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Feature Extraction, Transformation, and Selection - SparkML |
<a href="ml-guide.html">ML</a> - Features |
This section covers algorithms for working with features, roughly divided into these groups:
- Extraction: Extracting features from "raw" data
- Transformation: Scaling, converting, or modifying features
- Selection: Selecting a subset from a larger set of features
Table of Contents
- This will become a table of contents (this text will be scraped). {:toc}
Term Frequency-Inverse Document Frequency (TF-IDF) is a common text pre-processing step. In Spark ML, TF-IDF is separate into two parts: TF (+hashing) and IDF.
TF: HashingTF is a Transformer which takes sets of terms and converts those sets into fixed-length feature vectors. In text processing, a "set of terms" might be a bag of words.
The algorithm combines Term Frequency (TF) counts with the hashing trick for dimensionality reduction.
IDF: IDF is an Estimator which fits on a dataset and produces an IDFModel. The IDFModel takes feature vectors (generally created from HashingTF) and scales each column. Intuitively, it down-weights columns which appear frequently in a corpus.
Please refer to the MLlib user guide on TF-IDF for more details on Term Frequency and Inverse Document Frequency. For API details, refer to the HashingTF API docs and the IDF API docs.
In the following code segment, we start with a set of sentences. We split each sentence into words using Tokenizer. For each sentence (bag of words), we use HashingTF to hash the sentence into a feature vector. We use IDF to rescale the feature vectors; this generally improves performance when using text as features. Our feature vectors could then be passed to a learning algorithm.
val sentenceData = sqlContext.createDataFrame(Seq( (0, "Hi I heard about Spark"), (0, "I wish Java could use case classes"), (1, "Logistic regression models are neat") )).toDF("label", "sentence") val tokenizer = new Tokenizer().setInputCol("sentence").setOutputCol("words") val wordsData = tokenizer.transform(sentenceData) val hashingTF = new HashingTF().setInputCol("words").setOutputCol("rawFeatures").setNumFeatures(20) val featurizedData = hashingTF.transform(wordsData) val idf = new IDF().setInputCol("rawFeatures").setOutputCol("features") val idfModel = idf.fit(featurizedData) val rescaledData = idfModel.transform(featurizedData) rescaledData.select("features", "label").take(3).foreach(println) {% endhighlight %}
import org.apache.spark.api.java.JavaRDD; import org.apache.spark.ml.feature.HashingTF; import org.apache.spark.ml.feature.IDF; import org.apache.spark.ml.feature.Tokenizer; import org.apache.spark.mllib.linalg.Vector; import org.apache.spark.sql.DataFrame; import org.apache.spark.sql.Row; import org.apache.spark.sql.RowFactory; import org.apache.spark.sql.types.DataTypes; import org.apache.spark.sql.types.Metadata; import org.apache.spark.sql.types.StructField; import org.apache.spark.sql.types.StructType;
JavaRDD jrdd = jsc.parallelize(Lists.newArrayList( RowFactory.create(0, "Hi I heard about Spark"), RowFactory.create(0, "I wish Java could use case classes"), RowFactory.create(1, "Logistic regression models are neat") )); StructType schema = new StructType(new StructField[]{ new StructField("label", DataTypes.DoubleType, false, Metadata.empty()), new StructField("sentence", DataTypes.StringType, false, Metadata.empty()) }); DataFrame sentenceData = sqlContext.createDataFrame(jrdd, schema); Tokenizer tokenizer = new Tokenizer().setInputCol("sentence").setOutputCol("words"); DataFrame wordsData = tokenizer.transform(sentenceData); int numFeatures = 20; HashingTF hashingTF = new HashingTF() .setInputCol("words") .setOutputCol("rawFeatures") .setNumFeatures(numFeatures); DataFrame featurizedData = hashingTF.transform(wordsData); IDF idf = new IDF().setInputCol("rawFeatures").setOutputCol("features"); IDFModel idfModel = idf.fit(featurizedData); DataFrame rescaledData = idfModel.transform(featurizedData); for (Row r : rescaledData.select("features", "label").take(3)) { Vector features = r.getAs(0); Double label = r.getDouble(1); System.out.println(features); } {% endhighlight %}
sentenceData = sqlContext.createDataFrame([ (0, "Hi I heard about Spark"), (0, "I wish Java could use case classes"), (1, "Logistic regression models are neat") ], ["label", "sentence"]) tokenizer = Tokenizer(inputCol="sentence", outputCol="words") wordsData = tokenizer.transform(sentenceData) hashingTF = HashingTF(inputCol="words", outputCol="rawFeatures", numFeatures=20) featurizedData = hashingTF.transform(wordsData) idf = IDF(inputCol="rawFeatures", outputCol="features") idfModel = idf.fit(featurizedData) rescaledData = idfModel.transform(featurizedData) for features_label in rescaledData.select("features", "label").take(3): print(features_label) {% endhighlight %}
Word2Vec is an Estimator which takes sequences of words that represents documents and trains a Word2VecModel. The model is a Map(String, Vector) essentially, which maps each word to an unique fix-sized vector. The Word2VecModel transforms each documents into a vector using the average of all words in the document, which aims to other computations of documents such as similarity calculation consequencely. Please refer to the MLlib user guide on Word2Vec for more details on Word2Vec.
