How it works...

You might ask, why did we take out class 2 out of the target set?

The reason is that, to use a regressor, our intent has to be to predict a real number. The categories had to have real number properties: that they are ordered (informally, if we have three ordered categories x, y, z and x < y and y < z then x < z). By eliminating the third category, the remaining flowers (Setosa and Versicolor) became ordered by a property we invented: Setosaness or Versicolorness.

The next time you encounter categories, you can consider whether they can be ordered. For example, if the dataset consists of shoe sizes, they can be ordered and a regressor can be applied, even though no one has a shoe size of 12.125.