code: public RegressionDataSet asRegressionDataSet(int index)
      {
          if(index < 0)
              throw new IllegalArgumentException("Index must be a non-negative value");
          else if(getNumNumericalVars()== 0)
              throw new IllegalArgumentException("Dataset has no numeric variables, can not create regression dataset");
          else if(index >= getNumNumericalVars())
              throw new IllegalArgumentException("Index " + index + " i larger than number of numeric features " + getNumNumericalVars());
          
          RegressionDataSet rds = new RegressionDataSet(this.datapoints.toList(), index);
  	for(int i = 0; i < size(); i++)
  	    rds.setWeight(i, this.getWeight(i));
  	return rds;
      }
docs: Converts this dataset into one meant for regression problems. The
  given numeric feature index is removed from the data and made the
  target variable for the regression problem.
  
  @param index the regression variable index, should be in the range
  [0, {@link #getNumNumericalVars() })
  @return a new dataset where one numeric variable is removed and made
  the target of a regression dataset