How to drop factor levels in a subsetted data frame

Question

Consider a data frame containing a factor.

When I create a subset using subset() or any other indexing function, then a new data frame is created.

I have observed that the factor variable retains all of its original levels, even if they do not exist in the new data frame.

This creates problems while plotting or using the functions that rely on factor levels.

Is there any way to remove levels from a factor in the new data frame i.e. the data frame I have taken a subset of

Below is my example:

data <- data.frame(letters=letters[1:10],
                    numbers=seq(1:10))

levels(data$letters)
## [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j"

subdata <- subset(data, numbers <= 5)
##   letters numbers
## 1       a       1
## 2       b       2
## 3       c       3
## 4       d       4
## 5       e       5 
## But the  levels are still there!
levels(subdata$letters)
## [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j"

Answer 1

You can use factor(ff) to drop levels that do not occur

factor(ff)      
# drops the levels that do not occur

For dropping levels from all factor columns in a data frame, you can use:

subdata <- subset(data, numbers <= 5)

##   letters numbers
## 1       a       1
## 2       b       2
## 3       c       3 
## 4       d       4
## 5       e       5 

subdata[] <- lapply(subdata, function(x) if(is.factor(x)) factor(x) else x)

NOTE: You can use the above command in a single dataframe with minimal columns. But, if you have large number of columns then you have to use drop.levels() from gdata.