Word2Vec is implemented in Word2Vec. In the following code segment, we start with a set of documents, each of them is represented as a sequence of words. For each document, we transform it into a feature vector. This feature vector could then be passed to a learning algorithm.
// Input data: Each row is a bag of words from a sentence or document. val documentDF = sqlContext.createDataFrame(Seq( "Hi I heard about Spark".split(" "), "I wish Java could use case classes".split(" "), "Logistic regression models are neat".split(" ") ).map(Tuple1.apply)).toDF("text")
// Learn a mapping from words to Vectors. val word2Vec = new Word2Vec() .setInputCol("text") .setOutputCol("result") .setVectorSize(3) .setMinCount(0) val model = word2Vec.fit(documentDF) val result = model.transform(documentDF) result.select("result").take(3).foreach(println) {% endhighlight %}
import org.apache.spark.api.java.JavaRDD; import org.apache.spark.api.java.JavaSparkContext; import org.apache.spark.sql.DataFrame; import org.apache.spark.sql.Row; import org.apache.spark.sql.RowFactory; import org.apache.spark.sql.SQLContext; import org.apache.spark.sql.types.*;
JavaSparkContext jsc = ... SQLContext sqlContext = ...
// Input data: Each row is a bag of words from a sentence or document. JavaRDD jrdd = jsc.parallelize(Lists.newArrayList( RowFactory.create(Lists.newArrayList("Hi I heard about Spark".split(" "))), RowFactory.create(Lists.newArrayList("I wish Java could use case classes".split(" "))), RowFactory.create(Lists.newArrayList("Logistic regression models are neat".split(" "))) )); StructType schema = new StructType(new StructField[]{ new StructField("text", new ArrayType(DataTypes.StringType, true), false, Metadata.empty()) }); DataFrame documentDF = sqlContext.createDataFrame(jrdd, schema);
// Learn a mapping from words to Vectors. Word2Vec word2Vec = new Word2Vec() .setInputCol("text") .setOutputCol("result") .setVectorSize(3) .setMinCount(0); Word2VecModel model = word2Vec.fit(documentDF); DataFrame result = model.transform(documentDF); for (Row r: result.select("result").take(3)) { System.out.println(r); } {% endhighlight %}
documentDF = sqlContext.createDataFrame([ ("Hi I heard about Spark".split(" "), ), ("I wish Java could use case classes".split(" "), ), ("Logistic regression models are neat".split(" "), ) ], ["text"])
word2Vec = Word2Vec(vectorSize=3, minCount=0, inputCol="text", outputCol="result") model = word2Vec.fit(documentDF) result = model.transform(documentDF) for feature in result.select("result").take(3): print(feature) {% endhighlight %}
Tokenization is the process of taking text (such as a sentence) and breaking it into individual terms (usually words). A simple Tokenizer class provides this functionality. The example below shows how to split sentences into sequences of words.
Note: A more advanced tokenizer is provided via RegexTokenizer.
