How do I perform a summation of `n` rows at a time in pandas? [duplicate]

Question

Given a data frame

     A
0   14
1   59
2   38
3   40
4   99
5   89
6   70
7   64
8   84
9   40
10  30
11  94
12  65
13  29
14  48
15  26
16  80
17  79
18  74
19  69

This data frame has 20 columns. I would like to group n=5 rows at a time and sum them up. So, my output would look like this:

     A
0  250
1  347
2  266
3  328 

df.rolling_sum will not help because it does not allow you to vary the stride when summing.

What other ways are there to do this?

Answer 1

df.set_index(df.index // 5).sum(level=0)

Answer 2

If you can manage an ndarray with the sums as opposed to a Series (you could always construct a Series again anyhow), you could use np.add.reduceat.

np.add.reduceat(df.A.values, np.arange(0, df.A.size, 5))

Which in this case returns

array([250, 347, 266, 328])