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https://issues.apache.org/jira/browse/MAHOUT-406?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12873659#action_12873659
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Sean Owen commented on MAHOUT-406:
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Yeah I looked into this and I think all other implementations ultimately manage
to throw an exception. This one is a special case since it actually has no
knowledge of the underlying data.
I update the docs to be more correct: an exception is thrown if the user or
item is known to not exist, and returns NaN if the similarity is otherwise
unknown. These are both valid outcomes and yes you should be prepared to deal
with either.
The only twist is that just because the method doesn't throw an exception
doesn't mean the item/user definitely exists.
> 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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