val sentenceDataFrame = sqlContext.createDataFrame(Seq( (0, "Hi I heard about Spark"), (0, "I wish Java could use case classes"), (1, "Logistic regression models are neat") )).toDF("label", "sentence") val tokenizer = new Tokenizer().setInputCol("sentence").setOutputCol("words") val wordsDataFrame = tokenizer.transform(sentenceDataFrame) wordsDataFrame.select("words", "label").take(3).foreach(println) {% endhighlight %}
import org.apache.spark.api.java.JavaRDD; import org.apache.spark.ml.feature.Tokenizer; import org.apache.spark.mllib.linalg.Vector; import org.apache.spark.sql.DataFrame; import org.apache.spark.sql.Row; import org.apache.spark.sql.RowFactory; import org.apache.spark.sql.types.DataTypes; import org.apache.spark.sql.types.Metadata; import org.apache.spark.sql.types.StructField; import org.apache.spark.sql.types.StructType;
JavaRDD jrdd = jsc.parallelize(Lists.newArrayList( RowFactory.create(0, "Hi I heard about Spark"), RowFactory.create(0, "I wish Java could use case classes"), RowFactory.create(1, "Logistic regression models are neat") )); StructType schema = new StructType(new StructField[]{ new StructField("label", DataTypes.DoubleType, false, Metadata.empty()), new StructField("sentence", DataTypes.StringType, false, Metadata.empty()) }); DataFrame sentenceDataFrame = sqlContext.createDataFrame(jrdd, schema); Tokenizer tokenizer = new Tokenizer().setInputCol("sentence").setOutputCol("words"); DataFrame wordsDataFrame = tokenizer.transform(sentenceDataFrame); for (Row r : wordsDataFrame.select("words", "label").take(3)) { java.util.List words = r.getList(0); for (String word : words) System.out.print(word + " "); System.out.println(); } {% endhighlight %}
sentenceDataFrame = sqlContext.createDataFrame([ (0, "Hi I heard about Spark"), (0, "I wish Java could use case classes"), (1, "Logistic regression models are neat") ], ["label", "sentence"]) tokenizer = Tokenizer(inputCol="sentence", outputCol="words") wordsDataFrame = tokenizer.transform(sentenceDataFrame) for words_label in wordsDataFrame.select("words", "label").take(3): print(words_label) {% endhighlight %}
Binarization is the process of thresholding numerical features to binary features. As some probabilistic estimators make assumption that the input data is distributed according to Bernoulli distribution, a binarizer is useful for pre-processing the input data with continuous numerical features.
A simple Binarizer class provides this functionality. Besides the common parameters of inputCol and outputCol, Binarizer has the parameter threshold used for binarizing continuous numerical features. The features greater than the threshold, will be binarized to 1.0. The features equal to or less than the threshold, will be binarized to 0.0. The example below shows how to binarize numerical features.
val data = Array( (0, 0.1), (1, 0.8), (2, 0.2) ) val dataFrame: DataFrame = sqlContext.createDataFrame(data).toDF("label", "feature")
val binarizer: Binarizer = new Binarizer() .setInputCol("feature") .setOutputCol("binarized_feature") .setThreshold(0.5)
val binarizedDataFrame = binarizer.transform(dataFrame) val binarizedFeatures = binarizedDataFrame.select("binarized_feature") binarizedFeatures.collect().foreach(println) {% endhighlight %}
import org.apache.spark.api.java.JavaRDD; import org.apache.spark.ml.feature.Binarizer; import org.apache.spark.sql.DataFrame; import org.apache.spark.sql.Row; import org.apache.spark.sql.RowFactory; import org.apache.spark.sql.types.DataTypes; import org.apache.spark.sql.types.Metadata; import org.apache.spark.sql.types.StructField; import org.apache.spark.sql.types.StructType;
JavaRDD jrdd = jsc.parallelize(Lists.newArrayList( RowFactory.create(0, 0.1), RowFactory.create(1, 0.8), RowFactory.create(2, 0.2) )); StructType schema = new StructType(new StructField[]{ new StructField("label", DataTypes.DoubleType, false, Metadata.empty()), new StructField("feature", DataTypes.DoubleType, false, Metadata.empty()) }); DataFrame continuousDataFrame = jsql.createDataFrame(jrdd, schema); Binarizer binarizer = new Binarizer() .setInputCol("feature") .setOutputCol("binarized_feature") .setThreshold(0.5); DataFrame binarizedDataFrame = binarizer.transform(continuousDataFrame); DataFrame binarizedFeatures = binarizedDataFrame.select("binarized_feature"); for (Row r : binarizedFeatures.collect()) { Double binarized_value = r.getDouble(0); System.out.println(binarized_value); } {% endhighlight %}
continuousDataFrame = sqlContext.createDataFrame([ (0, 0.1), (1, 0.8), (2, 0.2) ], ["label", "feature"]) binarizer = Binarizer(threshold=0.5, inputCol="feature", outputCol="binarized_feature") binarizedDataFrame = binarizer.transform(continuousDataFrame) binarizedFeatures = binarizedDataFrame.select("binarized_feature") for binarized_feature, in binarizedFeatures.collect(): print(binarized_feature) {% endhighlight %}
Polynomial expansion is the process of expanding your features into a polynomial space, which is formulated by an n-degree combination of original dimensions. A PolynomialExpansion class provides this functionality. The example below shows how to expand your features into a 3-degree polynomial space.
