In R, how do I create a long frequency table for every unique combination of the other columns

Question

Here are the first few rows of my table...

enter image description here

In R, how do I create a long table where Col A and Col B show every possible combination and Col C contains either a or b (or c or d or whatever its value might be), and Col D shows the frequency of Col C value for each combination of Col A and B.

Using the example above, the results should look like this (showing just the first few rows)...

enter image description here

I tried combinations of melt() and table() with little success.

Answer 1

as.data.frame.table(xtabs(~ ColA + ColB + ColC, dat))
#   ColA ColB ColC Freq
# 1    1    1    a    2
# 2    2    1    a    0
# 3    1    2    a    1
# 4    2    2    a    0
# 5    1    1    b    1
# 6    2    1    b    2
# 7    1    2    b    1
# 8    2    2    b    1

---

Data

dat <- structure(list(ColA = c(1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L), ColB = c(1L, 1L, 1L, 2L, 2L, 1L, 1L, 2L), ColC = c("a", "a", "b", "b", "a", "b", "b", "b")), class = "data.frame", row.names = c(NA, -8L))