[GitHub] spark issue #16740: [SPARK-19400] Allow GLM to handle intercept only model

[actuaryzhang <gi...@git.apache.org>]

Github user actuaryzhang commented on the issue:

    https://github.com/apache/spark/pull/16740
  
    The following is a simple example to illustrate the issue. 
    
    ```
    val dataset = Seq(
              (1.0, 1.0, 2.0, 0.0, 5.0),
              (0.5, 2.0, 1.0, 1.0, 2.0),
              (1.0, 3.0, 0.5, 2.0, 1.0),
              (2.0, 4.0, 1.5, 3.0, 3.0)
            ).toDF("y", "w", "off", "x1", "x2")
    
    val formula = new RFormula().setFormula("y ~ 1")
    val output = formula.fit(dataset).transform(dataset)
    val glr = new GeneralizedLinearRegression().setFamily("poisson")
    val model = glr.fit(output)
    ```
    
    The above prints out the following error message:
    ```
    java.lang.UnsupportedOperationException: empty.reduceLeft
      at scala.collection.TraversableOnce$class.reduceLeft(TraversableOnce.scala:180)
      at scala.collection.mutable.ArrayOps$ofDouble.scala$collection$IndexedSeqOptimized$$super$reduceLeft(ArrayOps.scala:270)
      at scala.collection.IndexedSeqOptimized$class.reduceLeft(IndexedSeqOptimized.scala:74)
    ```


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