omit empty cells in crosstab?

sjaffe <sjaffe <at> riskspan.com> writes:

I have data with many factors, each taking many values. However, only
relatively few combinations appear in the data, ie have nonzero counts, in
other words the resulting table is sparse. Say we have 10 factors each with
10 levels. The result of table() would exceed the memory space (on a 32bit
machine). Is there any way to produce a table with empty cells omitted?
(without first producing the whole table and then removing rows.)

It would be easier if you had a reproducible base example, but I 
suggest to create ONE new factor of the pasted levels using unique(), 
and  creating a table of these.