Package org.apache.spark.ml.feature

package org.apache.spark.ml.feature

Feature transformers The `ml.feature` package provides common feature transformers that help convert raw data or features into more suitable forms for model fitting. Most feature transformers are implemented as Transformers, which transforms one Dataset into another, e.g., HashingTF. Some feature transformers are implemented as Estimator}s, because the transformation requires some aggregated information of the dataset, e.g., document frequencies in IDF. For those feature transformers, calling Estimator.fit(org.apache.spark.sql.Dataset<?>, org.apache.spark.ml.param.ParamPair<?>, org.apache.spark.ml.param.ParamPair<?>...) is required to obtain the model first, e.g., IDFModel, in order to apply transformation. The transformation is usually done by appending new columns to the input Dataset, so all input columns are carried over. We try to make each transformer minimal, so it becomes flexible to assemble feature transformation pipelines. Pipeline can be used to chain feature transformers, and VectorAssembler can be used to combine multiple feature transformations, for example:

 
   import java.util.Arrays;

   import org.apache.spark.api.java.JavaRDD;
   import static org.apache.spark.sql.types.DataTypes.*;
   import org.apache.spark.sql.types.StructType;
   import org.apache.spark.sql.Dataset;
   import org.apache.spark.sql.RowFactory;
   import org.apache.spark.sql.Row;

   import org.apache.spark.ml.feature.*;
   import org.apache.spark.ml.Pipeline;
   import org.apache.spark.ml.PipelineStage;
   import org.apache.spark.ml.PipelineModel;

  // a DataFrame with three columns: id (integer), text (string), and rating (double).
  StructType schema = createStructType(
    Arrays.asList(
      createStructField("id", IntegerType, false),
      createStructField("text", StringType, false),
      createStructField("rating", DoubleType, false)));
  JavaRDD<Row> rowRDD = jsc.parallelize(
    Arrays.asList(
      RowFactory.create(0, "Hi I heard about Spark", 3.0),
      RowFactory.create(1, "I wish Java could use case classes", 4.0),
      RowFactory.create(2, "Logistic regression models are neat", 4.0)));
  Dataset<Row> dataset = jsql.createDataFrame(rowRDD, schema);
  // define feature transformers
  RegexTokenizer tok = new RegexTokenizer()
    .setInputCol("text")
    .setOutputCol("words");
  StopWordsRemover sw = new StopWordsRemover()
    .setInputCol("words")
    .setOutputCol("filtered_words");
  HashingTF tf = new HashingTF()
    .setInputCol("filtered_words")
    .setOutputCol("tf")
    .setNumFeatures(10000);
  IDF idf = new IDF()
    .setInputCol("tf")
    .setOutputCol("tf_idf");
  VectorAssembler assembler = new VectorAssembler()
    .setInputCols(new String[] {"tf_idf", "rating"})
    .setOutputCol("features");

  // assemble and fit the feature transformation pipeline
  Pipeline pipeline = new Pipeline()
    .setStages(new PipelineStage[] {tok, sw, tf, idf, assembler});
  PipelineModel model = pipeline.fit(dataset);

  // save transformed features with raw data
  model.transform(dataset)
    .select("id", "text", "rating", "features")
    .write().format("parquet").save("/output/path");
 
 

Some feature transformers implemented in MLlib are inspired by those implemented in scikit-learn. The major difference is that most scikit-learn feature transformers operate eagerly on the entire input dataset, while MLlib's feature transformers operate lazily on individual columns, which is more efficient and flexible to handle large and complex datasets.

See Also:

• scikit-learn.preprocessing

Related Packages

Package	Description
org.apache.spark.ml	DataFrame-based machine learning APIs to let users quickly assemble and configure practical machine learning pipelines.

Class	Description
Binarizer	Binarize a column of continuous features given a threshold.
BucketedRandomProjectionLSH	This BucketedRandomProjectionLSH implements Locality Sensitive Hashing functions for Euclidean distance metrics.
BucketedRandomProjectionLSHModel	Model produced by BucketedRandomProjectionLSH, where multiple random vectors are stored.
BucketedRandomProjectionLSHParams	Params for BucketedRandomProjectionLSH.
Bucketizer	Bucketizer maps a column of continuous features to a column of feature buckets.
ChiSqSelector	Deprecated. use UnivariateFeatureSelector instead.
ChiSqSelectorModel	Model fitted by ChiSqSelector.
ChiSqSelectorModel.ChiSqSelectorModelWriter
ColumnPruner	Utility transformer for removing temporary columns from a DataFrame.
CountVectorizer	Extracts a vocabulary from document collections and generates a CountVectorizerModel.
CountVectorizerModel	Converts a text document to a sparse vector of token counts.
CountVectorizerParams	Params for CountVectorizer and CountVectorizerModel.
DCT	A feature transformer that takes the 1D discrete cosine transform of a real vector.
Dot
ElementwiseProduct	Outputs the Hadamard product (i.e., the element-wise product) of each input vector with a provided "weight" vector.
EmptyTerm	Placeholder term for the result of undefined interactions, e.g. '1:1' or 'a:1'
FeatureHasher	Feature hashing projects a set of categorical or numerical features into a feature vector of specified dimension (typically substantially smaller than that of the original feature space).
HashingTF	Maps a sequence of terms to their term frequencies using the hashing trick.
IDF	Compute the Inverse Document Frequency (IDF) given a collection of documents.
IDFBase	Params for IDF and IDFModel.
IDFModel	Model fitted by IDF.
Imputer	Imputation estimator for completing missing values, using the mean, median or mode of the columns in which the missing values are located.
ImputerModel	Model fitted by Imputer.
ImputerParams	Params for Imputer and ImputerModel.
IndexToString	A Transformer that maps a column of indices back to a new column of corresponding string values.
InteractableTerm	A term that may be part of an interaction, e.g.
Interaction	Implements the feature interaction transform.
LabeledPoint	Class that represents the features and label of a data point.
LSHParams	Params for LSH.
MaxAbsScaler	Rescale each feature individually to range [-1, 1] by dividing through the largest maximum absolute value in each feature.
MaxAbsScalerModel	Model fitted by MaxAbsScaler.
MaxAbsScalerParams	Params for MaxAbsScaler and MaxAbsScalerModel.
MinHashLSH	LSH class for Jaccard distance.
MinHashLSHModel	Model produced by MinHashLSH, where multiple hash functions are stored.
MinMaxScaler	Rescale each feature individually to a common range [min, max] linearly using column summary statistics, which is also known as min-max normalization or Rescaling.
MinMaxScalerModel	Model fitted by MinMaxScaler.
MinMaxScalerParams	Params for MinMaxScaler and MinMaxScalerModel.
NGram	A feature transformer that converts the input array of strings into an array of n-grams.
Normalizer	Normalize a vector to have unit norm using the given p-norm.
OneHotEncoder	A one-hot encoder that maps a column of category indices to a column of binary vectors, with at most a single one-value per row that indicates the input category index.
OneHotEncoderBase	Private trait for params and common methods for OneHotEncoder and OneHotEncoderModel
OneHotEncoderCommon	Provides some helper methods used by OneHotEncoder.
OneHotEncoderModel	param: categorySizes Original number of categories for each feature being encoded.
PCA	PCA trains a model to project vectors to a lower dimensional space of the top PCA!.