I have a deal dataframe with three columns and I have sorted by the type and date, It looks like:
type date price
A 2020-05-01 4
A 2020-06-04 6
A 2020-06-08 8
A 2020-07-03 5
B 2020-02-01 3
B 2020-04-02 4
There are many types (A, B, C,D,E…), I want to calculate the previous mean price of the same type of product. For example: the pre_mean_price value of third row A is (4+6)/2=5. I want to get a dataframe like this:
type date price pre_mean_price
A 2020-05-01 4 .
A 2020-06-04 6 4
A 2020-06-08 8 5
A 2020-07-03 5 6
B 2020-02-01 3 .
B 2020-04-02 4 3
How can I calculate the pre_mean_price? Thanks a lot!
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