[R] drop unused levels in subset.data.frame

[Marc Schwartz]

On Nov 10, 2009, at 9:49 AM, baptiste auguie wrote:

> Dear list,
>
> subset has a 'drop' argument that I had often mistaken for the one in
> [.factor which removes unused levels.
> Clearly it doesn't work that way, as shown below,
>
> d <- data.frame(x = factor(letters[1:15]), y = factor(LETTERS[1:3]))
> s <- subset(d, y=="A", drop=TRUE)
> str(s)
> 'data.frame':	5 obs. of  2 variables:
> $ x: Factor w/ 15 levels "a","b","c","d",..: 1 4 7 10 13
> $ y: Factor w/ 3 levels "A","B","C": 1 1 1 1 1
>
> The subset still retains all the unused factor levels. I wonder how
> people usually get rid of all unused levels in a data.frame after
> subsetting? I came up with this but I may have missed a better
> built-in solution,
>
> dropit <- function (d, columns = names(d), ...)
> {
>    d[columns] = lapply(d[columns], "[", drop=TRUE, ...)
>    d
> }
>
> str(dropit(s))
> 'data.frame':	5 obs. of  2 variables:
> $ x: Factor w/ 5 levels "a","d","g","j",..: 1 2 3 4 5
> $ y: Factor w/ 1 level "A": 1 1 1 1 1

There is a page in the R wiki here:

   http://wiki.r-project.org/rwiki/doku.php?id=tips:data-manip:drop_unused_levels

that has some approaches.

HTH,

Marc Schwartz