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I have a problem understanding the behavior of rpart function of R.

Here is the r code part :

library(rpart);

library(rpart.plot)

data(ptitanic); 

attr(ptitanic$age,"class") <- NULL;

class(ptitanic$age);

#This is a function which draws the tree 

arbre=function(data){

  set.seed(415)

   ptitanicTree  = rpart(survived~., data = data, 
                        control = rpart.control(minsplit = 5, cp = 0), 
                        method = "class")       


   ptitanicOptimal = prune(ptitanicTree, 
                           cp = ptitanicTree$cptable[which.min(ptitanicTree$cptable[,4]),1] )


   prp(ptitanicOptimal,extra=1)
}

data1=ptitanic[,c(1,2,3,4,5,6)];

arbre(data1)

# I change the position switching column 5 and 6
data2=ptitanic[,c(1,2,3,4,6,5)];

# Here i get an another tree : why ?
arbre(data2)

Is anyone able to explain why we get two different trees we have only changed the position of columns in data input ?

First Tree Second Tree

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  • $\begingroup$ You did not provide a reproducible example, there is no ptitanic data in base R, nor in rpart package. As about the results, they are nearly the same -- the only difference is that pruning returned one extra level. $\endgroup$
    – Tim
    Commented Jun 16, 2017 at 14:24
  • $\begingroup$ @tim : thank's I have edited the post, the data frame is in the R package rpart.plot (also used to plot the trees). Best regards $\endgroup$
    – TheBridge
    Commented Jun 16, 2017 at 14:35
  • $\begingroup$ By the way the parameter "cp" seems involved in this curious behavior but I can't explain how. regards $\endgroup$
    – TheBridge
    Commented Jun 16, 2017 at 14:38
  • $\begingroup$ Please indent your code properly when asking others to read it. $\endgroup$
    – Matthew Drury
    Commented Jun 16, 2017 at 15:19
  • $\begingroup$ You might get more responses if your change your title. Your title Understanding rpart package is too vague. A better title would be something like Why does rpart generate a different tree when the order of the variables in the data is changed?. $\endgroup$
    – Stephen Milborrow
    Commented Aug 10, 2018 at 2:02

1 Answer 1

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The rpart algorithm can generate different trees when the order of the variables is changed because it's a greedy algorithm. At each step when when growing the tree, it may so happen that the optimum Gini Impurity may be achieved by more than one potential split-variable -- which of those variables is actually selected by the algorithm will depend on the order in which the algorithm scans the variables.

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  • $\begingroup$ Thanks for changing the title to be more informative for others who have the same question. $\endgroup$
    – Stephen Milborrow
    Commented Aug 10, 2018 at 17:19

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