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From "Robin Anil (JIRA)" <j...@apache.org>
Subject [jira] [Resolved] (MAHOUT-1117) Vectors are not hashable
Date Sat, 01 Jun 2013 17:42:25 GMT

     [ https://issues.apache.org/jira/browse/MAHOUT-1117?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Robin Anil resolved MAHOUT-1117.
--------------------------------

    Resolution: Won't Fix
    
> Vectors are not hashable
> ------------------------
>
>                 Key: MAHOUT-1117
>                 URL: https://issues.apache.org/jira/browse/MAHOUT-1117
>             Project: Mahout
>          Issue Type: Improvement
>    Affects Versions: 1.0
>            Reporter: Dan Filimon
>            Priority: Minor
>
> No *Vector classes (DenseVector, WeightedVector, etc.) implement hashCode().
> In working on improving clustering in Mahout, Ted Dunning wrote prototype code for Streaming
KMeans and Ball KMeans, that I'm working with him on. These need to be used together in the
MapReduce version.
> However, in Ball KMeans, we initialize the clusters using a probabilistic approach similar
to k-means++. This however requires a Multinomial<WeightedVector> distribution of the
points we want to cluster to pick the centroids.
> Internally, the Multinomial<T> uses a HashMap to keep track of the values it can
sample from.
> Since Vectors don't override Object's hashCode(), it is possible to get the same value
multiple times in the map (as long as the references differ).
> This is less of an issue because of how we're adding the vectors to the multinomial (we
can guarantee that the references will be unique) and once MAHOUT-1116 is resolved the hashing
will work okay for our needs.
> It still seems that it would be useful to have hashable vectors.
> What do you think? And what would a hash function look like?

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