Github user mengxr commented on a diff in the pull request:

    https://github.com/apache/spark/pull/1155#discussion_r14779320
  
    --- Diff: 
mllib/src/main/scala/org/apache/spark/mllib/evaluation/MulticlassMetrics.scala 
---
    @@ -0,0 +1,181 @@
    +/*
    + * Licensed to the Apache Software Foundation (ASF) under one or more
    + * contributor license agreements.  See the NOTICE file distributed with
    + * this work for additional information regarding copyright ownership.
    + * The ASF licenses this file to You under the Apache License, Version 2.0
    + * (the "License"); you may not use this file except in compliance with
    + * the License.  You may obtain a copy of the License at
    + *
    + *    http://www.apache.org/licenses/LICENSE-2.0
    + *
    + * Unless required by applicable law or agreed to in writing, software
    + * distributed under the License is distributed on an "AS IS" BASIS,
    + * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    + * See the License for the specific language governing permissions and
    + * limitations under the License.
    + */
    +
    +package org.apache.spark.mllib.evaluation
    +
    +import org.apache.spark.Logging
    +import org.apache.spark.SparkContext._
    +import org.apache.spark.annotation.Experimental
    +import org.apache.spark.rdd.RDD
    +
    +import scala.collection.Map
    +
    +/**
    + * ::Experimental::
    + * Evaluator for multiclass classification.
    + *
    + * @param predictionAndLabels an RDD of (prediction, label) pairs.
    + */
    +@Experimental
    +class MulticlassMetrics(predictionAndLabels: RDD[(Double, Double)]) 
extends Logging {
    +
    +  private lazy val labelCountByClass: Map[Double, Long] = 
predictionAndLabels.values.countByValue()
    +  private lazy val labelCount: Long = labelCountByClass.values.sum
    +  private lazy val tpByClass: Map[Double, Int] = predictionAndLabels
    +    .map { case (prediction, label) =>
    +    (label, if (label == prediction) 1 else 0)
    +  }.reduceByKey(_ + _)
    +    .collectAsMap()
    +  private lazy val fpByClass: Map[Double, Int] = predictionAndLabels
    +    .map { case (prediction, label) =>
    +    (prediction, if (prediction != label) 1 else 0)
    +  }.reduceByKey(_ + _)
    +    .collectAsMap()
    +  private lazy val confusions = predictionAndLabels.map {
    +    case (prediction, label) => ((prediction, label), 1)
    +  }.reduceByKey(_ + _).collectAsMap()
    +
    +  /**
    +   * Returns confusion matrix:
    +   * predicted classes are in columns,
    +   * they are ordered by class label ascending,
    +   * as in "labels"
    +   */
    +  lazy val confusionMatrix: Array[Array[Int]] = {
    --- End diff --
    
    Should return `mllib.linalg.Matrix`


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