R help - Dec 2007 - adjusting "levels" after subset a table

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On Sat, 2007-12-08 at 19:26 -0800, Milton Cezar Ribeiro
wrote:
> Dear all,
> 
> I have a data.frame with a factor collumn with about 10 levels.
> After extract a subset of this data.frame, by selecting 2 of my 10
> levels, the new data.frame continue with original number of levels.
> How can I adjust it in a manner that when I try levels(my.df) I
> receive the actualyzed number of levels?
> 
> By the way, I read my file using reab.table.
> 
> I tryed solve it with :  levels(my.df$my.var)<-unique(my.df$my.var)
> but the problem remain.
> 
> Many thanks,
> 
> miltinho
> Brazil
The default when subsetting factors (which happens when you subset a
data frame) is to retain the original set of levels, even if they don't
occur in the resultant subset. This is described in ?"[.factor" where
the 'drop' argument is FALSE by default.

To subset the factor and only retain levels for those values that are
still present, you can use:

  MyFactor <- factor(MyFactor)

or

  MyFactor <- MyFactor[, drop = TRUE]

after subsetting the data frame.

There is also a page in the R Wiki that describes some additional
approaches:

http://wiki.r-project.org/rwiki/doku.php?id=tips:data-manip:drop_unused_levels

HTH,

Marc Schwartz

running
my.df$my.var <- factor(my.df$my.var)

seems to do the job

P.J.

On Sat, 8 Dec 2007, Milton Cezar Ribeiro wrote:
> Dear all,
>
> I have a data.frame with a factor collumn with about 10 levels.
> After extract a subset of this data.frame, by selecting 2 of my 10 levels,
the new data.frame continue with original number of levels. How can I adjust it
in a manner that when I try levels(my.df) I receive the actualyzed number of
levels?
>
> By the way, I read my file using reab.table.
>
> I tryed solve it with :  levels(my.df$my.var)<-unique(my.df$my.var) but
the problem remain.
>
> Many thanks,
>
> miltinho
> Brazil
>
>
>
>  para armazenamento!
>
> 	[[alternative HTML version deleted]]
>
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> PLEASE do read the posting guide
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> and provide commented, minimal, self-contained, reproducible code.
>
Paulo Justiniano Ribeiro Jr
LEG (Laboratorio de Estatistica e Geoinformacao)
Universidade Federal do Parana
Caixa Postal 19.081
CEP 81.531-990
Curitiba, PR  -  Brasil
Tel: (+55) 41 3361 3573
Fax: (+55) 41 3361 3141
e-mail: paulojus AT  ufpr  br
http://www.leg.ufpr.br/~paulojus

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