Convert factor to numeric in the same order of the factor from 0 to length of the unique values

Question

I am able to convert the new_target column into numerical form. But as the factor form is already numerical, I am left with a bunch of numbers. I want them ordered and reassigned to their equivalent from 0 to the length of the factor. I have a numerical target at first, then I quantize it to 20 bins. As a result, I obtain new_target column which consists of the unique values (0,1,3,14,16,18,19) . Instead of these unique values I need values ordered from 0 to length of the unique values in new_target . Which are c(0,1,2,3,4,5,6) . The expected output is given in new_target_expected column. How can I create new_target_expected column without manually creating it? I have a bigger dataframe I am dealing with and it is not possible to do this manually.

require(stringr)
require(data.table)

cat_var <- c("rock", "indie", "rock", "rock", "pop", "indie", "pop", "rock", "pop")
cat_var_2 <- c("blue", "green", "red", "red", "blue", "red", "green", "blue", "green")
target_var <- c(30, 10, 27, 14, 29, 25, 27, 12, 10)
df <- data.table("categorical_variable" = cat_var, "categorical_variable_2" = cat_var_2, "target_variable" =  target_var)

targetVariable <- "target_variable"

number_of_buckets = 20
# Each bucket should contain equal number of objects
a <- cut(df[[targetVariable]] , breaks = number_of_buckets, labels = 0:(number_of_buckets - 1)) 

df[["new_target"]] <- a
df[["new_target"]] <- as.numeric(as.character(df[["new_target"]]))
df[["new_target_expected"]] <- c(6, 0, 4, 2, 5, 3, 4, 1, 0)

Answer 1

We could remove the unused levels with droplevels and coerce the factor to integer . Indexing in R starts from 1, so subtract 1 to make the values start from 0.

library(data.table)
df[, (targetVariable) := as.integer(droplevels(a))-1]

-output

> df
   categorical_variable categorical_variable_2 target_variable
1:                 rock                   blue               6
2:                indie                  green               0
3:                 rock                    red               4
4:                 rock                    red               2
5:                  pop                   blue               5
6:                indie                    red               3
7:                  pop                  green               4
8:                 rock                   blue               1
9:                  pop                  green               0