val data = Array( Vectors.dense(-2.0, 2.3), Vectors.dense(0.0, 0.0), Vectors.dense(0.6, -1.1) ) val df = sqlContext.createDataFrame(data.map(Tuple1.apply)).toDF("features") val polynomialExpansion = new PolynomialExpansion() .setInputCol("features") .setOutputCol("polyFeatures") .setDegree(3) val polyDF = polynomialExpansion.transform(df) polyDF.select("polyFeatures").take(3).foreach(println) {% endhighlight %}
import org.apache.spark.api.java.JavaRDD; import org.apache.spark.api.java.JavaSparkContext; import org.apache.spark.mllib.linalg.Vector; import org.apache.spark.mllib.linalg.VectorUDT; import org.apache.spark.mllib.linalg.Vectors; import org.apache.spark.sql.DataFrame; import org.apache.spark.sql.Row; import org.apache.spark.sql.RowFactory; import org.apache.spark.sql.SQLContext; import org.apache.spark.sql.types.Metadata; import org.apache.spark.sql.types.StructField; import org.apache.spark.sql.types.StructType;
JavaSparkContext jsc = ... SQLContext jsql = ... PolynomialExpansion polyExpansion = new PolynomialExpansion() .setInputCol("features") .setOutputCol("polyFeatures") .setDegree(3); JavaRDD data = jsc.parallelize(Lists.newArrayList( RowFactory.create(Vectors.dense(-2.0, 2.3)), RowFactory.create(Vectors.dense(0.0, 0.0)), RowFactory.create(Vectors.dense(0.6, -1.1)) )); StructType schema = new StructType(new StructField[] { new StructField("features", new VectorUDT(), false, Metadata.empty()), }); DataFrame df = jsql.createDataFrame(data, schema); DataFrame polyDF = polyExpansion.transform(df); Row[] row = polyDF.select("polyFeatures").take(3); for (Row r : row) { System.out.println(r.get(0)); } {% endhighlight %}
df = sqlContext.createDataFrame( [(Vectors.dense([-2.0, 2.3]), ), (Vectors.dense([0.0, 0.0]), ), (Vectors.dense([0.6, -1.1]), )], ["features"]) px = PolynomialExpansion(degree=2, inputCol="features", outputCol="polyFeatures") polyDF = px.transform(df) for expanded in polyDF.select("polyFeatures").take(3): print(expanded) {% endhighlight %}
StringIndexer encodes a string column of labels to a column of label indices.
The indices are in [0, numLabels), ordered by label frequencies.
So the most frequent label gets index 0.
If the input column is numeric, we cast it to string and index the string values.
Examples
Assume that we have the following DataFrame with columns id and category:
id | category
----|----------
0 | a
1 | b
2 | c
3 | a
4 | a
5 | c
category is a string column with three labels: "a", "b", and "c".
Applying StringIndexer with category as the input column and categoryIndex as the output
column, we should get the following:
id | category | categoryIndex
----|----------|---------------
0 | a | 0.0
1 | b | 2.0
2 | c | 1.0
3 | a | 0.0
4 | a | 0.0
5 | c | 1.0
"a" gets index 0 because it is the most frequent, followed by "c" with index 1 and "b" with
index 2.
StringIndexer takes an input
column name and an output column name.
{% highlight scala %} import org.apache.spark.ml.feature.StringIndexer
val df = sqlContext.createDataFrame( Seq((0, "a"), (1, "b"), (2, "c"), (3, "a"), (4, "a"), (5, "c")) ).toDF("id", "category") val indexer = new StringIndexer() .setInputCol("category") .setOutputCol("categoryIndex") val indexed = indexer.fit(df).transform(df) indexed.show() {% endhighlight %}
{% highlight java %} import java.util.Arrays;
import org.apache.spark.api.java.JavaRDD; import org.apache.spark.ml.feature.StringIndexer; import org.apache.spark.sql.DataFrame; import org.apache.spark.sql.Row; import org.apache.spark.sql.RowFactory; import org.apache.spark.sql.types.StructField; import org.apache.spark.sql.types.StructType; import static org.apache.spark.sql.types.DataTypes.*;
JavaRDD jrdd = jsc.parallelize(Arrays.asList( RowFactory.create(0, "a"), RowFactory.create(1, "b"), RowFactory.create(2, "c"), RowFactory.create(3, "a"), RowFactory.create(4, "a"), RowFactory.create(5, "c") )); StructType schema = new StructType(new StructField[] { createStructField("id", DoubleType, false), createStructField("category", StringType, false) }); DataFrame df = sqlContext.createDataFrame(jrdd, schema); StringIndexer indexer = new StringIndexer() .setInputCol("category") .setOutputCol("categoryIndex"); DataFrame indexed = indexer.fit(df).transform(df); indexed.show(); {% endhighlight %}
StringIndexer takes an input
column name and an output column name.
