Convert ranger model to party object — as.party.ranger • lorax

Convert a single tree from a ranger random forest model to a party object for use with partykit visualization and analysis tools.

Usage

# S3 method for class 'ranger'
as.party(obj, tree = 1L, data = NULL, ...)

Arguments

obj: A ranger object from the ranger package.
tree: Integer specifying which tree to convert (1-based indexing, default is 1). Must be between 1 and the number of trees in the forest.
data: Data.frame containing the training data, including both predictors and response variable. Required for proper party object creation with fitted values and node summaries.
...: Not currently used.

Value

A party object from the partykit package.

Details

Ranger tree storage format

The ranger package stores trees in obj$forest with parallel vectors:

split.varIDs[[tree]]: 0-based variable indices for splits
split.values[[tree]]: threshold values for splits
child.nodeIDs[[tree]]: matrix with 2 columns (left, right child IDs)
is.ordered[[tree]]: whether split variable is ordered (for categoricals)
All node IDs are 0-based (root = 0)

Node indexing

Internally, ranger uses 0-based node indices (root is node 0)
User-facing tree parameter uses 1-based indexing (R convention)
Leaf nodes have split.varIDs entry of NA or large sentinel value

Split encoding

For numeric variables: left child when feature < threshold, right child when feature >= threshold
partykit split created with right = TRUE (right interval closed)

Child node references

child.nodeIDs is a matrix with 2 columns: left_child, right_child
Value 0 indicates no child (terminal node)
Both children 0 means current node is terminal

The party object will use 1-based node IDs and variable indices as required by partykit.

Examples

if (rlang::is_installed(c("ranger", "palmerpenguins"))) {
  # Classification example
  data(penguins, package = "palmerpenguins")
  penguins <- na.omit(penguins)

  set.seed(2847)
  rf <- ranger::ranger(species ~ ., data = penguins, num.trees = 3)

