Extract the factor's values positions in level

Question

I'm returning to R after some time, and the following has me stumped:

I'd like to build a list of the positions factor values have in the facor levels list. Example:

> data = c("a", "b", "a","a","c")
> fdata = factor(data)
> fdata
[1] a b a a c
Levels: a b c
> fdata$lvl_idx <- ????

Such that:

> fdata$lvl_idx
[1] 1 2 1 1 3

Appreciate any hints or tips.

Answer 1

The solution provided years ago by Matthew Lundberg is not robust. It could be that the as.integer() function was defined for a specific S3 type of factors. Imagine someone would create a new factor class to keep operators like >=.

as.myfactor <- function(x, ...) {
  structure(as.factor(x), class = c("myfactor", "factor"))
}

# and that someone would create an S3 method for integers - it should
# only remove the operators, which makes sense...
as.integer.myfactor <- function(x, ...) {
  as.integer(gsub("(<|=|>)+", "", as.character(x)))
}

Now this is not working anymore, - it just removes operators:

f <- as.myfactor(">=2")
as.integer(f)
#> [1] 2

But this is robust with any factor you want to know the index of the level of, using which():

f <- factor(2:4)
which(levels(f) == 2)
#> [1] 1

Answer 2

If you convert a factor to integer, you get the position in the levels:

as.integer(fdata)
## [1] 1 2 1 1 3

In certain situations, this is counter-intuitive:

f <- factor(2:4)
f
## [1] 2 3 4
## Levels: 2 3 4
as.integer(f)
## [1] 1 2 3

Also if you silently coerce to integer, for example by using a factor as a vector index:

LETTERS[2:4]
## [1] "B" "C" "D"
LETTERS[f]
## [1] "A" "B" "C"

Converting to character before converting to integer gives the expected values. See ?factor for details.