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From "Christian Semrau (JIRA)" <j...@apache.org>
Subject [jira] Commented: (MATH-248) Multiplying sparse matrices is slow
Date Sun, 22 Feb 2009 19:55:05 GMT

    [ https://issues.apache.org/jira/browse/MATH-248?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12675698#action_12675698
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Christian Semrau commented on MATH-248:
---------------------------------------

According to my little tests, walking in optimized order and adding the pairwise products
to the result matrix is much faster for very sparse matrices, but for a full matrix (every
entry set to 1), it is about 50% slower than the current implementation. So the implementation
might switch between the two algorithms. Also one might wish to walk the other matrix if it
is more sparse than this.


> Multiplying sparse matrices is slow
> -----------------------------------
>
>                 Key: MATH-248
>                 URL: https://issues.apache.org/jira/browse/MATH-248
>             Project: Commons Math
>          Issue Type: Improvement
>            Reporter: Christian Semrau
>            Priority: Minor
>
> The multiplication of sparse real matrices is very slow compared to real matrices: Ten
times as slow for size 200, four times as slow for size 400. The time is independent of the
number of nonzero entries, because the general algorithm inherited from AbstractRealMatrix
is used. I suggest using a specialized multiplication algorithm for matrices that are "sparse
enough", walking only over the nonzero entries in one of the matrices.

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