Why doesn't subset() work with a logical and && operator combining two conditions?
> subset(tt, (customer_id==177 && visit_date=="2010-08-26"))
<0 rows> (or 0-length row.names)
but they each work individually:
> subset(tt, customer_id==177)
> subset(tt, visit_date=="2010-08-26")
(Want to avoid using large temporary variables - my dataset is huge)