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# 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"