Question

In an answer to another question, @Marek posted the following solution:
https://stackoverflow.com/a/10432263/636656

dat <- structure(list(product = c(11L, 11L, 9L, 9L, 6L, 1L, 11L, 5L, 
                                  7L, 11L, 5L, 11L, 4L, 3L, 10L, 7L, 10L, 5L, 9L, 8L)), .Names = "product", row.names = c(NA, -20L), class = "data.frame")

`levels<-`(
  factor(dat$product),
  list(Tylenol=1:3, Advil=4:6, Bayer=7:9, Generic=10:12)
  )

Which produces as output:

 [1] Generic Generic Bayer   Bayer   Advil   Tylenol Generic Advil   Bayer   Generic Advil   Generic Advil   Tylenol
[15] Generic Bayer   Generic Advil   Bayer   Bayer  

This is just the printout of a vector; so to store it, you can do the even more confusing:

res <- `levels<-`(
  factor(dat$product),
  list(Tylenol=1:3, Advil=4:6, Bayer=7:9, Generic=10:12)
  )

Clearly this is some kind of call to the levels function, but I have no idea what’s being done here. What is the term for this kind of sorcery, and how do I increase my magical ability in this domain?

Answer 1

The answers here are good, but they are missing an important point. Let me try and describe it.

R is a functional language and does not like to mutate its objects. But it does allow assignment statements, using replacement functions:

levels(x) <- y

is equivalent to

x <- `levels<-`(x, y)

The trick is, this rewriting is done by <-; it is not done by levels<-. levels<- is just a regular function that takes an input and gives an output; it does not mutate anything.

One consequence of that is that, according to the above rule, <- must be recursive:

levels(x)[1] <- "a"

is

levels(x) <- `[<-`(levels(x), 1, "a")

is

x <- `levels<-`(x, `[<-`(levels(x), 1, "a"))

It’s kind of beautiful that this pure-functional transformation (up until the very end, where the assignment happens) is equivalent to what an assignment would be in an imperative language. This construct in functional languages is called a lens. Lenses can be awkward to use in some programming languages, but in R they just work.

But then, once you have defined replacement functions like levels<-, you get another, unexpected windfall: you don’t just have the ability to make assignments, you have a handy function that takes in a factor, and gives out another factor with different levels. There’s really nothing "assignment" about it!

So, the code you’re describing is just making use of this other interpretation of levels<-. I admit that the name levels<- is a little confusing because it suggests an assignment, but this is not what is going on. The code is simply setting up a sort of pipeline:

• Start with dat$product

• Convert it to a factor

• Change the levels

• Store that in res

Personally, I think that line of code is beautiful 😉