Change factor levels conditionally using dplyr and forcats

forcats_ifelse_mutate.R

library(dplyr)
library(forcats)

# encode the 'Month' column of the 'airquality' dataset as a factor
airquality2 <- airquality %>%
  dplyr::mutate(
    Month = factor(Month),
    Month = forcats::fct_recode(Month, May = "5", June = "6", July = "7",
                                August = "8", September = "9")
    )
levels(airquality2$Month)

# recode 'Month' as 'X' whenever the 'Day' column is even
airquality2 %>%
  dplyr::mutate(
    Month = dplyr::if_else(
    Day %% 2 == 0,
    # must return a factor
    factor("X", levels = c("X", levels(Month))),  # with the right levels
    factor(Month)
    ))