Can we get factor matrices in R?

Question

It seems not possible to get matrices of factor in R. Is it true? If yes, why? If not, how should I do?

f <- factor(sample(letters[1:5], 20, rep=TRUE), le         


        

Answer 1

Unfortunately factor support is not completely universal in R, so many R functions default to treating factors as their internal storage type, which is integer:

> typeof(factor(letters[1:3]))
[1] "integer  

This is what happens with matrix, cbind. They don't know how to handle factors, but they do know what to do with integers, so they treat your factor like an integer. head is actually the opposite. It does know how to handle a factor, but it never bothers to check that your factor is also a matrix so just treats it like a normal dimensionless factor vector.

Your best bet to operate as if you had factors with your matrix is to coerce it to character. Once you are done with your operations, you can restore it back to factor form. You could also do this with the integer form, but then you risk weird stuff (you could for example do matrix multiplication on an integer matrix, but that makes no sense for factors).

Note that if you add class "matrix" to your factor some (but not all) things start working:

f <- factor(letters[1:9])
dim(f) <- c(3, 3)
class(f) <- c("factor", "matrix")
head(f, 2)

Produces:

     [,1] [,2] [,3]
[1,] a    d    g   
[2,] b    e    h   
Levels: a b c d e f g h i

This doesn't fix rbind, etc.