# Path from root to leaf node in mvpart

December 1, 2011
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(This article was first published on indiacrunchin » R, and kindly contributed to R-bloggers)

I was recently asked by a R user about how one could extract the “rule” in a classification/regression tree. The requirement was to obtain the path traced from the root node to the leaf nodes and obtain all the paths or “rules”

path.rpart() function in the mvpart package provides this convenience

library(mvpart)

# Create a classification tree
ozone <- mvpart(Ozone ~ ., data=airquality)
print(ozone) # Gives you the various splits in the tree

# Issue the two commands below to see the graphical representation
plot(ozone)
text(ozone)

# To obtain the summary of the created tree
summary(ozone)

# To obtain the path to the leaf nodes
leafnodeRows <- grepl("leaf",ozone$frame$var)
nodevals <- as.numeric(rownames(ozone\$frame)[leafnodeRows])
rules <- path.rpart(ozone,nodevals)
rulesdf <- do.call("rbind",lapply(rules,function(x)paste(x,collapse = " -AND- ")))
rulesdf <- data.frame(nodeNumber=rownames(rulesdf),rule=rulesdf[,1],stringsAsFactors=FALSE)