# [R] drop levels problem

Joshua Wiley jwiley.psych at gmail.com
Mon Nov 29 20:18:45 CET 2010

```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
>
>
>
>
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