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Martin Illecker commented on MAHOUT1266:

Thanks Sebastian, I got it.
You are calculating A' * B not A * B.
But then it's not a matrix multiplication defined as A * B!
Normally when executing a matrix multiplication of matrix A and matrix B, you would believe
to get A * B not A' * B?
So, how can I do a matrix multiplication (A * B) in Mahout, because I think there is no way
to get A * B from A' * B?
> Two minor problems in DistributedRowMatrix using MatrixMultiplication
> 
>
> Key: MAHOUT1266
> URL: https://issues.apache.org/jira/browse/MAHOUT1266
> Project: Mahout
> Issue Type: Bug
> Components: Math
> Affects Versions: 0.7
> Reporter: Martin Illecker
> Priority: Trivial
> Labels: newbie
> Original Estimate: 10m
> Remaining Estimate: 10m
>
> Hello,
> I think I have found two minor problems in *DistributedRowMatrix*.
> In [1] the condition is wrong, because (l x m) * (m x n) = (l x n).
> The condition should be like in [2].
> And in *times*[3] the {{this.transpose()}} seems to be missing? (See [4])
> Do you have any benchmark results for Mahout MatrixMultiplication?
> Thanks!
> Martin
> [1] [https://github.com/apache/mahout/blob/trunk/core/src/main/java/org/apache/mahout/math/hadoop/DistributedRowMatrix.java#L191193]
> [2] [https://github.com/millecker/applications/blob/master/hadoop/rootbeer/matrixmultiplication/src/at/illecker/hadoop/rootbeer/examples/matrixmultiplication/DistributedRowMatrix.java#L221225]
> [3] [https://github.com/apache/mahout/blob/trunk/core/src/main/java/org/apache/mahout/math/hadoop/DistributedRowMatrix.java#L190206]
> [4] [https://github.com/millecker/applications/blob/master/hadoop/rootbeer/matrixmultiplication/src/at/illecker/hadoop/rootbeer/examples/matrixmultiplication/DistributedRowMatrix.java#L230231]

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