library(data.table)
(dt <- data.table(id = as.numeric(1:5),
                 x = c(NA, "ab.cde", "co.hij.ab", "cox.cde.kl", NA)))

dcast(dt, id ~ x, value.var = "id")
dt[dcast(dt, id ~ x, value.var = "id"), on=.(id = id)]

dt[, unique(unlist(strsplit(dt$x, ".", fixed = TRUE))) :=
tstrsplit(dt$x, ".", fixed = TRUE)]

dt[, unique(unlist(strsplit(dt$x, ".", fixed = TRUE))) := character()]