{% highlight python %} from pyspark.ml.feature import StringIndexer
df = sqlContext.createDataFrame( [(0, "a"), (1, "b"), (2, "c"), (3, "a"), (4, "a"), (5, "c")], ["id", "category"]) indexer = StringIndexer(inputCol="category", outputCol="categoryIndex") indexed = indexer.fit(df).transform(df) indexed.show() {% endhighlight %}
One-hot encoding maps a column of label indices to a column of binary vectors, with at most a single one-value. This encoding allows algorithms which expect continuous features, such as Logistic Regression, to use categorical features
val df = sqlContext.createDataFrame(Seq( (0, "a"), (1, "b"), (2, "c"), (3, "a"), (4, "a"), (5, "c") )).toDF("id", "category")
val indexer = new StringIndexer() .setInputCol("category") .setOutputCol("categoryIndex") .fit(df) val indexed = indexer.transform(df)
val encoder = new OneHotEncoder().setInputCol("categoryIndex"). setOutputCol("categoryVec") val encoded = encoder.transform(indexed) encoded.select("id", "categoryVec").foreach(println) {% endhighlight %}
import org.apache.spark.api.java.JavaRDD; import org.apache.spark.ml.feature.OneHotEncoder; import org.apache.spark.ml.feature.StringIndexer; import org.apache.spark.ml.feature.StringIndexerModel; import org.apache.spark.sql.DataFrame; import org.apache.spark.sql.Row; import org.apache.spark.sql.RowFactory; import org.apache.spark.sql.types.DataTypes; import org.apache.spark.sql.types.Metadata; import org.apache.spark.sql.types.StructField; import org.apache.spark.sql.types.StructType;
JavaRDD jrdd = jsc.parallelize(Lists.newArrayList( RowFactory.create(0, "a"), RowFactory.create(1, "b"), RowFactory.create(2, "c"), RowFactory.create(3, "a"), RowFactory.create(4, "a"), RowFactory.create(5, "c") )); StructType schema = new StructType(new StructField[]{ new StructField("id", DataTypes.DoubleType, false, Metadata.empty()), new StructField("category", DataTypes.StringType, false, Metadata.empty()) }); DataFrame df = sqlContext.createDataFrame(jrdd, schema); StringIndexerModel indexer = new StringIndexer() .setInputCol("category") .setOutputCol("categoryIndex") .fit(df); DataFrame indexed = indexer.transform(df);
OneHotEncoder encoder = new OneHotEncoder() .setInputCol("categoryIndex") .setOutputCol("categoryVec"); DataFrame encoded = encoder.transform(indexed); {% endhighlight %}
df = sqlContext.createDataFrame([ (0, "a"), (1, "b"), (2, "c"), (3, "a"), (4, "a"), (5, "c") ], ["id", "category"])
stringIndexer = StringIndexer(inputCol="category", outputCol="categoryIndex") model = stringIndexer.fit(df) indexed = model.transform(df) encoder = OneHotEncoder(includeFirst=False, inputCol="categoryIndex", outputCol="categoryVec") encoded = encoder.transform(indexed) {% endhighlight %}
VectorIndexer helps index categorical features in datasets of Vectors.
It can both automatically decide which features are categorical and convert original values to category indices. Specifically, it does the following:
- Take an input column of type Vector and a parameter
maxCategories. - Decide which features should be categorical based on the number of distinct values, where features with at most
maxCategoriesare declared categorical. - Compute 0-based category indices for each categorical feature.
- Index categorical features and transform original feature values to indices.
Indexing categorical features allows algorithms such as Decision Trees and Tree Ensembles to treat categorical features appropriately, improving performance.
Please refer to the VectorIndexer API docs for more details.
In the example below, we read in a dataset of labeled points and then use VectorIndexer to decide which features should be treated as categorical. We transform the categorical feature values to their indices. This transformed data could then be passed to algorithms such as DecisionTreeRegressor that handle categorical features.
