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From Jake Mannix <jake.man...@gmail.com>
Subject Re: Taste-GenericItemBasedRecommender
Date Thu, 03 Dec 2009 18:31:49 GMT
Wait, this is just doing A'A?  Am I misunderstanding, or is this not most
easily done by first transposing A into A', and then doing the outer
products
instead of the inner products:

class Map extends Mapper<IntWritable, Vector, NullWritable, Matrix> {
  map(IntWritable i, Vector v, Context c) { c.write(NullWritable.get(),
v.cross(v)); }
}

class Reduce extends Reducer<NullWritable, Matrix, NullWritable, Matrix> {
  Matrix out;
  reduce(NullWritable n, Iterable<Matrix> it, Context c) {
    for(Matrix m : it) out = out.plus(m);
    c.write(n.get(), m);
  }
}

Then the outer products are computed in the mapper, and the sum is done in
the
combiners and single reducer.

Not sure how you'd fold in sparsification to that though.

  -jake

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