Use ?split()
split(dat[,-4], dat$Year_Month) #dat is the dataset.
A.K.
?? Country? Product?? Price? Year_Month
???? AE???????? 1?????????? 20??? 201204
???? DE???????? 1?????????? 20??? 201204
???? CN???????? 1?????????? 28??? 201204
???? AE???????? 2?????????? 28??? 201204
???? DE???????? 2?????????? 28??? 201204
???? CN???????? 2?????????? 22??? 201204
???? AE???????? 3?????????? 28??? 201204
???? CN???????? 3?????????? 28??? 201204
???? AE???????? 1?????????? 20??? 201205
???? DE???????? 1?????????? 20??? 201205
???? CN???????? 1?????????? 28??? 201205
???? AE???????? 2?????????? 28??? 201205
???? DE???????? 2?????????? 28??? 201205
How to create the list? which has:
[[201204]]
? Country? Product?? Price?
???? AE???????? 1?????????? 20???
???? DE???????? 1?????????? 20?
???? CN???????? 1?????????? 28???
???? AE???????? 2?????????? 28???
???? DE???????? 2?????????? 28???
???? CN???????? 2?????????? 22???
???? AE???????? 3?????????? 28?
???? CN???????? 3?????????? 28???
[[201205]]
? Country? Product?? Price?
???? AE???????? 1?????????? 20?
???? DE???????? 1?????????? 20?
???? CN???????? 1?????????? 28???
???? AE???????? 2?????????? 28???
???? DE???????? 2?????????? 28?