[R] persistance of factor levels in a data frame

[Peter Dalgaard]

Lefebure Tristan <Tristan.Lefebure at univ-lyon1.fr> writes:

> 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 ?

Because it is a property of the definition, not of the data. E.g. if
you tabulate it, you generally want to get a zero entry if there are
no "b"s in the data. If, for some reason, you want to reduce the
factor to only those levels that are present, factor() gets you there
soon enough:

>  levels(factor(data_ac$F1))
[1] "a" "c"


-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
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