How to find the last occurrence of a certain observation in grouped data in R?

Question

I have data that is grouped using dplyr in R. I would like to find the last occurrence of observations (\'B\') equal to or greater than 1 (1, 2, 3 or 4) in each group (\'A\'

Answer 1

We can try

library(data.table)
setDT(df1)[, last := day[tail(which(B>=1),1)] , A]
df1
#    A B day last
# 1: a 2   1    5
# 2: a 2   2    5
# 3: a 1   5    5
# 4: a 0   8    5
# 5: b 3   1    6
# 6: b 3   4    6
# 7: b 3   6    6
# 8: b 0   7    6
# 9: b 0   9    6
#10: c 1   2    4
#11: c 1   3    4
#12: c 1   4    4

---

Or using dplyr

library(dplyr)
df1 %>%
   group_by(A) %>%
   mutate(last = day[max(which(B>=1))])

Or use the last function from dplyr (as @docendo discimus suggested)

df1 %>%
   group_by(A) %>%
   mutate(last= last(day[B>=1]))

---

For the second question,

setDT(df1)[, dayafter:= if(all(!!B)) NA_integer_  else 
             day[max(which(B!=0))+1L] , A]
#    A B day dayafter
# 1: a 2   1        8
# 2: a 2   2        8
# 3: a 1   5        8
# 4: a 0   8        8
# 5: b 3   1        7
# 6: b 3   4        7
# 7: b 3   6        7
# 8: b 0   7        7
# 9: b 0   9        7
#10: c 1   2       NA
#11: c 1   3       NA
#12: c 1   4       NA