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https://issues.apache.org/jira/browse/MAHOUT-406?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12873668#action_12873668
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Sebastian Schelter commented on MAHOUT-406:
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I'm confused now, sorry. GenericItemSimilarity and AbstractJDBCItemSimilarity
both manage a list of precomputed similarities and both don't have access to a
DataModel, so they cannot know which items actually exist. So why should one
throw a NoSuchItemException and the other return NaN? Other similarity
implementations that compute the similarities from a DataModel can throw
NoSuchItemExceptions as they know all the preference data, but not these. Did I
get something wrong here?
> 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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