[R] persistance of factor levels in a data frame

[Lefebure Tristan]

Hi,
Just something I don't understand:

data <- data.frame(V1=c(1:12),F1=c(rep("a",4),rep("b",4),rep("c",4)))
data_ac <- data[which(data$F1 !="b"), ]  
levels(data_ac$F1)    

Why the level "b" is always present ?

thanks

Tristan, R 2.0.1 for Linux Fedora 3

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Tristan LEFEBURE
Laboratoire d'écologie des hydrosystèmes fluviaux (UMR 5023)
Université Lyon I - Campus de la Doua
Bat. Darwin C 69622 Villeurbanne - France

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