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])|
+-----+----------------------------------------------------------------------------------------------------------------------+
*/
​