Extract variable labels from rpart decision tree - r

Question

I've used part to build a decision tree on a dataset that has categorical variables with hundreds of levels. The tree splits these variables based on select values of the variable. I would like to examine the labels on which the split is made.
If I just run the decision tree result, the display listing the splits in the console gets truncated and either way, it is not in an easily-interpretable format (separated by commas). Is there a way to access this as an R object? I'm open to using another package to build the tree.

Answer 1

One issue here is that some of the functions in the rpart package are not exported. It appears you're looking to capture the output of the function rpart:::print.rpart. So, beginning with a reproducible example:
set.seed(1)
df1 <- data.frame(y=rbinom(n=100, size=1, prob=0.5),
x1=rbinom(n=100, size=1, prob=0.25),
x2=rbinom(n=100, size=1, prob=0.75))
(r1 <- rpart(y ~ ., data=df1))
giving
n= 100
node), split, n, deviance, yval
* denotes terminal node
1) root 100 24.960000 0.4800000
2) x1< 0.5 78 19.179490 0.4358974
4) x2>=0.5 66 15.954550 0.4090909 *
5) x2< 0.5 12 2.916667 0.5833333 *
3) x1>=0.5 22 5.090909 0.6363636
6) x2< 0.5 7 1.714286 0.4285714 *
7) x2>=0.5 15 2.933333 0.7333333 *
Now, looking at rpart:::print.rpart, we see a call to rpart:::labels.rpart, giving us the splits (or names of the 'rows' in the output above). The value of n, deviance, yval and more are stored in r1$frame, which can be seen by inspecting the output from unclass(r1).
Thus we could extract the above with
(df2 <- data.frame(split=rpart:::labels.rpart(r1), n=r1$frame$n))
giving
split n
1 root 100
2 x1< 0.5 78
3 x2>=0.5 66
4 x2< 0.5 12
5 x1>=0.5 22
6 x2< 0.5 7
7 x2>=0.5 15