I would like to inspect all the observations that reached some node in an rpart decision tree. For example, in the following code:
fit <- rpart(Kyphosis ~ Age + Start, data = kyphosis)
fit
n= 81
node), split, n, loss, yval, (yprob)
* denotes terminal node
1) root 81 17 absent (0.79012346 0.20987654)
2) Start>=8.5 62 6 absent (0.90322581 0.09677419)
4) Start>=14.5 29 0 absent (1.00000000 0.00000000) *
5) Start< 14.5 33 6 absent (0.81818182 0.18181818)
10) Age< 55 12 0 absent (1.00000000 0.00000000) *
11) Age>=55 21 6 absent (0.71428571 0.28571429)
22) Age>=111 14 2 absent (0.85714286 0.14285714) *
23) Age< 111 7 3 present (0.42857143 0.57142857) *
3) Start< 8.5 19 8 present (0.42105263 0.57894737) *
I would like to see all the observations in node (5) (i.e.: the 33 observations for which Start>=8.5 & Start< 14.5). Obviously I could manually get to them. But I would like to have some function like (say) "get_node_date". For which I could just run get_node_date(5) - and get the relevant observations.
Any suggestions on how to go about this?
There seems to be no such function which enables an extraction of the observations from a specific node. I would solve it as follows: first determine which rule/s is/are used for the node you are insterested in. You can use path.rpart for it. Then you could apply the rule/s one after the other to extract the observations.
This approach as a function:
get_node_date <- function(tree = fit, node = 5){
rule <- path.rpart(tree, node)
rule_2 <- sapply(rule[[1]][-1], function(x) strsplit(x, '(?<=[><=])(?=[^><=])|(?<=[^><=])(?=[><=])', perl = TRUE))
ind <- apply(do.call(cbind, lapply(rule_2, function(x) eval(call(x[2], kyphosis[,x[1]], as.numeric(x[3]))))), 1, all)
kyphosis[ind,]
}
For node 5 you get:
get_node_date()
node number: 5
root
Start>=8.5
Start< 14.5
Kyphosis Age Number Start
2 absent 158 3 14
10 present 59 6 12
11 present 82 5 14
14 absent 1 4 12
18 absent 175 5 13
20 absent 27 4 9
23 present 96 3 12
26 absent 9 5 13
28 absent 100 3 14
32 absent 125 2 11
33 absent 130 5 13
35 absent 140 5 11
37 absent 1 3 9
39 absent 20 6 9
40 present 91 5 12
42 absent 35 3 13
46 present 139 3 10
48 absent 131 5 13
50 absent 177 2 14
51 absent 68 5 10
57 absent 2 3 13
59 absent 51 7 9
60 absent 102 3 13
66 absent 17 4 10
68 absent 159 4 13
69 absent 18 4 11
71 absent 158 5 14
72 absent 127 4 12
74 absent 206 4 10
77 present 157 3 13
78 absent 26 7 13
79 absent 120 2 13
81 absent 36 4 13