add missing rows to a data table

Question

I have a data table:

library(data.table)
(f <- data.table(id1=c(1,2,3,1,2,3),
                 id2=as.factor(c(\"a\",\"a\",\"b\",\"c\",\"b\",\"d\")),
             


        

Answer 1

f[, {
  tab = table(id2)
  x = as.numeric(tab)
  x[x != 0] = v
  list(id2 = names(tab), v = x)
}, by = id1]

##    id1 id2 v
##  1:   1   a 1
##  2:   1   b 0
##  3:   1   c 4
##  4:   1   d 0
##  5:   2   a 2
##  6:   2   b 5
##  7:   2   c 0
##  8:   2   d 0
##  9:   3   a 0
## 10:   3   b 3
## 11:   3   c 0
## 12:   3   d 6

Answer 2

I'd get the unique values in id1 and id2 and do a join using data.table's cross join function CJ as follows:

# if you've already set the key:
ans <- f[CJ(id1, id2, unique=TRUE)][is.na(v), v := 0L][]

# or, if f is not keyed:
ans <- f[CJ(id1 = id1, id2 = id2, unique=TRUE), on=.(id1, id2)][is.na(v), v := 0L][]

ans