R Under development (unstable) (2024-08-16 r87026 ucrt) -- "Unsuffered Consequences"
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> suppressWarnings(RNGversion("3.5.2"))
> 
> ## package
> library("partykit")
Loading required package: grid
Loading required package: libcoin
Loading required package: mvtnorm
> 
> ## iris data
> data("iris", package = "datasets")
> irisct <- ctree(Species ~ ., data = iris)
> print(irisct)

Model formula:
Species ~ Sepal.Length + Sepal.Width + Petal.Length + Petal.Width

Fitted party:
[1] root
|   [2] Petal.Length <= 1.9: setosa (n = 50, err = 0.0%)
|   [3] Petal.Length > 1.9
|   |   [4] Petal.Width <= 1.7
|   |   |   [5] Petal.Length <= 4.8: versicolor (n = 46, err = 2.2%)
|   |   |   [6] Petal.Length > 4.8: versicolor (n = 8, err = 50.0%)
|   |   [7] Petal.Width > 1.7: virginica (n = 46, err = 2.2%)

Number of inner nodes:    3
Number of terminal nodes: 4
> table(fit = predict(irisct), true = iris$Species)
            true
fit          setosa versicolor virginica
  setosa         50          0         0
  versicolor      0         49         5
  virginica       0          1        45
> 
> ## airquality data
> data("airquality", package = "datasets")
> airq <- subset(airquality, !is.na(Ozone))
> airqct <- ctree(Ozone ~ ., data = airq)
> print(airqct)

Model formula:
Ozone ~ Solar.R + Wind + Temp + Month + Day

Fitted party:
[1] root
|   [2] Temp <= 82
|   |   [3] Wind <= 6.9: 55.600 (n = 10, err = 21946.4)
|   |   [4] Wind > 6.9
|   |   |   [5] Temp <= 77: 18.479 (n = 48, err = 3956.0)
|   |   |   [6] Temp > 77: 31.143 (n = 21, err = 4620.6)
|   [7] Temp > 82
|   |   [8] Wind <= 10.3: 81.633 (n = 30, err = 15119.0)
|   |   [9] Wind > 10.3: 48.714 (n = 7, err = 1183.4)

Number of inner nodes:    4
Number of terminal nodes: 5
> sum((airq$Ozone - predict(airqct))^2)
[1] 46825.35
> 
> ### split in one variable only: Temp is selected freely in the root node
> ### but none of the other variables is allowed deeper in the tree
> airqct1 <- ctree(Ozone ~ ., data = airq, 
+                  control = ctree_control(maxvar = 1L))
> psplitids <- unique(do.call("c", 
+         nodeapply(node_party(airqct1), 
+                   ids = nodeids(node_party(airqct1)),
+                   FUN = function(x) split_node(x)$varid)))
> stopifnot(length(psplitids) == 1L)
> 
> proc.time()
   user  system elapsed 
   0.89    0.14    1.01