  # Convert first tree
  party_tree <- as.party(rf, tree = 1L, data = penguins)
  print(party_tree)
  plot(party_tree)

  # Predictions from party object
  predict(party_tree, newdata = penguins[1:5, ])

  # Regression example
  data(mtcars)
  set.seed(5193)
  rf_reg <- ranger::ranger(mpg ~ ., data = mtcars, num.trees = 3)
  party_tree_reg <- as.party(rf_reg, tree = 1L, data = mtcars)
  print(party_tree_reg)
}
#> 
#> Model formula:
#> ~island + bill_length_mm + bill_depth_mm + flipper_length_mm + 
#>     body_mass_g + sex + year
#> 
#> Fitted party:
#> [1] root
#> |   [2] flipper_length_mm <= 206.5
#> |   |   [3] bill_length_mm <= 43.15
#> |   |   |   [4] flipper_length_mm <= 187.5
#> |   |   |   |   [5] flipper_length_mm <= 186.5
#> |   |   |   |   |   [6] bill_length_mm <= 41.75: Adelie (n = 40, err = 0.0%)
#> |   |   |   |   |   [7] bill_length_mm > 41.75: Adelie (n = 2, err = 50.0%)
#> |   |   |   |   [8] flipper_length_mm > 186.5
#> |   |   |   |   |   [9] body_mass_g <= 3325: Adelie (n = 5, err = 20.0%)
#> |   |   |   |   |   [10] body_mass_g > 3325
#> |   |   |   |   |   |   [11] bill_length_mm <= 40.3: Adelie (n = 5, err = 0.0%)
#> |   |   |   |   |   |   [12] bill_length_mm > 40.3: Chinstrap (n = 3, err = 33.3%)
#> |   |   |   [13] flipper_length_mm > 187.5: Adelie (n = 87, err = 0.0%)
#> |   |   [14] bill_length_mm > 43.15
#> |   |   |   [15] sex in female
#> |   |   |   |   [16] bill_length_mm <= 47.65: Chinstrap (n = 23, err = 0.0%)
#> |   |   |   |   [17] bill_length_mm > 47.65
#> |   |   |   |   |   [18] bill_length_mm <= 49.1: Chinstrap (n = 2, err = 50.0%)
#> |   |   |   |   |   [19] bill_length_mm > 49.1: Chinstrap (n = 6, err = 0.0%)
#> |   |   |   [20] sex in male
#> |   |   |   |   [21] bill_length_mm <= 47.25: Adelie (n = 6, err = 0.0%)
#> |   |   |   |   [22] bill_length_mm > 47.25: Chinstrap (n = 29, err = 0.0%)
#> |   [23] flipper_length_mm > 206.5
#> |   |   [24] bill_length_mm <= 40.85: Adelie (n = 1, err = 0.0%)
#> |   |   [25] bill_length_mm > 40.85
#> |   |   |   [26] bill_length_mm <= 54.15
#> |   |   |   |   [27] flipper_length_mm <= 212.5
#> |   |   |   |   |   [28] island in Biscoe: Gentoo (n = 31, err = 0.0%)
#> |   |   |   |   |   [29] island in Dream, Torgersen
#> |   |   |   |   |   |   [30] year <= 2008.5: Chinstrap (n = 2, err = 0.0%)
#> |   |   |   |   |   |   [31] year > 2008.5
#> |   |   |   |   |   |   |   [32] island in Biscoe, Dream: Chinstrap (n = 2, err = 0.0%)
#> |   |   |   |   |   |   |   [33] island in Torgersen: Adelie (n = 1, err = 0.0%)
#> |   |   |   |   [34] flipper_length_mm > 212.5: Gentoo (n = 83, err = 0.0%)
#> |   |   |   [35] bill_length_mm > 54.15
#> |   |   |   |   [36] bill_length_mm <= 55.85: Gentoo (n = 3, err = 33.3%)
#> |   |   |   |   [37] bill_length_mm > 55.85: Gentoo (n = 2, err = 0.0%)
#> 
#> Number of inner nodes:    18
#> Number of terminal nodes: 19

#> 
#> Model formula:
#> ~cyl + disp + hp + drat + wt + qsec + vs + am + gear + carb
#> 
#> Fitted party:
#> [1] root
#> |   [2] wt <= 2.38
#> |   |   [3] hp <= 65.5: 32.150 (n = 2, err = 6.1)
#> |   |   [4] hp > 65.5: 27.780 (n = 5, err = 56.4)
#> |   [5] wt > 2.38
#> |   |   [6] drat <= 3.815
#> |   |   |   [7] hp <= 83.5: 24.400 (n = 1, err = 0.0)
#> |   |   |   [8] hp > 83.5
#> |   |   |   |   [9] carb <= 3.5
#> |   |   |   |   |   [10] qsec <= 16.945: 15.500 (n = 1, err = 0.0)
#> |   |   |   |   |   [11] qsec > 16.945
#> |   |   |   |   |   |   [12] disp <= 317.9: 17.871 (n = 7, err = 42.4)
#> |   |   |   |   |   |   [13] disp > 317.9: 18.950 (n = 2, err = 0.1)
#> |   |   |   |   [14] carb > 3.5
#> |   |   |   |   |   [15] drat <= 3.635: 14.083 (n = 6, err = 59.9)
#> |   |   |   |   |   [16] drat > 3.635: 13.300 (n = 1, err = 0.0)
#> |   |   [17] drat > 3.815
#> |   |   |   [18] wt <= 3.16
#> |   |   |   |   [19] am <= 0.5: 22.800 (n = 1, err = 0.0)
#> |   |   |   |   [20] am > 0.5
#> |   |   |   |   |   [21] drat <= 4.005: 21.000 (n = 2, err = 0.0)
#> |   |   |   |   |   [22] drat > 4.005: 21.400 (n = 1, err = 0.0)
#> |   |   |   [23] wt > 3.16: 17.600 (n = 3, err = 5.8)
#> 
#> Number of inner nodes:    11
#> Number of terminal nodes: 12