That makes sense.

Thanks all!


2013/7/24 David Carlson <dcarlson@tamu.edu>

> Benchmark is probably a subset from a larger dataframe. R does
> not automatically remove empty levels but you can do it:
>
> set.seed(42)
> dataset <- data.frame(Benchmark=factor(sample(LETTERS[1:26],
> 50,
>     replace=TRUE), levels=LETTERS[1:26]))
> levels(dataset$Benchmark)
> # [1] "A" "B" "C" "D" "E" "F" "G" "H" "I" "J" "K" "L" "M" "N"
> "O" "P" "Q" "R" "S"
> # [20] "T" "U" "V" "W" "X" "Y" "Z"
> dataset$Benchmark <- factor(dataset$Benchmark)
> levels(dataset$Benchmark)
> # [1] "A" "C" "D" "F" "G" "H" "J" "K" "L" "M" "N" "O" "P" "Q"
> "R" "S" "T" "V" "X"
> # [20] "Y" "Z"
>
> There are times when you want to know if certain factor levels
> do not appear in a subset of the original data.
>
> -------------------------------------
> David L Carlson
> Associate Professor of Anthropology
> Texas A&M University
> College Station, TX 77840-4352
>
> ----Original Message-----
> From: r-help-bounces@r-project.org
> [mailto:r-help-bounces@r-project.org] On Behalf Of Borja
> Rivier
> Sent: Wednesday, July 24, 2013 8:25 AM
> To: r-help@r-project.org
> Subject: [R] Levels of a factor
>
> Hi all,
>
> I am having a bit of trouble using the levels() function.
> I have a factor with many elements, and when I use the
> function levels() to
> extract the list of unique elements, some of the elements
> returned are not
> actually in the factor.
>
> For example I would have this:
>
> > vector <- dataset$Benchmark
> > class(vector)
> [1] "factor"
> > length(vector)
> [1] 35615
> > vector2 <- levels(vector)
> > length(which(!(vector2 %in% vector)))
> [1] 235
>
> Does anyone know how this is possible?
>
> Many thanks!
>
> Borja
>
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>
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>

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