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https://issues.apache.org/jira/browse/MAHOUT-406?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12873677#action_12873677
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Sean Owen commented on MAHOUT-406:
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You are right. Really for consistency the JDBC-backed one needs to return NaN.
How about that? because the same logic applies -- just because it's not in the
similarity table doesn't mean it doesn't exist in the model.
> Inconsistent return values of different ItemSimilarity implementations
> ----------------------------------------------------------------------
>
> Key: MAHOUT-406
> URL: https://issues.apache.org/jira/browse/MAHOUT-406
> Project: Mahout
> Issue Type: Bug
> Components: Collaborative Filtering
> Reporter: Sebastian Schelter
> Assignee: Sean Owen
> Priority: Minor
> Fix For: 0.4
>
>
> org.apache.mahout.cf.taste.impl.similarity.GenericItemSimilarity returns
> Double.NaN if you request the similarity for two itemIDs it doesn't know,
> while
> org.apache.mahout.cf.taste.impl.similarity.jdbc.AbstractJDBCItemSimilarity
> throws a NoSuchItemException.
> They should act consistently (and both return Double.NaN, I guess).
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