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ppforest2

ppforest2 provides projection pursuit oblique decision trees and random forests for classification. Instead of splitting on single variables, each node projects the data onto a linear combination of features, capturing structure that axis-aligned trees miss.

The package wraps a high-performance C++ core and is intended as a modern successor to PPforest.

Key capabilities: oblique splits via projection pursuit, multi-threaded forest training (OpenMP), cross-platform reproducibility, three variable importance measures (projection-based, weighted, permutation), LDA/PDA optimisation, OOB error estimation, and parsnip / tidymodels integration.

Installation 

# install.packages("devtools")
devtools::install_github("andres-vidal/ppforest2", subdir = "bindings/R", build = FALSE)

Usage

Single tree 

library(ppforest2)

model <- pptr(Species ~ ., data = iris)
predict(model, iris[1:5, ])
summary(model)

Random forest 

forest <- pprf(Species ~ ., data = iris, size = 500)
predict(forest, iris[1:5, ])
predict(forest, iris[1:5, ], type = "prob")   # vote proportions
summary(forest)

Regularisation (PDA)

When classes are highly correlated or the number of variables is large relative to the sample size, penalised discriminant analysis can improve separation:

pptr(Species ~ ., data = iris, lambda = 0.5)

Visualisation

ppforest2 provides four diagnostic plot types (requires ggplot2):

# Mosaic overview: structure + importance + boundaries
plot(model)

# Individual plot types
plot(model, type = "structure")     # tree diagram with per-node histograms
plot(model, type = "importance")    # variable importance bar chart
plot(model, type = "projection")    # projected data at each split
plot(model, type = "boundaries")    # decision boundaries in feature space

# Forest: importance across all trees, or inspect individual trees
plot(forest)
plot(forest, type = "structure", tree_index = 1)
plot(forest, type = "boundaries", tree_index = 1)

tidymodels integration

ppforest2 integrates with parsnip:

library(parsnip)

# Single tree
spec <- pp_tree(lambda = 0) |> set_engine("ppforest2") |> set_mode("classification")
fit  <- fit(spec, Species ~ ., data = iris)

# Random forest
spec <- pp_rand_forest(trees = 50, mtry = 2) |> set_engine("ppforest2")
fit  <- spec |> fit(Species ~ ., data = iris)
predict(fit, iris, type = "prob")

JSON serialisation

Models can be saved and loaded in JSON format, enabling interoperability with the C++ CLI and other language bindings:

save_json(model, "model.json")
restored <- load_json("model.json")

Learning more