Question

There is an interesting option drop = TRUE in data.frame filtering, see excerpt from help('[.data.frame'):

Usage

S3 method for class ‘data.frame’

x[i, j, drop = ]

But when I try it on data.frame, it doesn’t work!

> df = data.frame(a = c("europe", "asia", "oceania"), b = c(1, 2, 3))
>
> df[1:2,, drop = TRUE]$a
[1] europe asia  
Levels: asia europe oceania     <--- oceania shouldn't be here!!
>

I know there are other ways like

df2 <- droplevels(df[1:2,])

but the documentation promised much more elegant way to do this, so why it doesn’t work? Is it a bug? Because I don’t understand how this could be a feature…

EDIT: I was confused by drop = TRUE dropping factor levels for vectors, as you can see here. It is not very intuitive that [i, drop = TRUE] drops factor levels and [i, j, drop = TRUE] does not!!

Answer 1

The documentation clearly states:

drop : logical. If TRUE the result is coerced to the lowest possible
dimension. The default is to drop if only one column is left, but not
to drop if only one row is left.

This means that if drop = TRUE and the filtered data.frame results in a single column or row, the result is coerced to a vector/list instead of returning a single-column/single-row data.frame.

Therefore, this argument has no relation with levels dropping, and so the right way to eliminate exceeding levels is the one you mentioned (i.e. using droplevels function).