[R] levels of factor when subsetting the factor

[Sundar Dorai-Raj]

Yes. I do this periodically:

dat.new <- dat[1:6, ]
dat.new[] <- lapply(dat.new, function(x)
                     if(is.factor(x)) factor(x) else x)

HTH,

--sundar

Afshartous, David said the following on 9/12/2006 11:00 AM:
> thanks to all for the quick replies!
> 
> if the factor is part of a dataframe, I can apply the subsetting
> to the entire dataframe, and then use drop=True to the factor
> separately and then put it back into the new dataframe (code below).  is there a way
> to do this in a single step? 
> 
> dat <-data.frame(fact = as.factor(c(rep("A", 3),rep("B", 3), rep("C", 3))),Y = rnorm(9))
> dat.new = dat[1:6, ]
> dat.new$fact = dat$fact[1:6, drop = T]
> 
> 
>  
> 
> -----Original Message-----
> From: pd at pubhealth.ku.dk [mailto:pd at pubhealth.ku.dk] On Behalf Of Peter Dalgaard
> Sent: Tuesday, September 12, 2006 11:45 AM
> To: Afshartous, David
> Cc: r-help at stat.math.ethz.ch
> Subject: Re: [R] levels of factor when subsetting the factor
> 
> "Afshartous, David" <afshart at exchange.sba.miami.edu> writes:
> 
>>  
>> All,
>>
>> When I take a subset of a factor the reduced factor still maintains 
>> all the original levels of the factor when say forming the key in a plot.
>> The data is correct, but the variable still "remembers" the original 
>> levels.  See below for reproducible code.  Does anyone know how to fix 
>> this?
>> cheers,
>> dave
>>
>> fact = as.factor(c(rep("A", 3),rep("B", 3), rep("C", 3))) new.fact = 
>> fact[1:6]
>>> new.fact
>> [1] A A A B B B
>> Levels: A B C    ## should only show A B
> 
> Just use
> 
>> factor(new.fact)
> [1] A A A B B B
> Levels: A B
> 
> or
> 
>> fact[1:6, drop=T]
> [1] A A A B B B
> Levels: A B
> 
> 
> And, no, it is not a bug. The fact that a subsample happens to consist only of males does not turn gender into a one-level factor... (Apart from the philosophy, it makes a real difference in tabulation.) 
> 
>