ddply to count frequency of combinations

Here's a non plyr approach:

tab.df <- as.data.frame(table(df))
tab.df[tab.df$Freq > 0, ]

? x y Freq
1 ?1 1 ? ?2
4 ?4 1 ? ?1
7 ?2 2 ? ?2
13 3 3 ? ?1
18 3 4 ? ?1
19 4 4 ? ?1
25 5 5 ? ?1

But look at str(tab.df) - x and y are now factors (or characters).  I
wrote count to avoid this problem.

Also compare:

table(mtcars)
# Error in table(mtcars) : attempt to make a table with >= 2^31 elements

count(mtcars)
#     mpg cyl  disp  hp drat    wt  qsec vs am gear carb freq
# 1  10.4   8 460.0 215 3.00 5.424 17.82  0  0    3    4    1
# 2  10.4   8 472.0 205 2.93 5.250 17.98  0  0    3    4    1
# ...

Hadley

Assistant Professor / Dobelman Family Junior Chair
Department of Statistics / Rice University
http://had.co.nz/