val data = MLUtils.loadLibSVMFile(sc, "data/mllib/sample_libsvm_data.txt").toDF() val indexer = new VectorIndexer() .setInputCol("features") .setOutputCol("indexed") .setMaxCategories(10) val indexerModel = indexer.fit(data) val categoricalFeatures: Set[Int] = indexerModel.categoryMaps.keys.toSet println(s"Chose ${categoricalFeatures.size} categorical features: " + categoricalFeatures.mkString(", "))
// Create new column "indexed" with categorical values transformed to indices val indexedData = indexerModel.transform(data) {% endhighlight %}
import org.apache.spark.api.java.JavaRDD; import org.apache.spark.ml.feature.VectorIndexer; import org.apache.spark.ml.feature.VectorIndexerModel; import org.apache.spark.mllib.regression.LabeledPoint; import org.apache.spark.mllib.util.MLUtils; import org.apache.spark.sql.DataFrame;
JavaRDD rdd = MLUtils.loadLibSVMFile(sc.sc(), "data/mllib/sample_libsvm_data.txt").toJavaRDD(); DataFrame data = sqlContext.createDataFrame(rdd, LabeledPoint.class); VectorIndexer indexer = new VectorIndexer() .setInputCol("features") .setOutputCol("indexed") .setMaxCategories(10); VectorIndexerModel indexerModel = indexer.fit(data); Map<Integer, Map<Double, Integer>> categoryMaps = indexerModel.javaCategoryMaps(); System.out.print("Chose " + categoryMaps.size() + "categorical features:"); for (Integer feature : categoryMaps.keySet()) { System.out.print(" " + feature); } System.out.println();
// Create new column "indexed" with categorical values transformed to indices DataFrame indexedData = indexerModel.transform(data); {% endhighlight %}
data = MLUtils.loadLibSVMFile(sc, "data/mllib/sample_libsvm_data.txt").toDF() indexer = VectorIndexer(inputCol="features", outputCol="indexed", maxCategories=10) indexerModel = indexer.fit(data)
indexedData = indexerModel.transform(data) {% endhighlight %}
Normalizer is a Transformer which transforms a dataset of Vector rows, normalizing each Vector to have unit norm. It takes parameter p, which specifies the p-norm used for normalization. (
The following example demonstrates how to load a dataset in libsvm format and then normalize each row to have unit
val data = MLUtils.loadLibSVMFile(sc, "data/mllib/sample_libsvm_data.txt") val dataFrame = sqlContext.createDataFrame(data)
// Normalize each Vector using
// Normalize each Vector using
JavaRDD data = MLUtils.loadLibSVMFile(jsc.sc(), "data/mllib/sample_libsvm_data.txt").toJavaRDD(); DataFrame dataFrame = jsql.createDataFrame(data, LabeledPoint.class);
// Normalize each Vector using
// Normalize each Vector using
data = MLUtils.loadLibSVMFile(sc, "data/mllib/sample_libsvm_data.txt") dataFrame = sqlContext.createDataFrame(data)
normalizer = Normalizer(inputCol="features", outputCol="normFeatures", p=1.0) l1NormData = normalizer.transform(dataFrame)
lInfNormData = normalizer.transform(dataFrame, {normalizer.p: float("inf")}) {% endhighlight %}
StandardScaler transforms a dataset of Vector rows, normalizing each feature to have unit standard deviation and/or zero mean. It takes parameters:
withStd: True by default. Scales the data to unit standard deviation.withMean: False by default. Centers the data with mean before scaling. It will build a dense output, so this does not work on sparse input and will raise an exception.
StandardScaler is a Model which can be fit on a dataset to produce a StandardScalerModel; this amounts to computing summary statistics. The model can then transform a Vector column in a dataset to have unit standard deviation and/or zero mean features.
Note that if the standard deviation of a feature is zero, it will return default 0.0 value in the Vector for that feature.
More details can be found in the API docs for StandardScaler and StandardScalerModel.
The following example demonstrates how to load a dataset in libsvm format and then normalize each feature to have unit standard deviation.
val data = MLUtils.loadLibSVMFile(sc, "data/mllib/sample_libsvm_data.txt") val dataFrame = sqlContext.createDataFrame(data) val scaler = new StandardScaler() .setInputCol("features") .setOutputCol("scaledFeatures") .setWithStd(true) .setWithMean(false)
// Compute summary statistics by fitting the StandardScaler val scalerModel = scaler.fit(dataFrame)
// Normalize each feature to have unit standard deviation. val scaledData = scalerModel.transform(dataFrame) {% endhighlight %}
JavaRDD data = MLUtils.loadLibSVMFile(jsc.sc(), "data/mllib/sample_libsvm_data.txt").toJavaRDD(); DataFrame dataFrame = jsql.createDataFrame(data, LabeledPoint.class); StandardScaler scaler = new StandardScaler() .setInputCol("features") .setOutputCol("scaledFeatures") .setWithStd(true) .setWithMean(false);
// Compute summary statistics by fitting the StandardScaler StandardScalerModel scalerModel = scaler.fit(dataFrame);
// Normalize each feature to have unit standard deviation. DataFrame scaledData = scalerModel.transform(dataFrame); {% endhighlight %}
data = MLUtils.loadLibSVMFile(sc, "data/mllib/sample_libsvm_data.txt") dataFrame = sqlContext.createDataFrame(data) scaler = StandardScaler(inputCol="features", outputCol="scaledFeatures", withStd=True, withMean=False)
scalerModel = scaler.fit(dataFrame)
scaledData = scalerModel.transform(dataFrame) {% endhighlight %}
Bucketizer transforms a column of continuous features to a column of feature buckets, where the buckets are specified by users. It takes a parameter:
splits: Parameter for mapping continuous features into buckets. With n+1 splits, there are n buckets. A bucket defined by splits x,y holds values in the range [x,y) except the last bucket, which also includes y. Splits should be strictly increasing. Values at -inf, inf must be explicitly provided to cover all Double values; Otherwise, values outside the splits specified will be treated as errors. Two examples ofsplitsareArray(Double.NegativeInfinity, 0.0, 1.0, Double.PositiveInfinity)andArray(0.0, 1.0, 2.0).
