Github user feynmanliang commented on a diff in the pull request:
https://github.com/apache/spark/pull/7244#discussion_r34188842
--- Diff: docs/ml-features.md ---
@@ -288,6 +288,88 @@ for words_label in wordsDataFrame.select("words",
"label").take(3):
</div>
+## $n$-gram
+
+An [n-gram](https://en.wikipedia.org/wiki/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](api/scala/index.html#org.apache.spark.ml.feature.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.
--- End diff --
OK
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