Counting non-empty levels of a factor

From: sylvain willart <sylvain.willart at gmail.com>
Subject: [R] Counting non-empty levels of a factor
To: r-help at r-project.org
Received: Sunday, November 8, 2009, 8:38 AM
Hi everyone,

I'm struggling with a little problem for a while, and I'm
wondering if
anyone could help...

I have a dataset (from retailing industry) that indicates
which brands
are present in a panel of 500 stores,

store , brand
1 , B1
1 , B2
1 , B3
2 , B1
2 , B3
3 , B2
3 , B3
3 , B4

I would like to know how many brands are present in each
store,

I tried:
result <- aggregate(MyData$brand , by=list(MyData$store)
, nlevels)

but I got:
Group.1 x
1 , 4
2 , 4
3 , 4

which is not exactly the result I expected
I would like to get sthg like:
Group.1 x
1 , 3
2 , 2
3 , 3

Looking around, I found I can delete empty levels of factor
using:
problem.factor <- problem.factor[,drop=TRUE]
But this solution isn't handy for me as I have many stores
and should
make a subset of my data for each store before dropping
empty factor

I can't either counting the line for each store (N),
because the same
brand can appear several times in each store (several
products for the
same brand, and/or several weeks of observation)

I used to do this calculation using SAS with:
proc freq data = MyData noprint ; by store ;
 tables? brand / out = result ;
run ;
(the cool thing was I got a database I can merge with
MyData)

any idea for doing that in R ?

Thanks in advance,

King Regards,

Sylvain Willart,
PhD Marketing,
IAE Lille, France