Note that if you have no idea of the upper bound and lower bound of the targeted column, you would better add the Double.NegativeInfinity and Double.PositiveInfinity as the bounds of your splits to prevent a potenial out of Bucketizer bounds exception.
Note also that the splits that you provided have to be in strictly increasing order, i.e. s0 < s1 < s2 < ... < sn.
More details can be found in the API docs for Bucketizer.
The following example demonstrates how to bucketize a column of Doubles into another index-wised column.
val splits = Array(Double.NegativeInfinity, -0.5, 0.0, 0.5, Double.PositiveInfinity)
val data = Array(-0.5, -0.3, 0.0, 0.2) val dataFrame = sqlContext.createDataFrame(data.map(Tuple1.apply)).toDF("features")
val bucketizer = new Bucketizer() .setInputCol("features") .setOutputCol("bucketedFeatures") .setSplits(splits)
// Transform original data into its bucket index. val bucketedData = bucketizer.transform(dataFrame) {% endhighlight %}
import org.apache.spark.sql.DataFrame; import org.apache.spark.sql.Row; import org.apache.spark.sql.RowFactory; import org.apache.spark.sql.types.DataTypes; import org.apache.spark.sql.types.Metadata; import org.apache.spark.sql.types.StructField; import org.apache.spark.sql.types.StructType;
double[] splits = {Double.NEGATIVE_INFINITY, -0.5, 0.0, 0.5, Double.POSITIVE_INFINITY};
JavaRDD data = jsc.parallelize(Lists.newArrayList( RowFactory.create(-0.5), RowFactory.create(-0.3), RowFactory.create(0.0), RowFactory.create(0.2) )); StructType schema = new StructType(new StructField[] { new StructField("features", DataTypes.DoubleType, false, Metadata.empty()) }); DataFrame dataFrame = jsql.createDataFrame(data, schema);
Bucketizer bucketizer = new Bucketizer() .setInputCol("features") .setOutputCol("bucketedFeatures") .setSplits(splits);
// Transform original data into its bucket index. DataFrame bucketedData = bucketizer.transform(dataFrame); {% endhighlight %}
splits = [-float("inf"), -0.5, 0.0, 0.5, float("inf")]
data = [(-0.5,), (-0.3,), (0.0,), (0.2,)] dataFrame = sqlContext.createDataFrame(data, ["features"])
bucketizer = Bucketizer(splits=splits, inputCol="features", outputCol="bucketedFeatures")
bucketedData = bucketizer.transform(dataFrame) {% endhighlight %}
ElementwiseProduct multiplies each input vector by a provided "weight" vector, using element-wise multiplication. In other words, it scales each column of the dataset by a scalar multiplier. This represents the Hadamard product between the input vector, v and transforming vector, w, to yield a result vector.
\[ \begin{pmatrix} v_1 \\ \vdots \\ v_N \end{pmatrix} \circ \begin{pmatrix} w_1 \\ \vdots \\ w_N \end{pmatrix} = \begin{pmatrix} v_1 w_1 \\ \vdots \\ v_N w_N \end{pmatrix} \]
ElementwiseProduct takes the following parameter:
scalingVec: the transforming vector.
This example below demonstrates how to transform vectors using a transforming vector value.
// Create some vector data; also works for sparse vectors val dataFrame = sqlContext.createDataFrame(Seq( ("a", Vectors.dense(1.0, 2.0, 3.0)), ("b", Vectors.dense(4.0, 5.0, 6.0)))).toDF("id", "vector")
val transformingVector = Vectors.dense(0.0, 1.0, 2.0) val transformer = new ElementwiseProduct() .setScalingVec(transformingVector) .setInputCol("vector") .setOutputCol("transformedVector")
// Batch transform the vectors to create new column: val transformedData = transformer.transform(dataFrame)
{% endhighlight %}
import org.apache.spark.api.java.JavaRDD; import org.apache.spark.ml.feature.ElementwiseProduct; import org.apache.spark.mllib.linalg.Vector; import org.apache.spark.mllib.linalg.Vectors; import org.apache.spark.sql.DataFrame; import org.apache.spark.sql.Row; import org.apache.spark.sql.RowFactory; import org.apache.spark.sql.SQLContext; import org.apache.spark.sql.types.DataTypes; import org.apache.spark.sql.types.Metadata; import org.apache.spark.sql.types.StructField; import org.apache.spark.sql.types.StructType;
// Create some vector data; also works for sparse vectors JavaRDD jrdd = jsc.parallelize(Lists.newArrayList( RowFactory.create("a", Vectors.dense(1.0, 2.0, 3.0)), RowFactory.create("b", Vectors.dense(4.0, 5.0, 6.0)) )); List fields = new ArrayList(2); fields.add(DataTypes.createStructField("id", DataTypes.StringType, false)); fields.add(DataTypes.createStructField("vector", DataTypes.StringType, false)); StructType schema = DataTypes.createStructType(fields); DataFrame dataFrame = sqlContext.createDataFrame(jrdd, schema); Vector transformingVector = Vectors.dense(0.0, 1.0, 2.0); ElementwiseProduct transformer = new ElementwiseProduct() .setScalingVec(transformingVector) .setInputCol("vector") .setOutputCol("transformedVector"); // Batch transform the vectors to create new column: DataFrame transformedData = transformer.transform(dataFrame);
{% endhighlight %}
VectorAssembler is a transformer that combines a given list of columns into a single vector
column.
