R Notebook of Sanjay

How to add levels to an existing factor variable, without changing previous sequence

Let’s take a sequence of months. I like to take month names as factors. Always.

m<-factor(sample(month.name,5,replace = T))
m

[1] October  May      February November March   
Levels: February March May November October

as.integer(m)

[1] 5 3 1 4 2

now let’s change the levels to increase them to the full 12 months

n<-factor(m,levels = month.name,ordered = T)
n

[1] October  May      February November March   
12 Levels: January < February < March < April < May < June < July < August < September < October < ... < December

as.integer(n)

[1] 10  5  2 11  3

Bingo!