library
(
"data.table"
)
data <-
data.table
( items=
c
(
"chocos"
,
"milk"
,
"drinks"
,
"drinks"
,
"milk"
,
"milk"
,
"chocos"
,
"milk"
,
"honey"
,
"honey"
),
weight=
c
(10,20,34,23,12,45,23,
12,34,34),
cost=
c
(120,345,567,324,112,345,
678,100,45,67))
print
(data[,
lapply
(.SD, sum), by = items])
print
(data[,
lapply
(.SD, mean), by = items])