Q1:

The generic predict() function dispatches to a method according to class
name, in this case 'usrData'. If we also had a predict.usrData1(), it
would not get called.

Q2:

library(rectools)
getInstEval()
print(sum(ivl$d == 36))

Q3:

73421 X 3

Q4:

meanActivity <- function(db) {
   tmp <- sapply(db, function(dbElement) length(dbElement$itms))
   mean(tmp)
}

library(rectools) 
d <- data.frame(uID=c(1,2,1),iID=c(5,1,8),rat=c(3,3,5)) 
db <- formUserData(d)
print(meanActivity(db))  # should be 1.5