How can I efficiently construct a very long factor with few levels?

Question

In R, I want to create a factor with only a few levels, but with a length of almost 100 million. The "normal" way for me to create a factor is to call factor on a character vector, but I expect this method to be very inefficient. What is the proper way to construct a long factor without fully expanding the corresponding character vector.

Here is an example of the wrong way to do it: creating and then factoring a character vector:

long.char.vector = sample(c("left", "middle", "right"), replace=TRUE, 50000000)
long.factor = factor(long.char.vector)

How can I construct long.factor without first constructing long.char.vector? Yes, I know those two lines of code can be combined, but the resulting line of code still creates the gigantic char vector anyway.

Answer 1

It's not going to be much more efficient, but you can sample a factor vector:

big.factor <- sample(factor(c("left", "middle", "right")), replace=TRUE, 5e7)