[jira] Commented: (MAHOUT-268) Vector.getDistanceSquared() is incorrect for both SparseVector varieties

["Jake Mannix (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/MAHOUT-268?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12805562#action_12805562 ] 

Jake Mannix commented on MAHOUT-268:
------------------------------------

Oy, this is wrong in all three places it is implemented (in different ways, :\ ) - even in the "non-optimized" impl in AbstractVector:
{code}
  @Override
  public double getDistanceSquared(Vector v) {
    double d = 0;
    Iterator<Element> it = iterateNonZero();
    Element e;
    while(it.hasNext() && (e = it.next()) != null) {
      double diff = e.get() - v.getQuick(e.index());
      d += (diff * diff);
    }
    return d;
  }
{code}

Iterating over the nonzero entries of this vector doesn't make sure to iterate over the nonzero entries of the other one as well!

> Vector.getDistanceSquared() is incorrect for both SparseVector varieties
> ------------------------------------------------------------------------
>
>                 Key: MAHOUT-268
>                 URL: https://issues.apache.org/jira/browse/MAHOUT-268
>             Project: Mahout
>          Issue Type: Bug
>    Affects Versions: 0.2
>         Environment: all
>            Reporter: NOT_A_USER
>            Assignee: Jake Mannix
>             Fix For: 0.3
>
>
> I'm pretty sure that getDistanceSquared() should just return as if an optimized implementation of:
> {code}
>   public double getDistanceSquared(Vector v) { return this.minus(v).getLengthSquared(); }
> {code}
> In which case if some vector elements are negative, both SequentialAccessSparseVector (my fault!) and RandomAccessSparseVector return the wrong thing.  Very easy to write a failing unit test for this one.

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