How to fill NaN cells in a pandas dataframe, based on the previous column?

Question

I have the following DF

col1| col2
 123| NaN
 234| 234
 456| NaN
 567| 567

and what I want to do, is to copy the left cell to the right cell if the right cell is empty, so the output is

col1| col2
 123| 123
 234| 234
 456| 456
 567| 567

I tried to work something with fillna, but failed miserably

@jezrael Check [`this`](https://stackoverflow.com/questions/48521360/pandas-fill-nan-based-on-the-previous-value-of-another-cell) out. If this looks close to you? — Mayank Porwal, Nov 11 '20 at 14:17

score 2 · Answer 1 · answered Nov 11 '20 at 14:06

2

Use ffill by columns, so axis=1:

df = df.ffill(axis=1)
print (df)
    col1   col2
0  123.0  123.0
1  234.0  234.0
2  456.0  456.0
3  567.0  567.0

answered Nov 11 '20 at 14:06

jezrael

score 2 · Answer 2 · answered Nov 11 '20 at 14:16

2

df.fillna(method='ffill',axis=1)

answered Nov 11 '20 at 14:16

Ehtisham Ahmed

sophocles · Accepted Answer · 2020-11-17T20:25:15.360

1

You can use np.where from the numpy package

Assuming your dataframe is called df:

import numpy as np
df['col2'] = np.where(df['col2'].isnull(),df['col1'],df['col2'])

which will give you:

col1| col2
 123| 123
 234| 234
 456| 456
 567| 567

edited Nov 17 '20 at 20:25

answered Nov 11 '20 at 14:09

sophocles

3 Answers3