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Convert randomForest model to party object

Source: R/randomForest.R
as.party.randomForest.Rd

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

Usage

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

Arguments

obj

A randomForest object from the randomForest 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

Optional data.frame containing the training data. If NULL, a placeholder data.frame will be created with correct variable names but no observations. Providing data enables full party functionality including predictions.

...

Not currently used.

Value

A party object from the partykit package.

Details

randomForest tree storage format

The randomForest package stores trees in obj$forest as parallel matrices:

  • leftDaughter[i, tree]: 1-based row index of left child (0 = no child)

  • rightDaughter[i, tree]: 1-based row index of right child (0 = no child)

  • bestvar[i, tree]: 1-based variable index for split (0 for terminal)

  • xbestsplit[i, tree]: threshold value for split

  • nodestatus[i, tree]: node status (-1 = terminal, -3 = internal)

  • nodepred[i, tree]: prediction at node (for regression) or class (classification)

Node indexing

  • randomForest uses 1-based row indices for nodes (root is row 1)

  • Value 0 in leftDaughter/rightDaughter indicates no child

  • User-facing tree parameter uses 1-based indexing (R convention)

Split encoding

  • For numeric variables: left child when feature <= threshold, right child when feature > threshold

  • Note: randomForest uses <= for left (different from ranger's <)

  • partykit split created with right = FALSE to match this

Terminal node identification

  • nodestatus == -1 indicates terminal node

  • Alternatively: bestvar == 0 or both daughters == 0

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

Examples

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

  set.seed(2847)
  rf <- randomForest::randomForest(species ~ ., data = penguins, ntree = 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 <- randomForest::randomForest(mpg ~ ., data = mtcars, ntree = 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] island in Biscoe
#> |   |   |   [4] flipper_length_mm < 194.5
#> |   |   |   |   [5] bill_length_mm < 42.35
#> |   |   |   |   |   [6] body_mass_g < 3225
#> |   |   |   |   |   |   [7] flipper_length_mm < 186: Adelie (n = 10, err = 0.0%)
#> |   |   |   |   |   |   [8] flipper_length_mm >= 186
#> |   |   |   |   |   |   |   [9] body_mass_g < 3187.5: Adelie (n = 5, err = 0.0%)
#> |   |   |   |   |   |   |   [10] body_mass_g >= 3187.5: Adelie (n = 3, err = 33.3%)
#> |   |   |   |   |   [11] body_mass_g >= 3225: Adelie (n = 59, err = 0.0%)
#> |   |   |   |   [12] bill_length_mm >= 42.35
#> |   |   |   |   |   [13] sex in female, male: Chinstrap (n = 20, err = 0.0%)
#> |   |   |   |   |   [14] sex in female, male
#> |   |   |   |   |   |   [15] bill_length_mm < 47.25: Adelie (n = 2, err = 0.0%)
#> |   |   |   |   |   |   [16] bill_length_mm >= 47.25: Chinstrap (n = 6, err = 0.0%)
#> |   |   |   [17] flipper_length_mm >= 194.5
#> |   |   |   |   [18] flipper_length_mm < 196.5
#> |   |   |   |   |   [19] sex in female, male
#> |   |   |   |   |   |   [20] bill_length_mm < 41.05: Adelie (n = 3, err = 0.0%)
#> |   |   |   |   |   |   [21] bill_length_mm >= 41.05: Chinstrap (n = 6, err = 0.0%)
#> |   |   |   |   |   [22] sex in female, male
#> |   |   |   |   |   |   [23] bill_depth_mm < 20.3
#> |   |   |   |   |   |   |   [24] bill_depth_mm < 18.5
#> |   |   |   |   |   |   |   |   [25] bill_length_mm < 45: Adelie (n = 3, err = 0.0%)
#> |   |   |   |   |   |   |   |   [26] bill_length_mm >= 45: Chinstrap (n = 1, err = 0.0%)
#> |   |   |   |   |   |   |   [27] bill_depth_mm >= 18.5: Chinstrap (n = 4, err = 25.0%)
#> |   |   |   |   |   |   [28] bill_depth_mm >= 20.3: Adelie (n = 2, err = 0.0%)
#> |   |   |   |   [29] flipper_length_mm >= 196.5
#> |   |   |   |   |   [30] body_mass_g < 3962.5: Chinstrap (n = 19, err = 15.8%)
#> |   |   |   |   |   [31] body_mass_g >= 3962.5
#> |   |   |   |   |   |   [32] bill_length_mm < 45.35: Adelie (n = 8, err = 0.0%)
#> |   |   |   |   |   |   [33] bill_length_mm >= 45.35: Chinstrap (n = 11, err = 9.1%)
#> |   |   [34] island in Dream, Torgersen
#> |   |   |   [35] bill_depth_mm < 15.15: NA (n = 0, err = NA)
#> |   |   |   [36] bill_depth_mm >= 15.15: Adelie (n = 46, err = 0.0%)
#> |   [37] flipper_length_mm >= 206.5
#> |   |   [38] island in Biscoe, Dream, Torgersen: Gentoo (n = 118, err = 0.0%)
#> |   |   [39] island in Biscoe, Dream, Torgersen
#> |   |   |   [40] island in Biscoe: Chinstrap (n = 6, err = 16.7%)
#> |   |   |   [41] island in Dream, Torgersen: Adelie (n = 1, err = 0.0%)
#> 
#> Number of inner nodes:    20
#> Number of terminal nodes: 21

#> 
#> Model formula:
#> ~cyl + disp + hp + drat + wt + qsec + vs + am + gear + carb
#> 
#> Fitted party:
#> [1] root
#> |   [2] cyl < 5
#> |   |   [3] disp < 101.55: 30.880 (n = 5, err = 24.7)
#> |   |   [4] disp >= 101.55
#> |   |   |   [5] wt < 2.23: 26.000 (n = 1, err = 0.0)
#> |   |   |   [6] wt >= 2.23
#> |   |   |   |   [7] disp < 143.75: 22.125 (n = 4, err = 1.8)
#> |   |   |   |   [8] disp >= 143.75: 24.400 (n = 1, err = 0.0)
#> |   [9] cyl >= 5
#> |   |   [10] disp < 266.9
#> |   |   |   [11] drat < 3.91: 20.240 (n = 5, err = 7.4)
#> |   |   |   [12] drat >= 3.91: 18.500 (n = 2, err = 1.0)
#> |   |   [13] disp >= 266.9
#> |   |   |   [14] qsec < 17.71
#> |   |   |   |   [15] qsec < 16.945: 14.780 (n = 5, err = 4.0)
#> |   |   |   |   [16] qsec >= 16.945
#> |   |   |   |   |   [17] hp < 162.5: 15.200 (n = 1, err = 0.0)
#> |   |   |   |   |   [18] hp >= 162.5: 17.260 (n = 5, err = 13.1)
#> |   |   |   [19] qsec >= 17.71: 12.000 (n = 3, err = 15.4)
#> 
#> Number of inner nodes:     9
#> Number of terminal nodes: 10