Using mutate and case_when to create variables based on other vars
# example creates a new var (era) based on year var
df = df %>% mutate(era = case_when(year < 2000 ~ "pre-millenium",
year >= 2000 ~ "post-millenium",
TRUE ~ "unknown"))
# TRUE ~ assigns anything left over that's not been accounted for with above rules to "unknown"