# n-gram

An n-gram is a sequence of n tokens (typically words) for some integer n. The NGram class can be used to transform input features into n-grams.

NGram takes as input a sequence of strings (e.g. the output of a Tokenizer). The parameter n is used to determine the number of terms in each n-gram. The output will consist of a sequence of n-grams where each n-gram is represented by a space-delimited string of n consecutive words. If the input sequence contains fewer than n strings, no output is produced.

Concept: combining neighboring words can be more meaningful than individual words

```
import org.apache.spark.ml.feature.NGram

val wordDataFrame = spark.createDataFrame(Seq(
  (0, Array("Hi", "I", "heard", "about", "Spark")),
  (1, Array("I", "wish", "Java", "could", "use", "case", "classes")),
  (2, Array("Logistic", "regression", "models", "are", "neat"))
)).toDF("id", "words")

val ngram = new NGram().setN(2).setInputCol("words").setOutputCol("ngrams")

val ngramDataFrame = ngram.transform(wordDataFrame)
ngramDataFrame.select("ngrams").show(false)

/*

setN(2) means n=2, 2 neighboring words in each n-gram
+------------------------------------------------------------------+
|ngrams                                                            |
+------------------------------------------------------------------+
|[Hi I, I heard, heard about, about Spark]                         |
|[I wish, wish Java, Java could, could use, use case, case classes]|
|[Logistic regression, regression models, models are, are neat]    |
+------------------------------------------------------------------+

*/

```


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