Create Ranking based on Dates(Annual) in R - r

Question

We will start with the following DataTable:
id date
1: 1 2015-12-31
2: 1 2014-12-31
3: 1 2013-12-31
4: 1 2012-12-31
5: 1 2011-12-31
6: 2 2015-12-31
7: 2 2014-12-31
8: 2 2014-01-25
9: 2 2013-01-25
10: 2 2012-01-25
library(data.table)
DT <- data.table(c(1, 1, 1, 1, 1, 2, 2, 2, 2, 2),
as.IDate(c("2015-12-31", "2014-12-31", "2013-12-31", "2012-12-31",
"2011-12-31", "2015-12-31", "2014-12-31", "2014-01-25",
"2013-01-25", "2012-01-25")))
setnames(DT, c("id", "date"))
For every unique id, I want to create a ranking. The most current date for a specific id should have a rank of 0. After I should remove one year from that date to get the ranking -1. If the month is not the same as the date of rank 0, we should stop the ranking. For example, at the line 8, for the id=2, since the month is not december we should stop the ranking.
We would get the following result:
id date rank_year
1: 1 2015-12-31 0
2: 1 2014-12-31 -1
3: 1 2013-12-31 -2
4: 1 2012-12-31 -3
5: 1 2011-12-31 -4
6: 2 2015-12-31 0
7: 2 2014-12-31 -1
8: 2 2014-01-25 NA
9: 2 2013-01-25 NA
10: 2 2012-01-25 NA
I have the following code so far (given by #Frank and #akrun) :
DT <- DT[order(id, -date)]
DT <- DT[,rank_year := { z = month(date) + year(date)*12
as.integer( (z - z[1L])/12) # 12 months
}, by = id]
id date rank_year
1: 1 2015-12-31 0
2: 1 2014-12-31 -1
3: 1 2013-12-31 -2
4: 1 2012-12-31 -3
5: 1 2011-12-31 -4
6: 2 2015-12-31 0
7: 2 2014-12-31 -1
8: 2 2014-01-25 -1
9: 2 2013-01-25 -2
10: 2 2012-01-25 -3

Answer 1

Ok, I guess I would do it like
DT[, rank_year := replace(
year(date) - year(date)[1L],
month(date) != month(date[1L]),
NA_integer_
), by=id]
id date rank_year
1: 1 2015-12-31 0
2: 1 2014-12-31 -1
3: 1 2013-12-31 -2
4: 1 2012-12-31 -3
5: 1 2011-12-31 -4
6: 2 2015-12-31 0
7: 2 2014-12-31 -1
8: 2 2014-01-25 NA
9: 2 2013-01-25 NA
10: 2 2012-01-25 NA
See ?replace for details on how this works.
One way of extending the old answer is
DT[, r := {
z = month(date) + year(date)*12
res = (z - z[1L])/12
as.integer( replace(res, res %% 1 != 0, NA) )
}, by=id]