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https://issues.apache.org/jira/browse/SPARK-14886?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15262306#comment-15262306
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Apache Spark commented on SPARK-14886:
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User 'srowen' has created a pull request for this issue:
https://github.com/apache/spark/pull/12756
> RankingMetrics.ndcgAt throw java.lang.ArrayIndexOutOfBoundsException
> ----------------------------------------------------------------------
>
> Key: SPARK-14886
> URL: https://issues.apache.org/jira/browse/SPARK-14886
> Project: Spark
> Issue Type: Bug
> Components: MLlib
> Affects Versions: 1.6.1
> Reporter: lichenglin
> Priority: Minor
>
> {code}
> @Since("1.2.0")
> def ndcgAt(k: Int): Double = {
> require(k > 0, "ranking position k should be positive")
> predictionAndLabels.map { case (pred, lab) =>
> val labSet = lab.toSet
> if (labSet.nonEmpty) {
> val labSetSize = labSet.size
> val n = math.min(math.max(pred.length, labSetSize), k)
> var maxDcg = 0.0
> var dcg = 0.0
> var i = 0
> while (i < n) {
> val gain = 1.0 / math.log(i + 2)
> if (labSet.contains(pred(i))) {
> dcg += gain
> }
> if (i < labSetSize) {
> maxDcg += gain
> }
> i += 1
> }
> dcg / maxDcg
> } else {
> logWarning("Empty ground truth set, check input data")
> 0.0
> }
> }.mean()
> }
> {code}
> "if (labSet.contains(pred(i)))" will throw ArrayIndexOutOfBoundsException
> when pred's size less then k.
> That meas the true relevant documents has less size then the param k.
> just try this with sample_movielens_data.txt
> for example set pred.size to 5,labSetSize to 10,k to 20,then the n is 10.
> pred[10] not exists;
> precisionAt is ok just because it has
> val n = math.min(pred.length, k)
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