Python pandas get previous values row

Question

I created this dataframe with python pandas:

import numpy as np
import pandas as pd

my_df = pd.DataFrame(
    {'ColumnA':['Value A', '', 'Value B', '', '', 'Value C',''],
     'ColumnB':['', '', '', '', '', '', '']})

The dataframe is presented below:

    ColumnA | ColumnB
0   Value A | 
1           | 
2   Value B | 
3           | 
4           |  
5   Value C | 
6           |

To complete column B, I put this condition:

conditions = [
    my_df['ColumnA'] == '',
    my_df['ColumnA'] != '']
result = [my_df['ColumnA'].shift(1),
          my_df['ColumnA']]
my_df['ColumnB'] = np.select(conditions, result)

I have this as a result:

        ColumnA | ColumnB
0     Value A   | Value A
1               | Value A
2     Value B   | Value B
3               | Value B
4               | 
5     Value C   | Value C
6               | Value C

Now I want all the cells in columnB to be filled in like this:

        ColumnA | ColumnB
0       Value A | Value A
1               | Value A
2       Value B | Value B
3               | Value B
4               | Value B
5       Value C | Value C
6               | Value C

Thank you for your suggestions!

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. — Community
– Community Bot, Commented Nov 1, 2022 at 15:59

BeRT2me · Accepted Answer · 2022-11-01 16:09:15Z

1

Using real NaN values instead of empty strings makes many things easier...

import pandas as pd
import numpy as np

data = {'ColumnA':['Value A', '', 'Value B', '', '', 'Value C',''],
        'ColumnB':['', '', '', '', '', '', '']}
df = pd.DataFrame(data)

# Fix your null values:
df = df.replace('', np.nan)

# You appear to want ColumnB to be ColumnA if it were forward-filled.
df['ColumnB'] = df['ColumnA'].ffill()

print(df)

Output:

   ColumnA  ColumnB
0  Value A  Value A
1      NaN  Value A
2  Value B  Value B
3      NaN  Value B
4      NaN  Value B
5  Value C  Value C
6      NaN  Value C

answered Nov 1, 2022 at 16:09

BeRT2me

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1 Comment

Laurent Reynaud Over a year ago

Thank you very much for your reactivity!

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