Hi,
I agree with you that levels should not be automatically dropped after
subsetting.
However, I think there should/can be an extra argument to make it
possible (the default being no dropping). I have no example in mind, but
I guess it is possible that sometimes, one want to show only some
levels. Would it be a bad approach?
Anyway, it is not that complicated to use factor() again.
Ivan
Le 9/21/2010 23:22, Greg Snow a écrit :
> This comes up every now and then. The fact is that the behavior of R in not
> throwing away information unless explicitly told to, is a feature, and one
> that I don't want to see go away.
>
> Yes in your example doing a table or plot based on iris1$Species gives
> meaningless results, but anything you do with that column in now meaningless,
> why do you care if there is extra information in a column that you should not
> be doing anything further with anyways? Does it really make sense to use
> that column for anything now? It is a bit like a teacher bemoaning the fact
> that half of his/her students scored below the class median.
>
> Now some proposes that all factors should have levels dropped after
> subsetting, this is worse than useless, consider the following made up
> example:
>
> tmp1<- rep( c(1:5,1:5), c(10,20,30,20,0,0,10,20,30,20) )
> result<- factor(tmp1, levels=1:5, labels=c('Strongly Disagree',
> 'Disagree', 'No Opinion', 'Agree', 'Strongly Agree') )
>
> my.df<- data.frame( result=result, sex = rep( c('M','F'), each=80 ) )
>
> df.m.2<- df.m.1<- my.df[ my.df$sex=='M', ]
> df.f.2<- df.f.1<- my.df[ my.df$sex=='F', ]
>
> df.m.1[]<- lapply( df.m.1, factor )
> df.f.1[]<- lapply( df.f.1, factor )
>
>
> dev.new()
> par(mfrow=c(2,1))
> barplot(table(df.m.1$result), main='Males')
> barplot(table(df.f.1$result), main='Females')
>
> dev.new()
> par(mfrow=c(2,1))
> barplot(table(df.m.2$result), main='Males')
> barplot(table(df.f.2$result), main='Females')
>
>
> Which pair of plots is more meaningful? Easier to read? Not misleading?
>
>
>
--
Ivan CALANDRA
PhD Student
University of Hamburg
Biozentrum Grindel und Zoologisches Museum
Abt. Säugetiere
Martin-Luther-King-Platz 3
D-20146 Hamburg, GERMANY
+49(0)40 42838 6231
ivan.calan...@uni-hamburg.de
**********
http://www.for771.uni-bonn.de
http://webapp5.rrz.uni-hamburg.de/mammals/eng/mitarbeiter.php
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