spark-机器学习库-特征的提取，转换和选择

提取：从“原始”数据中提取特征
转换：缩放，转换或修改特征
选择：从较大的一组特征中选择一个子集
局部敏感哈希（LSH）：这类算法将特征变换的方面与其他算法相结合。

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特征提取

• 词频-逆向文档频率（TF-IDF）

import org.apache.spark.ml.feature.{HashingTF, IDF, Tokenizer}

val sentenceData = spark.createDataFrame(Seq(
  (0.0, "Hi I heard about Spark"),
  (0.0, "I wish Java could use case classes"),
  (1.0, "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)
// alternatively, CountVectorizer can also be used to get term frequency vectors

val idf = new IDF().setInputCol("rawFeatures").setOutputCol("features")
val idfModel = idf.fit(featurizedData)

val rescaledData = idfModel.transform(featurizedData)
rescaledData.select("label", "features").show()

• Word2Vec

import org.apache.spark.ml.feature.Word2Vec
import org.apache.spark.ml.linalg.Vector
import org.apache.spark.sql.Row

// Input data: Each row is a bag of words from a sentence or document.
val documentDF = spark.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.collect().foreach { case Row(text: Seq[_], features: Vector) =>
  println(s"Text: [${text.mkString(", ")}] => \nVector: $features\n") }

特征变换

• PCA（主元分析）

import org.apache.spark.ml.feature.PCA
import org.apache.spark.ml.linalg.Vectors

val data = Array(
  Vectors.sparse(5, Seq((1, 1.0), (3, 7.0))),
  Vectors.dense(2.0, 0.0, 3.0, 4.0, 5.0),
  Vectors.dense(4.0, 0.0, 0.0, 6.0, 7.0)
)
val df = spark.createDataFrame(data.map(Tuple1.apply)).toDF("features")

val pca = new PCA()
  .setInputCol("features")
  .setOutputCol("pcaFeatures")
  .setK(3)
  .fit(df)

val result = pca.transform(df).select("pcaFeatures")
result.show(false)

• MinMaxScaler（最大-最小规范化）

import org.apache.spark.ml.feature.MinMaxScaler
import org.apache.spark.ml.linalg.Vectors

val dataFrame = spark.createDataFrame(Seq(
  (0, Vectors.dense(1.0, 0.1, -1.0)),
  (1, Vectors.dense(2.0, 1.1, 1.0)),
  (2, Vectors.dense(3.0, 10.1, 3.0))
)).toDF("id", "features")

val scaler = new MinMaxScaler()
  .setInputCol("features")
  .setOutputCol("scaledFeatures")

// Compute summary statistics and generate MinMaxScalerModel
val scalerModel = scaler.fit(dataFrame)

// rescale each feature to range [min, max].
val scaledData = scalerModel.transform(dataFrame)
println(s"Features scaled to range: [${scaler.getMin}, ${scaler.getMax}]")
scaledData.select("features", "scaledFeatures").show()

• Bucketizer（分箱器）

import org.apache.spark.ml.feature.Bucketizer

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 = spark.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)
bucketedData.show()

val splitV = Array(60.0, 65,70, 75, 80, 85, 90, 95, 100,105)
val bucketizer = new Bucketizer().setInputCol("total_score").setOutputCol("period").setSplits(splitV).transform(data)