forward fill specific columns in pandas dataframe

Question

If I have a dataframe with multiple columns ['x', 'y', 'z'], how do I forward fill only one column 'x'? Or a group of columns ['x','y']?

I only know how to do it by axis.

Answer 1

tl;dr:

cols = ['X', 'Y'] df.loc[:,cols] = df.loc[:,cols].ffill() 

And I have also added a self containing example:

>>> import pandas as pd >>> import numpy as np >>>  >>> ## create dataframe ... ts1 = [0, 1, np.nan, np.nan, np.nan, np.nan] >>> ts2 = [0, 2, np.nan, 3, np.nan, np.nan] >>> d =  {'X': ts1, 'Y': ts2, 'Z': ts2} >>> df = pd.DataFrame(data=d) >>> print(df.head())     X   Y   Z 0   0   0   0 1   1   2   2 2 NaN NaN NaN 3 NaN   3   3 4 NaN NaN NaN >>>  >>> ## apply forward fill ... cols = ['X', 'Y'] >>> df.loc[:,cols] = df.loc[:,cols].ffill() >>> print(df.head())    X  Y   Z 0  0  0   0 1  1  2   2 2  1  2 NaN 3  1  3   3 4  1  3 NaN 

Answer 2

for col in ['X', 'Y']:     df[col] = df[col].ffill()