MATLAB: What is ‘categorical predictor’ in decision tree for regression

categorical predictorsdecision treeregression

Hi, I'd like to use Matlab's own example for the question. Please refer to https://uk.mathworks.com/help/stats/fitrtree.html for the original example.
>> load carsmall
>> whos
  Name                Size            Bytes  Class     Attributes
  Acceleration      100x1               800  double              
  Cylinders         100x1               800  double              
  Displacement      100x1               800  double              
  Horsepower        100x1               800  double              
  MPG               100x1               800  double              
  Mfg               100x13             2600  char                
  Model             100x33             6600  char                
  Model_Year        100x1               800  double              
  Origin            100x7              1400  char                
  Weight            100x1               800  double              
>> tree = fitrtree([Weight, Cylinders],MPG,...
                'categoricalpredictors',2,'MinParentSize',20,...
                'PredictorNames',{'W','C'})
tree = 
  RegressionTree
           PredictorNames: {'W'  'C'}
             ResponseName: 'Y'
    CategoricalPredictors: 2
        ResponseTransform: 'none'
          NumObservations: 94
  Properties, Methods
What exactly is the Categorical Predictors in this case and why it is 2?

Best Answer

Matlab's fitrtree() function returns a regression tree object. Read more about this object and its properties here:
https://www.mathworks.com/help/stats/classreg.learning.partition.regressionpartitionedmodel-class.html
As you'll read in the link above, the "CategoricalPredictors" contains index values corresponding to the columns of the categorical predictor data (if none of the predictors are categorical, this will be empty []).
So, why is it CategoricalPredictors equal to 2?
Now read about the function you're using fitrtree()
https://www.mathworks.com/help/stats/fitrtree.html#bt6cr84-tree
One of the name-value pairs (<- link) is 'CategoricalPredictors' which, is specified in your call to fitrtree() as 2. That's because you have two predictors being treated as categorical variables, [Weight, Cylinders].
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