Pandas sum of next n rows

Question

I have a DataFrame that looks like the following:

             ds         y
0    2017-02-07  0.154941
1    2017-02-08  0.110595
2    2017-02-09  0.044022
3    2017-02-10  0.283902
4    2017-02-11  0.121570
5    2017-02-12  0.000000
6    2017-02-13  0.020265
7    2017-02-14  0.053577
8    2017-02-15  0.080842
9    2017-02-16  0.022043

I am now trying to create a new column 'next_3' that is the sum of the y values for the next 3 days following the current day.

I am achieving this using:

df['next_3'] = df['y'].shift(-3).rolling(3).sum()

which produces this:

           ds         y     label
0  2017-02-07  0.154941       NaN
1  2017-02-08  0.110595       NaN
2  2017-02-09  0.044022  0.405472
3  2017-02-10  0.283902  0.141836
4  2017-02-11  0.121570  0.073842
5  2017-02-12  0.000000  0.154685
6  2017-02-13  0.020265  0.156462
7  2017-02-14  0.053577       NaN
8  2017-02-15  0.080842       NaN
9  2017-02-16  0.022043       NaN

I understand why the last 3 rows have NaN values since the next 3 rows aren't available, but why do the first 2 rows have NaN values when these values can be calculated?

How can I correct my shift().rolling().sum() call so that the first two rows are also calculated?

Answer 1

Use parameter min_periods=1:

df['next_3'] = df['y'].shift(-3).rolling(3, min_periods=1).sum()
print (df)
           ds         y    next_3
0  2017-02-07  0.154941  0.283902
1  2017-02-08  0.110595  0.405472
2  2017-02-09  0.044022  0.405472
3  2017-02-10  0.283902  0.141835
4  2017-02-11  0.121570  0.073842
5  2017-02-12  0.000000  0.154684
6  2017-02-13  0.020265  0.156462
7  2017-02-14  0.053577  0.102885
8  2017-02-15  0.080842  0.022043
9  2017-02-16  0.022043       NaN

Or first use rolling and then shifting:

df['next_3'] = df['y'].rolling(3).sum().shift(-3)
print (df)
           ds         y    next_3
0  2017-02-07  0.154941  0.438519
1  2017-02-08  0.110595  0.449494
2  2017-02-09  0.044022  0.405472
3  2017-02-10  0.283902  0.141835
4  2017-02-11  0.121570  0.073842
5  2017-02-12  0.000000  0.154684
6  2017-02-13  0.020265  0.156462
7  2017-02-14  0.053577       NaN
8  2017-02-15  0.080842       NaN
9  2017-02-16  0.022043       NaN