/**
* immutable multiply the vector times this: x' * A, i.e., colwise.
*
* @param v
* @return
*/
public SparseVector vectorTimes(SparseVector v) {
SparseVector w = new SparseVector();
// only the rows in A that v is nonzero
for (int i : v.keySet()) {
SparseVector a = get(i).copy();
a.factor(v.get(i));
w.add(a);
}
return w;
}
