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Selecting first row from each subgroup (pandas)

How to select the subset of rows where distance is lowest, grouping by date and p columns?

df
    v       p       distance    date
0   14.6    sst     22454.1     2021-12-30
1   14.9    sst     24454.1     2021-12-30
2   14.8    sst     33687.4     2021-12-30
3   1.67    wvht    23141.8     2021-12-30
4   1.9     wvht    24454.1     2021-12-30
5   1.8     wvht    24454.1     2021-12-30
6   1.7     wvht    23141.4     2021-12-31
7   2.1     wvht    24454.1     2021-12-31

Ideally, the returned dataframe should contain:

df
    v       p       distance    date
0   14.6    sst     22454.1     2021-12-30
3   1.67    wvht    23141.8     2021-12-30
6   1.7     wvht    23141.4     2021-12-31

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Answer

One way is to use groupby + idxmin to get the index of the smallest distance per group, then use loc to get the desired output:

out = df.loc[df.groupby(['date', 'p'])['distance'].idxmin()]

Output:

       v     p  distance        date
0  14.60   sst   22454.1  2021-12-30
3   1.67  wvht   23141.8  2021-12-30
6   1.70  wvht   23141.4  2021-12-31
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