Get first instance of condition in longitudinal dataset otherwise last instance-CodePudding

ll <- data.table(ID=rep(1:3,each=3),
    num=c(1:9),
    var=c("a","b","c","a","b","b","a","c","c"))
setorder(ll,ID,num,var)

Now I´d like to retrieve the first instance of "c" if present, but if not present I´s like to retrieve the last instance of not "c".

  ll[,.SD[var=="c"][1],by=.(ID)]

gives me the first instance of "c", but how can I also get the last instance of not "c"?

Desired result:

      ID   num    var
   <int> <int> <char>
1:     1     3      c
2:     2     6      b
3:     3     8      c

CodePudding user response：

One way could be:

ll[, .SD[if (any(var == 'c')) var == 'c' & rowid(var) == 1L else .N], by = .(ID)]

Output:

   ID num var
1:  1   3   c
2:  2   6   b
3:  3   8   c