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diff --git a/docs/ml-features.md b/docs/ml-features.md index f88c0248c1..54068debe2 100644 --- a/docs/ml-features.md +++ b/docs/ml-features.md @@ -288,6 +288,94 @@ 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](ml-features.html#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. + +<div class="codetabs"> +<div data-lang="scala" markdown="1"> +<div class="codetabs"> + +<div data-lang="scala" markdown="1"> + +[`NGram`](api/scala/index.html#org.apache.spark.ml.feature.NGram) takes an input column name, an output column name, and an optional length parameter n (n=2 by default). + +{% highlight scala %} +import org.apache.spark.ml.feature.NGram + +val wordDataFrame = sqlContext.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("label", "words") + +val ngram = new NGram().setInputCol("words").setOutputCol("ngrams") +val ngramDataFrame = ngram.transform(wordDataFrame) +ngramDataFrame.take(3).map(_.getAs[Stream[String]]("ngrams").toList).foreach(println) +{% endhighlight %} +</div> + +<div data-lang="java" markdown="1"> + +[`NGram`](api/java/org/apache/spark/ml/feature/NGram.html) takes an input column name, an output column name, and an optional length parameter n (n=2 by default). + +{% highlight java %} +import com.google.common.collect.Lists; + +import org.apache.spark.api.java.JavaRDD; +import org.apache.spark.ml.feature.NGram; +import org.apache.spark.mllib.linalg.Vector; +import org.apache.spark.sql.DataFrame; +import org.apache.spark.sql.Row; +import org.apache.spark.sql.RowFactory; +import org.apache.spark.sql.types.DataTypes; +import org.apache.spark.sql.types.Metadata; +import org.apache.spark.sql.types.StructField; +import org.apache.spark.sql.types.StructType; + +JavaRDD<Row> jrdd = jsc.parallelize(Lists.newArrayList( + RowFactory.create(0D, Lists.newArrayList("Hi", "I", "heard", "about", "Spark")), + RowFactory.create(1D, Lists.newArrayList("I", "wish", "Java", "could", "use", "case", "classes")), + RowFactory.create(2D, Lists.newArrayList("Logistic", "regression", "models", "are", "neat")) +)); +StructType schema = new StructType(new StructField[]{ + new StructField("label", DataTypes.DoubleType, false, Metadata.empty()), + new StructField("words", DataTypes.createArrayType(DataTypes.StringType), false, Metadata.empty()) +}); +DataFrame wordDataFrame = sqlContext.createDataFrame(jrdd, schema); +NGram ngramTransformer = new NGram().setInputCol("words").setOutputCol("ngrams"); +DataFrame ngramDataFrame = ngramTransformer.transform(wordDataFrame); +for (Row r : ngramDataFrame.select("ngrams", "label").take(3)) { + java.util.List<String> ngrams = r.getList(0); + for (String ngram : ngrams) System.out.print(ngram + " --- "); + System.out.println(); +} +{% endhighlight %} +</div> + +<div data-lang="python" markdown="1"> + +[`NGram`](api/python/pyspark.ml.html#pyspark.ml.feature.NGram) takes an input column name, an output column name, and an optional length parameter n (n=2 by default). + +{% highlight python %} +from pyspark.ml.feature import NGram + +wordDataFrame = sqlContext.createDataFrame([ + (0, ["Hi", "I", "heard", "about", "Spark"]), + (1, ["I", "wish", "Java", "could", "use", "case", "classes"]), + (2, ["Logistic", "regression", "models", "are", "neat"]) +], ["label", "words"]) +ngram = NGram(inputCol="words", outputCol="ngrams") +ngramDataFrame = ngram.transform(wordDataFrame) +for ngrams_label in ngramDataFrame.select("ngrams", "label").take(3): + print(ngrams_label) +{% endhighlight %} +</div> +</div> + + ## Binarizer Binarization is the process of thresholding numerical features to binary features. As some probabilistic estimators make assumption that the input data is distributed according to [Bernoulli distribution](http://en.wikipedia.org/wiki/Bernoulli_distribution), a binarizer is useful for pre-processing the input data with continuous numerical features. |