate location dependencies; however, crossing latitude and longitude into a single feature can pinpoint locations. Suppose we represent Atlanta as a grid of 100x100 rectangular sections, identifying each of the 10,000 sections by a feature cross of latitude and longitude. This feature cross enables the model to train on pricing conditions related to each individual section, which is a much stronger signal than latitude and longitude alone.

The following figure shows our plan, with the latitude & longitude values for the corners of the city in red text:

Map of Atlanta. Imagine this map divided into 10,000 sections of equal size.

For the solution, we used a combination of the bucketized_column we looked at earlier, with the step_crossed_column function.

spec <- feature_spec(dataset, target ~ latitute + longitude) %>% 
  step_numeric_column(latitude, longitude) %>% 
  step_bucketized_column(latitude, boundaries = c(latitude_edges)) %>% 
  step_bucketized_column(longitude, boundaries = c(longitude_edges)) %>% 
  step_crossed_column(latitude_longitude = c(latitude, longitude), hash_bucket_size = 100)

You may create a feature cross from either of the following:

When the feature columns latitude_bucket_fc and longitude_bucket_fc are crossed, TensorFlow will create (latitude_fc, longitude_fc) pairs for each example. This would produce a full grid of possibilities as follows:

(0,0),  (0,1)...  (0,99)
 (1,0),  (1,1)...  (1,99)
   ...     ...       ...
(99,0), (99,1)...(99, 99)

Except that a full grid would only be tractable for inputs with limited vocabularies. Instead of building this, potentially huge, table of inputs, the crossed_column only builds the number requested by the hash_bucket_size argument. The feature column assigns an example to a index by running a hash function on the tuple of inputs, followed by a modulo operation with hash_bucket_size.

As discussed earlier, performing the hash and modulo function limits the number of categories, but can cause category collisions; that is, multiple (latitude, longitude) feature crosses will end up in the same hash bucket. In practice though, performing feature crosses still adds significant value to the learning capability of your models.

Somewhat counterintuitively, when creating feature crosses, you typically still should include the original (uncrossed) features in your model (as in the preceding code snippet). The independent latitude and longitude features help the model distinguish between examples where a hash collision has occurred in the crossed feature.

Indicator and embedding columns

Indicator columns and embedding columns never work on features directly, but instead take categorical columns as input.

When using an indicator column, we’re telling TensorFlow to do exactly what we’ve seen in our categorical product_class example. That is, an indicator column treats each category as an element in a one-hot vector, where the matching category has value 1 and the rest have 0s:

Representing data in indicator columns.

Here’s how you create an indicator column:

spec <- feature_spec(dataset, target ~ .) %>% 
  step_categorical_column_with_vocabulary_list(product_class) %>% 

Now, suppose instead of having just three possible classes, we have a million. Or maybe a billion. For a number of reasons, as the number of categories grow large, it becomes infeasible to train a neural network using indicator columns.

We can use an embedding column to overcome this limitation. Instead of representing the data as a one-hot vector of many dimensions, an embedding column represents that data as a lower-dimensional, ordinary vector in which each cell can contain any number, not just 0 or 1. By permitting a richer palette of numbers for every cell, an embedding column contains far fewer cells than an indicator column.

Let’s look at an example comparing indicator and embedding columns. Suppose our input examples consist of different words from a limited palette of only 81 words. Further suppose that the data set provides the following input words in 4 separate examples:

In that case, the following figure illustrates the processing path for embedding columns or indicator columns.