Converting Map[Int, Double] to breeze.linalg.SparseVector

Question

I am new to the Breeze library and I would like to convert a Map[Int, Double] to breeze.linalg.SparseVector, and ideally without having to specify a fixed length of the SparseVector. I managed to achieve the goal with this clumsy code:

import breeze.linalg.{SparseVector => SBV}
val mySparseVector: SBV[Double] = new SBV[Double](Array.empty, Array.empty, 10000)
myMap foreach { e => mySparseVector(e._1) = e._2 }

Not only I have to specify a fixed length of 10,000, but the code runs in O(n), where n is the size of the map. Is there a better way?

Answer 1

You can use VectorBuilder. There's a (sadly) undocumented feature where if you tell it the length is -1, it will happily let you add things. You will have to (annoyingly) set the length before you construct the result...

val vb = new VectorBuilder(length = -1)
myMap foreach { e => vb.add(e._1, e._2) }
vb.length = myMap.keys.max + 1
vb.toSparseVector

(Your code is actually n^2 because SparseVector has to be sorted so you're repeatedly moving elements around in an array. VectorBuilder gives you n log n, which is the best you can do.)