很遗憾,在旧版的Spark MLlib 1.5.1中不提供该功能。
但是,在Spark MLlib 2.x的最近的Pipeline API中,您可以找到它,其名称为RandomForestClassifier
:
import org.apache.spark.ml.Pipeline
import org.apache.spark.ml.classification.RandomForestClassifier
import org.apache.spark.ml.feature.{IndexToString, StringIndexer, VectorIndexer}
import org.apache.spark.mllib.util.MLUtils
// Load and parse the data file, converting it to a DataFrame.
val data = MLUtils.loadLibSVMFile(sc, "data/mllib/sample_libsvm_data.txt").toDF
// Index labels, adding metadata to the label column.
// Fit on whole dataset to include all labels in index.
val labelIndexer = new StringIndexer()
.setInputCol("label")
.setOutputCol("indexedLabel").fit(data)
// Automatically identify categorical features, and index them.
// Set maxCategories so features with > 4 distinct values are treated as continuous.
val featureIndexer = new VectorIndexer()
.setInputCol("features")
.setOutputCol("indexedFeatures")
.setMaxCategories(4).fit(data)
// Split the data into training and test sets (30% held out for testing)
val Array(trainingData, testData) = data.randomSplit(Array(0.7, 0.3))
// Train a RandomForest model.
val rf = new RandomForestClassifier()
.setLabelCol(labelIndexer.getOutputCol)
.setFeaturesCol(featureIndexer.getOutputCol)
.setNumTrees(10)
// Convert indexed labels back to original labels.
val labelConverter = new IndexToString()
.setInputCol("prediction")
.setOutputCol("predictedLabel")
.setLabels(labelIndexer.labels)
// Chain indexers and forest in a Pipeline
val pipeline = new Pipeline()
.setStages(Array(labelIndexer, featureIndexer, rf, labelConverter))
// Fit model. This also runs the indexers.
val model = pipeline.fit(trainingData)
// Make predictions.
val predictions = model.transform(testData)
// predictions: org.apache.spark.sql.DataFrame = [label: double, features: vector, indexedLabel: double, indexedFeatures: vector, rawPrediction: vector, probability: vector, prediction: double, predictedLabel: string]
predictions.show(10)
// +-----+--------------------+------------+--------------------+-------------+-----------+----------+--------------+
// |label| features|indexedLabel| indexedFeatures|rawPrediction|probability|prediction|predictedLabel|
// +-----+--------------------+------------+--------------------+-------------+-----------+----------+--------------+
// | 0.0|(692,[124,125,126...| 1.0|(692,[124,125,126...| [0.0,10.0]| [0.0,1.0]| 1.0| 0.0|
// | 0.0|(692,[124,125,126...| 1.0|(692,[124,125,126...| [1.0,9.0]| [0.1,0.9]| 1.0| 0.0|
// | 0.0|(692,[129,130,131...| 1.0|(692,[129,130,131...| [1.0,9.0]| [0.1,0.9]| 1.0| 0.0|
// | 0.0|(692,[154,155,156...| 1.0|(692,[154,155,156...| [1.0,9.0]| [0.1,0.9]| 1.0| 0.0|
// | 0.0|(692,[154,155,156...| 1.0|(692,[154,155,156...| [1.0,9.0]| [0.1,0.9]| 1.0| 0.0|
// | 0.0|(692,[181,182,183...| 1.0|(692,[181,182,183...| [1.0,9.0]| [0.1,0.9]| 1.0| 0.0|
// | 1.0|(692,[99,100,101,...| 0.0|(692,[99,100,101,...| [4.0,6.0]| [0.4,0.6]| 1.0| 0.0|
// | 1.0|(692,[123,124,125...| 0.0|(692,[123,124,125...| [10.0,0.0]| [1.0,0.0]| 0.0| 1.0|
// | 1.0|(692,[124,125,126...| 0.0|(692,[124,125,126...| [10.0,0.0]| [1.0,0.0]| 0.0| 1.0|
// | 1.0|(692,[125,126,127...| 0.0|(692,[125,126,127...| [10.0,0.0]| [1.0,0.0]| 0.0| 1.0|
// +-----+--------------------+------------+--------------------+-------------+-----------+----------+--------------+
// only showing top 10 rows
注意: 这个例子来自于Spark MLlib的ML - 随机森林分类器官方文档。
以下是一些输出列的解释:
predictionCol
表示预测的标签。rawPredictionCol
是一个长度为 #classes 的向量,其中包含使预测的树节点上的训练实例标签计数(仅适用于分类)。probabilityCol
是长度为 # classes 的概率向量,等于将 rawPrediction
标准化到多项分布的向量(仅适用于分类)。您无法直接获取分类概率,但自己计算相对容易。RandomForest是树的集合体,其输出概率是这些树的大部分投票除以总数。
由于MLib中的RandomForestModel提供了训练后的树,因此自己计算很容易。以下代码给出了二元分类问题的概率。对于多类分类问题,其推广也很简单。
def predict(points: RDD[LabeledPoint], model: RandomForestModel) = {
val numTrees = model.trees.length
val trees = points.sparkContext.broadcast(model.trees)
points.map { point =>
trees.value
.map(_.predict(point.features))
.sum / numTrees
}
对于多分类情况,您只需将map替换为.map(_.predict(point.features)- > 1.0),以键分组而不是求和,最后取值的最大值即可。