replace NA value with the group value

Question

I have a df as follows which has 20 people across 5 households. Some people within the household have missing data for whether they have a med_card or not. I want to give th

Answer 1

data.table solution

library(data.table)
setDT(df)[, med_card2 := unique(med_card[!is.na(med_card)]), by = hhold_no]

#     person_id hhold_no med_card med_card2
#  1:         1        1        1         1
#  2:         2        1        1         1
#  3:         3        1       NA         1
#  4:         4        1       NA         1
#  5:         5        1       NA         1
#  6:         6        2        0         0
#  7:         7        2        0         0
#  8:         8        2        0         0
#  9:         9        2        0         0
# 10:        10        3       NA         1
# 11:        11        3       NA         1
# 12:        12        3       NA         1
# 13:        13        3        1         1
# 14:        14        3        1         1
# 15:        15        4        1         1
# 16:        16        4        1         1
# 17:        17        5        1         1
# 18:        18        5        1         1
# 19:        19        5       NA         1
# 20:        20        5       NA         1