It is useful for combining raw features and features generated by different feature transformers
into a single feature vector, in order to train ML models like logistic regression and decision
trees.
VectorAssembler accepts the following input column types: all numeric types, boolean type,
and vector type.
In each row, the values of the input columns will be concatenated into a vector in the specified
order.
Examples
Assume that we have a DataFrame with the columns id, hour, mobile, userFeatures,
and clicked:
id | hour | mobile | userFeatures | clicked
----|------|--------|------------------|---------
0 | 18 | 1.0 | [0.0, 10.0, 0.5] | 1.0
userFeatures is a vector column that contains three user features.
We want to combine hour, mobile, and userFeatures into a single feature vector
called features and use it to predict clicked or not.
If we set VectorAssembler's input columns to hour, mobile, and userFeatures and
output column to features, after transformation we should get the following DataFrame:
id | hour | mobile | userFeatures | clicked | features
----|------|--------|------------------|---------|-----------------------------
0 | 18 | 1.0 | [0.0, 10.0, 0.5] | 1.0 | [18.0, 1.0, 0.0, 10.0, 0.5]
VectorAssembler takes an array
of input column names and an output column name.
{% highlight scala %} import org.apache.spark.mllib.linalg.Vectors import org.apache.spark.ml.feature.VectorAssembler
val dataset = sqlContext.createDataFrame( Seq((0, 18, 1.0, Vectors.dense(0.0, 10.0, 0.5), 1.0)) ).toDF("id", "hour", "mobile", "userFeatures", "clicked") val assembler = new VectorAssembler() .setInputCols(Array("hour", "mobile", "userFeatures")) .setOutputCol("features") val output = assembler.transform(dataset) println(output.select("features", "clicked").first()) {% endhighlight %}
VectorAssembler takes an array
of input column names and an output column name.
{% highlight java %} import java.util.Arrays;
import org.apache.spark.api.java.JavaRDD; import org.apache.spark.mllib.linalg.VectorUDT; import org.apache.spark.mllib.linalg.Vectors; import org.apache.spark.sql.DataFrame; import org.apache.spark.sql.Row; import org.apache.spark.sql.RowFactory; import org.apache.spark.sql.types.; import static org.apache.spark.sql.types.DataTypes.;
StructType schema = createStructType(new StructField[] { createStructField("id", IntegerType, false), createStructField("hour", IntegerType, false), createStructField("mobile", DoubleType, false), createStructField("userFeatures", new VectorUDT(), false), createStructField("clicked", DoubleType, false) }); Row row = RowFactory.create(0, 18, 1.0, Vectors.dense(0.0, 10.0, 0.5), 1.0); JavaRDD rdd = jsc.parallelize(Arrays.asList(row)); DataFrame dataset = sqlContext.createDataFrame(rdd, schema);
VectorAssembler assembler = new VectorAssembler() .setInputCols(new String[] {"hour", "mobile", "userFeatures"}) .setOutputCol("features");
DataFrame output = assembler.transform(dataset); System.out.println(output.select("features", "clicked").first()); {% endhighlight %}
VectorAssembler takes a list
of input column names and an output column name.
{% highlight python %} from pyspark.mllib.linalg import Vectors from pyspark.ml.feature import VectorAssembler
dataset = sqlContext.createDataFrame( [(0, 18, 1.0, Vectors.dense([0.0, 10.0, 0.5]), 1.0)], ["id", "hour", "mobile", "userFeatures", "clicked"]) assembler = VectorAssembler( inputCols=["hour", "mobile", "userFeatures"], outputCol="features") output = assembler.transform(dataset) print(output.select("features", "clicked").first()) {% endhighlight %}