How to number nodes when creating decision trees in R? - r

Question

In R I am creating a data frame of the structure of decision trees. The issue I'm facing is, I have to number the nodes of the trees in a certain way that will allow me to plot them later. However, Im struggling to find a good way to number the nodes. Hopefully my example below will explain the issue.
For example, if I have a column in my data frame that describes the path or direction of the nodes, like so:
df <- data.frame(
var = c("P", "L", "R", "RL", "RR",
"P", "L", "R" , "RL", "RR", "LL", "LR", "RRL", "RRR")
)
Here, P means the parent node, L means left node, R means right node, RL means the left node from the previous right node etc... The diagram below shows what the decision trees made from df$var would look like:
So, as we can see, every time we reach a P in df$var, we start a new decision tree, as it is the parent.
Now, I want to try and number the nodes, so I can plot them. I initially tried numbering the nodes sequentially, like so:
df <- df %>%
group_by(newVal = cumsum(var == "P")) %>%
mutate(node = 1:length(var)) %>%
ungroup() %>%
select(-newVal)
df
var node
P 1
L 2
R 3
RL 4
RR 5
P 1
L 2
R 3
RL 4
RR 5
LL 6
LR 7
RRL 8
RRR 9
For clarity, that would look like this:
But as you can see (mainly in the 2nd tree), due to the original ordering of df$var, it results in a non-intuitive numbering of the nodes. This presents a problem when I try to plot the tree.
The issue is, when Im plotting the trees, I have to create data frames (for each tree) with 2 columns. That is, from and to, where we go from node x to node y. Using the 2nd image as an example, my data frames for plotting would look like this:
tree.1.Edges <- data.frame(
from = c(1,1,3,3),
to = c(2,3,4,5)
)
tree.2.edges <- data.frame(
from = c(1,1,2,2,3,3,5,5),
to = c(2,3,6,7,4,5,8,9)
)
Im finding it difficult to come up with a way to automate the process of creating the tree edges data frames using my method of sequentially numbering the nodes. Does anyone have any suggestions as to a better way I could do this?

Answer 1

Prefix
This is my solution. It returns a list of the edges with correctly numbered nodes.
The Nodes are numbered like this:
Parent node number < Child node number
Left node number < Right node number
Code
library(tidyverse)
df <- data.frame(
var = c("P", "L", "R", "RL", "RR",
"P", "L", "R" , "RL", "RR", "LL", "LR", "RRL", "RRR"),
stringsAsFactors = FALSE # important for character operations
)
#enumerate tree ids
# a new tree is initialized when a parent node "P" is initialized
df$tree <-cumsum(df$var=="P") # Cumsum increments for every TRUE by one
#sort nodes so that Left nodes are in front of Right nodes
# and every deeper level of the tree is numbered after
# the preceeding level
df <- df %>% group_by(tree) %>% mutate(level = nchar(var)) %>%
group_by(tree) %>% arrange(level, # arrange by level first
# custom alphabet function where P comes first
# As L comes in front of R in the alphabet
# longer strings are
# correctly sorted
ifelse(var=="P",1,match(LETTERS,var)+1),
.by_group = TRUE)
# define the nodes as row numbers resetting at every tree
df <- df %>% group_by(tree) %>% mutate(node = row_number())
## At this point the nodes are numbered according to your specifications
# Find out parent node by deleting the last character from every node name (var)
df <- df %>% group_by(tree) %>% mutate(parent_node_name=substr(var,0,nchar(var)-1))
# define parent node of P as NA
df$parent_node_name[df$var=="P"] <- NA
# define parent nodes vars with still empty parent node name as "P"
df$parent_node_name[df$parent_node_name==""] <- "P"
# Match parent node names to node numbers
df <- df %>%
group_by(tree) %>%
mutate(parent_node_num = match(parent_node_name,var))
# split the dataframe into a list of dfs, one for each tree
list_edges <- split(df,df$tree)
# for every dataframe in the list, replace by a result dataframe (res)
list_edges <- lapply(list_edges, function(df_tree){
res <- data.frame(
from = df_tree$parent_node_num,
to = df_tree$node
)
# delete NAs from result
res <- res[!is.na(res$from),]
return(res)
})
# Show result
list_edges
# $`1`
# from to
# 2 1 2
# 3 1 3
# 4 3 4
# 5 3 5
#
# $`2`
# from to
# 2 1 2
# 3 1 3
# 4 3 4
# 5 3 5
# 6 2 6
# 7 2 7
# 8 5 8
# 9 5 9
The code is quite convoluted, but you can insert df at any point to look at the intermediate results. Or simply post a comment.