-egen- has all sorts of cool things for you to play with. In particular, whenever you're thinking about doing something that spans multiple columns or rows, -egen- is usually the preferred solution. It's especially useful in combination with the -by:- prefix.
* Want to sum across rows? egen poptotal = rsum(pop1-pop10)
* Want to figure out how many apples are in each household (assuming each row is a person and the apple variable contains the number of apples they own?
bysort householdID: egen applestotal = apples
egen tag = tag(householdID)
keep if tag == 1
keep householdID applestotal