Combining factor level in R

Question

I would like combine level "A","B" into "A+B". I successfully did this by the following:

x <- factor(c("A","B","A","C","D","E","A","E","C"))
x
#[1] A B A C D E A E C
#Levels: A B C D E
l <- c("A+B","A+B","C","D+E","D+E")
factor(l[as.numeric(x)])
#[1] A+B A+B A+B C   D+E D+E A+B D+E C  
#Levels: A+B C D+E

Is there any more trivial way to do this? (ie more explainable function name such as combine.factor(f, old.levels, new.levels) would help to understand the code easier.)

Also, I try to find a well named function which probably work with data frame in dplyr package but no luck. The closest implementation is

df %>% mutate(x = factor(l[as.numeric(x)]))

Answer 1

This is now easy to do with fct_collapse() from the forcats package.

x <- factor(c("A","B","A","C","D","E","A","E","C"))

library(forcats)
fct_collapse(x, AB = c("A","B"), DE = c("D","E"))

#[1] AB AB AB C  DE DE AB DE C 
#Levels: AB C DE

Answer 2

One option is recode from car

library(car)
recode(x, "c('A', 'B')='A+B';c('D', 'E') = 'D+E'")
#[1] A+B A+B A+B C   D+E D+E A+B D+E C  
#Levels: A+B C D+E

It should also work with dplyr

library(dplyr)
df %>%
   mutate(x= recode(x, "c('A', 'B')='A+B';c('D', 'E') = 'D+E'"))
#    x
#1 A+B
#2 A+B
#3 A+B
#4   C
#5 D+E
#6 D+E
#7 A+B
#8 D+E
#9   C

data

df <- data.frame(x)

Answer 3

What about using ifelse() to create a new factor?

x = factor(c("A","B","A","C","D","E","A","E","C"))
# chained comparisons, a single '|' works on the whole vector
y = as.factor(
    ifelse(x=='A'|x=='B',
        'A+B',
        ifelse(x=='D'|x=='E','D+E','C')
    )
)
> y
[1] A+B A+B A+B C   D+E D+E A+B D+E C  
Levels: A+B C D+E

# using %in% to search
z = as.factor(
    ifelse(x %in% c('A','B'),
        'A+B',
        ifelse(x %in% c('D','E'),'D+E','C'))
)
> z
[1] A+B A+B A+B C   D+E D+E A+B D+E C  
Levels: A+B C D+E

If you don't want to hard-code in the factor level C above, or if you have more than one factor level that does not need to be combined, you can use the following.

# Added new factor levels
x = factor(c("A","B","A","C","D","E","A","E","C","New","Stuff","Here"))
w = as.factor(
    ifelse(x %in% c('A','B'),
        'A+B',
        ifelse(x %in% c('D','E'),
            'D+E',
            as.character(x) # without the cast it's numeric
        )
    )
)
> w
[1] A+B   A+B   A+B   C     D+E   D+E   A+B   D+E   C     New   Stuff Here
Levels: A+B C D+E Here New Stuff