How to get the row numbers n days prior to last observation in R?

Question

I have a big data-set with over 1000 subjects, a small piece of the data-set looks like:

mydata <- read.table(header=TRUE, text="
  Id   DAYS   QS   Event 
  01    50    1      1
  01    57    4      1
  01    70    1      1
  01    78    2      1
  01    85    3      1
  02    70    2      1
  02    92    4      1
  02    98    5      1
  02   105    6      1
  02   106    7      0
")

I would like to get row number of the observation 28 or more days prior to last observation, eg. for id=01; last observation is 85 minus 28 would be 57 which is row number 2. For id=02; last observation is 106 minus 28; 78 and because 78 does not exist, we will use row number of 70 which is 1 (I will be getting the row number for each observation separately) or first observation for id=02.

Answer 1

This should work:

mydata %>% group_by(Id) %>% 
  mutate(row_number = last(which(DAYS <= max(DAYS) - 28)))

# A tibble: 10 x 6
# Groups:   Id [2]
      Id  DAYS    QS Event   max row_number
   <int> <int> <int> <int> <dbl>      <int>
 1     1    50     1     1    57          2
 2     1    57     4     1    57          2
 3     1    70     1     1    57          2
 4     1    78     2     1    57          2
 5     1    85     3     1    57          2
 6     2    70     2     1    78          1
 7     2    92     4     1    78          1
 8     2    98     5     1    78          1
 9     2   105     6     1    78          1
10     2   106     7     0    78          1