Combining factor level in R

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

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)