code: public void setR(double r)
      {
          if(Double.isNaN(r) || Double.isInfinite(r) || r <= 0)
              throw new IllegalArgumentException("r must be a postive constant, not " + r);
          this.r = r;
      }
docs: Sets the r parameter of AROW, which controls the regularization. Larger
  values reduce the change in the model on each update.
  @param r the regularization parameter in (0, Inf)