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https://issues.apache.org/jira/browse/MAHOUT-1078?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13463552#comment-13463552
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Smita Wadhwa commented on MAHOUT-1078:
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Ya, the present matrix multiplication works like this a.transpose().times(b)
where a,b are input matrix.
If you give the transpose of a then u get a.times(b). Hope this helps.
> matrixmult gives wrong answer
> -----------------------------
>
> Key: MAHOUT-1078
> URL: https://issues.apache.org/jira/browse/MAHOUT-1078
> Project: Mahout
> Issue Type: Bug
> Components: Math
> Affects Versions: 0.7
> Reporter: Paul Hubenig
>
> Create a simple matrix:
> 1 2
> 3 4
> package exp
> import org.apache.hadoop.conf.Configuration
> import org.apache.hadoop.fs.Path
> import org.apache.hadoop.io.{IOUtils, IntWritable, SequenceFile}
> import org.apache.mahout.math.{SequentialAccessSparseVector, VectorWritable,
> RandomAccessSparseVector}
> object MakeMatrix extends App with CommonMethods {
> val matrixFileOutput = "/Users/phubenig/matrixFile.sf"
> val conf = new Configuration()
> val fs = getFileSystem(true, conf)
> val path = new Path(matrixFileOutput)
> val arr = Array(Array(1, 2), Array(3, 4))
> val rank = 2
> val key = new org.apache.hadoop.io.IntWritable()
> val writer = SequenceFile.createWriter(fs, conf, path,
> classOf[IntWritable], classOf[VectorWritable])
> for (i <- 0 until rank) {
> key.set(i)
> val v = new SequentialAccessSparseVector(rank)
> for (j <- 0 until rank) {
> v.setQuick(j, arr(i)(j))
> }
> writer.append(key, new VectorWritable(v))
> }
> IOUtils.closeStream(writer)
> }
> Engage mahout to square this matrix.
> $ mahout matrixmult --numRowsA 2 --numColsA 2 --numRowsB 2 --numColsB 2
> --inputPathA matrixFile.sf --inputPathB matrixFile.sf
> Read back the result:
> I get the answer:??
> (0, 0): 10.0
> (0, 1): 14.0
> (1, 0): 14.0
> (1, 1): 20.0
> i.e.,
> 10 14
> 14 20
> Correct answer??
> 1 2 1 2 7 10
> x =
> 3 4 3 4 15 22
> I tried mahout transpose and that seems to work.
> Am I specifying the matrix wrong somehow?
> 10 =|(1, 3)| * |(1,3)|
> 20 =|(2,4)| * |(2,4)|
> 14 = (1,3) , (2,4)
> but why is it doing this?
> Instead of two vectors in the matrix creation program, I also tried one
> vector consisting of: (1, 2, 3, 4) - matrixmult also give the wrong answer.
> Regardless of convention used (???) that I don't understand, here is no
> documentation and no error checking.
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