# [R] drop levels problem

Felipe Carrillo mazatlanmexico at yahoo.com
Mon Nov 29 20:26:35 CET 2010

```Thanks Joshua, I get it now, levels sometimes drive me loco....

Felipe D. Carrillo
Supervisory Fishery Biologist
Department of the Interior
US Fish & Wildlife Service
California, USA

----- Original Message ----
> From: Joshua Wiley <jwiley.psych at gmail.com>
> To: Felipe Carrillo <mazatlanmexico at yahoo.com>
> Cc: r-help at stat.math.ethz.ch
> Sent: Mon, November 29, 2010 11:18:45 AM
> Subject: Re: [R] drop levels problem
>
> Hi Felipe,
>
> On Mon, Nov 29, 2010 at 11:01 AM, Felipe Carrillo
> <mazatlanmexico at yahoo.com> wrote:
> > Hi all:
> > I am having trouble dropping levels, got a few hints online without success.
> > Please consider the dataset below:
> >  I was under the inpression that subset(......drop=TRUE) would work but it
> > doesn't
>
> Here drop is referring to:
>
> data.frame(1:10)[, 1]
> data.frame(1:10)[, 1, drop = FALSE]
>
> not to levels of a factor.
>
> >
> > library(ggplot2)
> >     library(hmisc)
> >
> > x <- structure(list(first = c(38.2086, 43.1768, 43.146, 41.8044, 42.4232,
> > 46.3646, 38.0813, 40.0745, 40.4889, 38.6246, 40.2826, 41.6056,
> > 34.5353, 40.0768), second = c(43.3295, 42.4326, 38.8994, 37.0894,
> > 42.3218, 46.1726, 39.1206, 41.2072, 42.4874, 40.2657, 38.7766,
> > 40.8822, 42.0165, 49.2055), third = c(42.24, 42.992, 37.7419,
> > 42.3448, 41.9131, 44.385, 42.7811, 44.1963, 40.8088, 43.9634,
> > 38.7079, 38.0791, 44.3136, 39.5333)), .Names = c("first", "second",
> > "third"), class = "data.frame", row.names = c(NA, -14L))
>
> Thanks for the nice example!
>
> >
> > xmelt <- melt(x)
> >  names(xmelt) <- c("year","fatPerc")
> >
> >   # Year variable is a factor with three levels
> >  # Subset to plot only 'first' year
> > firstyear <- subset(xmelt,year=='first');str(firstyear)
> > # Plot showing three levels still after I made the subset
> >   ggplot(firstyear,aes(year,fatPerc)) + geom_boxplot() + geom_jitter()
>
> right, because it is possible to have levels of a factor that have no
> observations---sometimes these are the most interesting (e.g., if you
> subset by smoking and found that there were no instances of lung
> cancer in non-smokers (not that extreme, but you get the point)).
>
> >
> > # Try to drop the levels but dropUnusedLevels() doesn't seem to work here
> >   dropUnusedLevels()
>
> sorry, I have had some difficulty installing Hmisc on my linux system
> and never gotten around to working it out.
>
> > ggplot(firstyear,aes(year,fatPerc)) + geom_boxplot() + geom_jitter()
> >
> > # code below also should drop levels but it doesn't
> > #data.frame(lapply(firstyear, function(x) if (is.factor(x)){ factor(x)}
> > else{x}))
>
> it would if you assigned it back to firstyear.  You do it, and then
> just print to screen and the changed data goes off to oblivion.
>
> firstyear <- data.frame(lapply(firstyear, function(x) if(is.factor(x))
> {factor(x)} else {x}))
> str(firstyear) # should now just have one level
>
> Cheers,
>
> Josh
>
> > str(firstyear)
> >
> > Felipe D. Carrillo
> > Supervisory Fishery Biologist
> > Department of the Interior
> > US Fish & Wildlife Service
> > California, USA
> >
> >
> >
> >
> > ______________________________________________
> > R-help at r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > and provide commented, minimal, self-contained, reproducible code.
>
>
> --
> Joshua Wiley
> Ph.D. Student, Health Psychology
> University of California, Los Angeles
> http://www.joshuawiley.com/
>

```