[R] Using lapply in R data table

[Frank S.]

Dear all,

I have a R data table like this:

DT <- data.table(
  id = rep(c(2, 5, 7), c(3, 2, 2)),
  fini = rep(as.Date(c('2005-04-20', '2006-02-19', '2006-10-08')), c(3, 2, 2)),
  group = rep(c("A", "B", "A"), c(3, 2, 2))  )


I want to construct a new variable "exposure" defined as follows:

1) If "fini" earlier than 2006-01-01 --> "exposure" = 1
2) If "fini" in [2006-01-01, 2006-06-30] --> "exposure" = "2007-01-01" - "fini"
3) If "fini" in [2006-07-01, 2006-12-31] --> "exposure" = 0.5


So the desired output would be the following data table:

   id                fini exposure group
1:  2 2005-04-20        1.00        A
2:  2 2005-04-20        1.00        A
3:  2 2005-04-20        1.00        A
4:  5 2006-02-19        0.87        B
5:  5 2006-02-19        0.87        B
6:  7 2006-10-08        0.50        A
7:  7 2006-10-08        0.50        A


I have tried:

DT <- DT[ , list(id, fini, exposure = 0, group)]
DT.new <- lapply(DT, function(exposure){
      exposure[fini < as.Date("2006-01-01")] <- 1   # 1st case
      exposure[fini >= as.Date("2006-01-01") & fini <= as.Date("2006-06-30")] <- difftime(as.Date("2007-01-01"), fini, units="days")/365.25 # 2nd case
    exposure[fini >= as.Date("2006-07-01") & fini <= as.Date("2006-12-31")] <- 0.5       # 3rd case
      exposure  # return value
  })


But I get an error message.

Thanks for any help!!


Frank S.


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