Sometimes, one or more preprocessing steps will need to be taken to condition data for a learner. Some examples of preprocessing steps include feature extraction, standardization, normalization, imputation, and dimensionality reduction. Preprocessing in Rubix ML is handled through Transformer objects whose logic is hidden behind an easy-to-use interface.

Stateful transformers are a type of transformer that must be fitted to a dataset before transforming. After fitting a stateful transformer, it will expect the features to be present in the same order when transforming subsequent datasets. A few transformers are supervised meaning they must be fitted with a Labeled dataset. Elastic transformers can have their fittings updated with new data after an initial fitting.

Transform a Dataset#

An example of a transformation is one that converts the categorical features of a dataset to continuous ones using a one hot encoding. To accomplish this with the library, pass a One Hot Encoder transformer instance to the Dataset object's apply() method. This method automatically handles fitting and transforming the samples.

use Rubix\ML\Transformers\OneHotEncoder;

$dataset->apply(new OneHotEncoder());

Transformations can be chained by calling the apply() method fluently.

use Rubix\ML\Transformers\RandomHotDeckImputer;
use Rubix\ML\Transformers\OneHotEncoder;
use Rubix\ML\Transformers\ZScaleStandardizer;

$dataset->apply(new RandomHotDeckImputer(5))
    ->apply(new OneHotEncoder())
    ->apply(new ZScaleStandardizer());

Note: Transformers do not alter the labels in a dataset. Instead, you can use the transformLabels() method on a Labeled dataset instance.

Sometimes, we might just want to transform a single column of the dataset. In the example below we use the transformColumn() method on the dataset to log transform a specified column.

$dataset->transformColumn(6, 'log1p');

Transformer Pipelines#

Pipeline meta-estimators help you automate a series of transformations. In addition, Pipeline objects are Persistable allowing you to save and load transformer fittings between processes. Whenever a dataset object is passed to a learner wrapped in a Pipeline, it will automatically be fitted and/or transformed before it arrives in the learner's context.

Let's apply the same 3 transformers as in the example above by passing the transformer instances in the order we want them applied along with a base estimator to the constructor of Pipeline like in the example below.

use Rubix\ML\Pipeline;
use Rubix\ML\Transformers\RandomHotDeckImputer;
use Rubix\ML\Transformers\OneHotEncoder;
use Rubix\ML\Transformers\ZScaleStandardizer;
use Rubix\ML\Classifiers\SoftmaxClassifier;

$estimator = new Pipeline([
    new RandomHotDeckImputer(5),
    new OneHotEncoder(),
    new ZScaleStandardizer(),
], new SoftmaxClassifier(200));

Calling train() or partial() will result in the transformers being fitted or updated before being passed to the Softmax Classifier.

$estimator->train($dataset); // Transformers fitted and applied automatically

$estimator->partial($dataset); // Transformers updated and applied

Any time a dataset is passed to the Pipeline it will automatically be transformed before being handed to the underlying estimator.

$predictions = $estimator->predict($dataset); // Dataset automatically transformed

Standardization and Normalization#

Oftentimes, the continuous features of a dataset will be on different scales because they were measured by different methods. For example, age (0 - 100) and income (0 - 9,999,999) are on two widely different scales. Standardization is the processes of transforming a dataset such that the features are all on one scale. Normalization is the special case where the transformed features have a range between 0 and 1. Depending on the transformer, it may operate on the columns or the rows of the dataset.

Transformer Operates On Range Stateful Elastic
L1 Normalizer Rows [0, 1]
L2 Normalizer Rows [0, 1]
Max Absolute Scaler Columns [-1, 1]
Min Max Normalizer Columns [min, max]
Robust Standardizer Columns [-∞, ∞]
Z Scale Standardizer Columns [-∞, ∞]

Feature Conversion#

Feature converters are transformers that convert feature columns of one type to another. Since learners can be compatible with different data types, it may be necessary sometimes to convert features of an incompatible type to a compatible one.

Transformer From To Stateful Elastic
Interval Discretizer Continuous Categorical
One Hot Encoder Categorical Continuous
Numeric String Converter Categorical Continuous

Dimensionality Reduction#

Dimensionality reduction in machine learning is analogous to compression in the context of sending data over a wire. It allows a learner to train and infer quicker by producing a dataset with fewer but more informative features.

Transformer Supervised Stateful Elastic
Dense Random Projector
Gaussian Random Projector
Linear Discriminant Analysis
Principal Component Analysis
Sparse Random Projector

Feature Selection#

Similarly to dimensionality reduction, feature selection aims to reduce the number of features in a dataset, however, feature selection seeks to keep the best features as-is and drop the less informative ones entirely. Adding feature selection can help speed up training and inference by creating a more parsimonious model. It can also improve the performance of the model by removing noise features and features that are uncorrelated with the outcome.

Transformer Supervised Stateful Elastic
Variance Threshold Filter


One technique for handling missing data is a preprocessing step called imputation. Imputation is the process of replacing missing values in the dataset with a pretty good substitution. Examples include the average value for a feature or the sample's nearest neighbor's value. Imputation allows you to get more value from your data and limits the introduction of bias in the process.

Transformer Compatibility Stateful Elastic
KNN Imputer Continuous, Categorical
Missing Data Imputer Continuous, Categorical
Random Hot Deck Imputer Continuous, Categorical

Text Transformers#

We provide a number of transformers for natural language processing (NLP) tasks such as those for text cleaning, normalization, and feature extraction. Cleaning the text will help eliminate noise such as stop words or other uninformative tokens like URLs and email addresses from the corpus. Normalizing the text ensures that words like therapist, Therapist, and ThErApIsT are recognized as the same word. Encoding the text into fixed-length feature vectors is the job of feature extractors such as Word Count Vectorizer.

Transformer Stateful Elastic
HTML Stripper
Regex Filter
Text Normalizer
Multibyte Text Normalizer
Stop Word Filter
TF-IDF Transformer
Whitespace Trimmer
Word Count Vectorizer

Image Transformers#

For computer vision tasks, images may need to be processed to ensure they are the correct size and shape.

Transformer Stateful Elastic
Image Resizer
Image Vectorizer

Other Transformers#

Here are a list of other transformers that do not fall into a category.

Transformer Stateful Elastic
Polynomial Expander