How to forward fill null values of one column from the values of another column?

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I am trying to fill the null values within column ‘beginning_daily_count’ with the previous index value from the ‘end_daily_count’. The starting dataset would be:

d = {
    'id': [1, 1, 1, 1, 1, 2, 2, 2, 2],
    'beginning_daily_count': [30, 33, 37, 46, None, 7, 1, None, 2],
    'end_daily_count': [33, 37, 46, 52, 33, 7, 1, 2, 3],
    'foils': [0, 0, 0, 0, 0, 0, 0, 1, 1]
}

and the desired dataset would be:

d = {
    'id': [1, 1, 1, 1, 1, 2, 2, 2, 2],
    'beginning_daily_count': [30, 33, 37, 46, 52, 33, 1, 1, 2],
    'end_daily_count': [33, 37, 46, 52, 33, 7, 1, 2, 3],
    'foils': [0, 0, 0, 0, 0, 0, 0, 1, 1]
}

I have attempted the following ffill() and iloc() methods, but to no avail. I admittedly have little experience with ffill and iloc.

d.iloc[beginning_daily_count.isna()].values = d.iloc[d.end_daily_count- 1].values
d['beginning_daily_count'].transform(lambda x: x.ffill(d['end_daily_count']))

>Solution :

You can fillna the column with the shifted other column per group (using GroupBy.shift to avoid leaking values from one group to the next one):

df['beginning_daily_count'] = (df['beginning_daily_count']
                               .fillna(df.groupby('id')['end_daily_count'].shift(),
                                       downcast='infer')
                              )

output:

   id  beginning_daily_count  end_daily_count  foils
0   1                     30               33      0
1   1                     33               37      0
2   1                     37               46      0
3   1                     46               52      0
4   1                     52               33      0
5   2                      7                7      0
6   2                      1                1      0
7   2                      1                2      1
8   2                      2                3      1

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