Replace NA values with a factor level, use multiple conditions with filter function

gistfile1.txt

df_subset <- df %>% 
  filter(spell.type == "Emergency" &  sex != "Not Specified", 
         end_datetime >= "2012-09-29" & end_datetime <= "2013-09-29") %>%
  mutate(as.Date(end_datetime)) %>%
  replace_na(list(patient.class = "Not_Admitted")) %>%
  select(pat_code, end_datetime, age_band, sex, patient.class, spell.type)