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

Put the rules in a table and do an update join:

gmap = data.table(old = 0:2, new = c("Unknown", "Male", "Female"))
trips[, Gender := gmap[copy(.SD), on=.(old = Gender), x.new]]

     Name  Gender
1:   John    Male
2:   Tina  Female
3:   Dave  Female
4: Casper Unknown