trees.R

library(data.tree)
data(acme)
acme$Set(len = runif(acme$totalCount))

# see ?as.data.frame.Node for more details
ToDataFrameTree(acme, 
                parent = function(node) node$parent$name, 
                child = "name", 
                len.parent = function(node) node$parent$len,
                len.child = "len", 
                filterFun = isNotRoot)[,-1 ]

#if you want to just display the difference:
print(acme, "len", diff = function(node) node$len - ifelse(node$isRoot, 0, node$parent$len))