Recode a variable using data.table package

Question

If I want to recode a variable in R using data.table, what is the syntax? I saw some ans but didn\'t find them appropriate.

e.g. if I have the variable cal

Answer 1

Once you have a data.table then it would be most efficient to use a vectorized translation strategy. The match function provides a method of creating a "selection vector" for a choosing a item from a set of character possibilities:

library(data.table)
setDT(trips)  # create a data.table from a dataframe

trips[ , Gender := c("Unknown", "male", "Female")[match(Gender, c(0,1,2))] ]
#-------------------
> trips
     Name  Gender
1:   John    male
2:   Tina  Female
3:   Dave  Female
4: Casper Unknown

For this specific case, a simpler solution could be (ht to @Chinsoon):

trips[, gender := c("Unknown", "Male", "Female")[gender + 1L] ]