Pandas：条件滚动 window 由另一个列元素？

Question

I have a dataframe with dates, id's and values.

For example:

date        id        value
2016-08-28   A          1
2016-08-28   B          1
2016-08-29   C          2
2016-09-02   B          0
2016-09-03   A          3
2016-09-06   C          1
2017-01-15   B          2
2017-01-18   C          3
2017-01-18   A          2

I want to apply a rolling mean by element, stating one after, so that the result would be like:

date        id        value    rolling_mean
2016-08-28   A          1           NaN
2016-08-28   B          1           NaN
2016-08-29   C          2           NaN
2016-09-02   B          0           0.5
2016-09-03   A          3           2.0
2016-09-06   C          1           1.5
2017-01-15   B          2           1.0
2017-01-18   C          3           2.0
2017-01-18   A          2           2.5

The closest I've come to this was:

grouped = df.groupby(["id", "value"])
df["rolling_mean"] = grouped["value"].shift(1).rolling(window = 2).mean()

But this gives me the wrong values back, as it keeps the order with the remaining elements.

Any ideia?

Thank you in advance,

Answer 1

You can just groupby id and use transform :

df['rolling_mean'] = df.groupby('id')['value'].transform(lambda x: x.rolling(2).mean())

Output:

         date id  value  rolling_mean
0  2016-08-28  A      1           NaN
1  2016-08-28  B      1           NaN
2  2016-08-29  C      2           NaN
3  2016-09-02  B      0           0.5
4  2016-09-03  A      3           2.0
5  2016-09-06  C      1           1.5
6  2017-01-15  B      2           1.0
7  2017-01-18  C      3           2.0
8  2017-01-18  A      2           2.5

Answer 2

Fix your code with groupby with id

grouped = df.groupby(["id"])
df['rolling_mean']=grouped["value"].rolling(window = 2).mean().reset_index(level=0,drop=True)
df
Out[67]: 
        date id  value  rolling_mean
0 2016-08-28  A      1           NaN
1 2016-08-28  B      1           NaN
2 2016-08-29  C      2           NaN
3 2016-09-02  B      0           0.5
4 2016-09-03  A      3           2.0
5 2016-09-06  C      1           1.5
6 2017-01-15  B      2           1.0
7 2017-01-18  C      3           2.0
8 2017-01-18  A      2           2.5

Answer 3

Like this:

df['rolling_mean'] = df.groupby('id')['value'].rolling(2).mean().reset_index(0,drop=True).sort_index()

Output:

         date id  value  rolling_mean
0  2016-08-28  A      1           nan
1  2016-08-28  B      1           nan
2  2016-08-29  C      2           nan
3  2016-09-02  B      0          0.50
4  2016-09-03  A      3          2.00
5  2016-09-06  C      1          1.50
6  2017-01-15  B      2          1.00
7  2017-01-18  C      3          2.00
8  2017-01-18  A      2          2.50