python rolling product on non-adjacent row

Question

I would like to calculate rolling product of non-adjacent row, such as product of values in every fifth row as shown in the photo (result in blue cell is the product of data in blue cell etc.)

The best way I can do now is the following;

temp = pd.DataFrame([range(20)]).transpose()
df = temp.copy()
df['shift1'] = temp.shift(5)
df['shift2'] = temp.shift(10)
df['shift3'] = temp.shift(15)

result = df.product(axis=1)

however, it looks to be cumbersome as I want to change the row step dynamically. can anyone tell me if there is a better way to navigate this? Thank you

Answer 1

You can use groupby.cumprod/groupby.prod with the modulo 5 as grouper:

import numpy as np
m = np.arange(len(df)) % 5

# option 1
df['result'] = df.groupby(m)['data'].cumprod()

# option 2
df.loc[~m.duplicated(keep='last'), 'result2'] = df.groupby(m)['data'].cumprod()
# or
# df.loc[~m.duplicated(keep='last'),
#        'result2'] = df.groupby(m)['data'].prod().to_numpy()

output:

    data  result  result2
0      0       0      NaN
1      1       1      NaN
2      2       2      NaN
3      3       3      NaN
4      4       4      NaN
5      5       0      NaN
6      6       6      NaN
7      7      14      NaN
8      8      24      NaN
9      9      36      NaN
10    10       0      NaN
11    11      66      NaN
12    12     168      NaN
13    13     312      NaN
14    14     504      NaN
15    15       0      0.0
16    16    1056   1056.0
17    17    2856   2856.0
18    18    5616   5616.0
19    19    9576   9576.0