How to get a series of highest values out of equal portions of another pandas series python

Question

I am working with a series in python, What I want to achieve is to get the highest value out of every n values in the series.

For example:

if n is 3

Series: 2, 1, 3, 5, 3, 6, 1, 6, 9

Expected Series: 3, 6, 9

I have tried nlargest function in pandas but it returns largest values in descending order, But I need the values in order of the original series.

Answer 1

There are various options. If the series is guaranteed to have a length of a multiple of n, you could drop down to numpy and do a .reshape followed by .max along an axis.

Otherwise, if the index is the default (0, 1, 2, ...), you can use groupby:

import pandas as pd

n = 3
ser = pd.Series([2, 1, 3, 5, 3, 6, 1, 6, 9])
out = ser.groupby(ser.index // n).max()

out:

0    3
1    6
2    9
dtype: int64