[jira] Assigned: (MAHOUT-182) New helper methods for Matrix: times(Vector), timesSquared(Vector), numRows() and numCols()

["Grant Ingersoll (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/MAHOUT-182?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Grant Ingersoll reassigned MAHOUT-182:
--------------------------------------

    Assignee: Grant Ingersoll

> New helper methods for Matrix: times(Vector), timesSquared(Vector), numRows() and numCols()
> -------------------------------------------------------------------------------------------
>
>                 Key: MAHOUT-182
>                 URL: https://issues.apache.org/jira/browse/MAHOUT-182
>             Project: Mahout
>          Issue Type: Improvement
>          Components: Matrix
>    Affects Versions: 0.2
>            Reporter: Jake Mannix
>            Assignee: Grant Ingersoll
>            Priority: Minor
>         Attachments: matrixTimes.patch
>
>
> numRows() { return size()[ROW]; } and numCols() { return size()[COL]; } are pretty much no-brainer methods, right?  Who wants to deal with a length-two array of ints all the time when getting the number of rows and columns of a matrix?
> Those are pretty trivial, but the key feature of a Matrix is to map Vector instances to Vector instances, and while you can do that currently by making a a row Matrix and doing Matrix.times(Matrix), it's silly to have to always do that.  Matrix.times(Vector) is pretty needed.
> Even less trivial, for really big sparse Matrices, if you need to get (M'M)v for some matrix M, then this can be computed in one pass through M without ever computing the transpose of M by a simple reordering of the limits of summation.
> Attaching a patch with these implementations, including unit tests (as well as an improvement in the Matrix.times(Matrix) unit test to actually check the math).

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