repeated searching of no-missing values

dear Hadley and Bert,
thank you very much for your suggestions. I asked one question and I
learned 2 things:

1. Hadley,

library(plyr)
ddply(data, .(V1), colwise(cl))

that is exactly what I was searching for.

2. Bert,

?tapply says that the first argument is an **atomic** vector. A
factor is not an atomic vector. So tapply interprets it as such by looking
only at its representation, which is as integer values.

I admit I payed not enough attention to the definition of **atomic** vector.
That implies a deeper understanding of structures of data. I'm working with!

Many thanks,

Patrizio