In pandas, how to replace all zero values with the last non zero value which is not more than 4 rows up, without a for loop?

Question

If one of my dataframe columns looks like this:

5
0
0
0
0
0
6
0

It should look like this

5
5
5
5
5
0
6
6

I know how to do it with a loop and a counter, but I am wondering how to do it without a for loop?

Answer 1

You don't even need to mask, there is the method replace that allows you to specify limit and method='ffill'. Going through Nanalso converts to float which is not needed.

import pandas as pd

df = pd.DataFrame({'a': [5, 0, 0, 0, 0, 0, 6, 0]})

# Replace 0s with forward fill and limit set to 4 elements
df2 = df.replace(0, limit=4, method='ffill')

print(df)

   a
0  5
1  5
2  5
3  5
4  5
5  0
6  6
7  6