[R] levels update

[Prof Brian Ripley]

On Fri, 5 Dec 2008, jim holtman wrote:

> try this:
>
>> df <- data.frame(factor(c("a","a","c","b","b")), c(4,5,6,7,8), c(9,1,2,3,4))
>> names(df) <- c("X1","X2","X3")
>>
>> my.sub <- subset(df, X1 == "a" | X1 == "b")
>> levels(my.sub$X1)
> [1] "a" "b" "c"
>> my.sub$X1 <- factor(my.sub$X1)

I find

my.sub$X1 <- my.sub$X1[drop=TRUE]

a lot more self-explanatory.  See ?"[.factor".  However, if you find 
yourself wanting to do this, ask why you have a factor (rather than a 
character vector) in the first place.


>> levels(my.sub$X1)
> [1] "a" "b"
>>
>
>
> On Fri, Dec 5, 2008 at 7:50 AM, Antje <niederlein-rstat at yahoo.de> wrote:
>> Hello,
>>
>> I hope this question is not too stupid. I would like to know how to update
>> levels after subsetting data from a data.frame.
>>
>> df <- data.frame(factor(c("a","a","c","b","b")), c(4,5,6,7,8), c(9,1,2,3,4))
>> names(df) <- c("X1","X2","X3")
>>
>> my.sub <- subset(df, X1 == "a" | X1 == "b")
>> levels(my.sub$X1)
>>
>> # still gives me "a","b","c", though the subset does not contain entries
>> with "c" anymore
>>
>> I guess, the solution is rather simple, but I cannot find it.
>>
>> Antje
>>
>> ______________________________________________
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>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>
>
>
> -- 
> Jim Holtman
> Cincinnati, OH
> +1 513 646 9390
>
> What is the problem that you are trying to solve?
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595