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https://issues.apache.org/jira/browse/MAHOUT-182?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12778772#action_12778772
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Jake Mannix commented on MAHOUT-182:
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Ah, fair enough, might as well save a millisecond or so here and there, that's
removable, sure.
> 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: MAHOUT-182.patch, 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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