Try using the Apache Mahout code that solves exactly this problem. Mahout has a distributed row-wise matrix that is read one row at a time. Dot products with the vector are computed and the results are collected. This capability is used extensively in the large scale SVD's in Mahout.
On Tue, May 17, 2011 at 1:13 PM, Alexandra Anghelescu < axanghele...@gmail.com> wrote: > Hi all, > > I was wondering how to go about doing a matrix-vector multiplication using > hadoop. I have my matrix in one file and my vector in another. All the map > tasks will need the vector file... basically they need to share it. > > Basically I want my map function to output key-value pairs (i,m[i,j]*v(j)), > where i is the row number, and j the column number; v(j) is the jth element > in v. And the reduce function will sum up all the values with the same key > - > i, and that will be the ith element of my result vector. > > I don't know how to format the input to do this.. even if I do it in 2 MR > iterations, first formatting the input and second the actual > matrix-vector-multiply, I don't have a clear idea. > > If you have any ideas/suggestions I would appreciate it! > > Thanks in advance, > Alexandra >
