code: public void unitize() {
          for (Datum<SparseArray> row : this) {
              double sum = 0.0;
  
              for (SparseArray.Entry e : row.x) {
                  sum += Math.sqr(e.x);
              }
  
              sum = Math.sqrt(sum);
  
              for (SparseArray.Entry e : row.x) {
                  e.x /= sum;
              }
          }
      }
docs: Unitize each row so that L2 norm of x = 1.