Pandas dataframe get first row of each group

StackOverflow

I have a pandas DataFrame like following.

df = pd.DataFrame({"id" : [1,1,1,2,2,3,3,3,3,4,4,5,6,6,6,7,7],
                "value"  : ["first";"second";"second";"first",
                            "second";"first";"third";"fourth",
                            "fifth";"second";"fifth";"first",
                            "first";"second";"third";"fourth";"fifth"]})

I want to group this by ["id";"value"] and get the first row of each group.

        id   value
0        1   first
1        1  second
2        1  second
3        2   first
4        2  second
5        3   first
6        3   third
7        3  fourth
8        3   fifth
9        4  second
10       4   fifth
11       5   first
12       6   first
13       6  second
14       6   third
15       7  fourth
16       7   fifth

Expected outcome

    id   value
     1   first
     2   first
     3   first
     4  second
     5  first
     6  first
     7  fourth

I tried following which only gives the first row of the DataFrame. Any help regarding this is appreciated.

In [25]: for index, row in df.iterrows():
   ....:     df2 = pd.DataFrame(df.groupby(["id","value"]).reset_index().ix[0])

Answer rating: 305

>>> df.groupby("id").first()
     value
id        
1    first
2    first
3    first
4   second
5    first
6    first
7   fourth

If you need id as column:

>>> df.groupby("id").first().reset_index()
   id   value
0   1   first
1   2   first
2   3   first
3   4  second
4   5   first
5   6   first
6   7  fourth

To get n first records, you can use head():

>>> df.groupby("id").head(2).reset_index(drop=True)
    id   value
0    1   first
1    1  second
2    2   first
3    2  second
4    3   first
5    3   third
6    4  second
7    4   fifth
8    5   first
9    6   first
10   6  second
11   7  fourth
12   7   fifth

Answer rating: 65

This will give you the second row of each group (zero indexed, nth(0) is the same as first()):

df.groupby("id").nth(1) 

Documentation: http://pandas.pydata.org/pandas-docs/stable/groupby.html#taking-the-nth-row-of-each-group





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