TF-IDF

TF-IDF

Term frequency-inverse document frequency (TF-IDF) is a feature vectorization method widely used in text mining to reflect the importance of a term to a document in the corpus.

TF: Both HashingTF and CountVectorizer can be used to generate the term frequency vectors.

HashingTF:

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. HashingTF utilizes the hashing trick.

   // $example on$
    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(false)
    // $example off$

/*
+-----+----------------------------------------------------------------------------------------------------------------------+
|label|features                                                                                                              |
+-----+----------------------------------------------------------------------------------------------------------------------+
|0.0  |(20,[0,5,9,17],[0.6931471805599453,0.6931471805599453,0.28768207245178085,1.3862943611198906])                        |
|0.0  |(20,[2,7,9,13,15],[0.6931471805599453,0.6931471805599453,0.8630462173553426,0.28768207245178085,0.28768207245178085]) |
|1.0  |(20,[4,6,13,15,18],[0.6931471805599453,0.6931471805599453,0.28768207245178085,0.28768207245178085,0.6931471805599453])|
+-----+----------------------------------------------------------------------------------------------------------------------+
*/