pandas dataframe groupby and get nth row

Question

I have a pandas DataFrame like following.

df = pd.DataFrame([[1.1, 1.1, 1.1, 2.6, 2.5, 3.4,2.6,2.6,3.4,3.4,2.6,1.1,1.1,3.3], list(\'AAABBBBABCBDDD\'), [1.1,          


        

Answer 1

I think the nth method is supposed to do just that:

In [10]: g = df.groupby('ID')
In [11]: g.nth(1).dropna()
Out[11]: 
    col1 col2  col3     col4 col5
ID                               
1    1.1    D   4.7    x/y/z  200
2    3.4    B   3.8    x/u/v  404
3    1.1    A   2.5  x/y/z/n  404
5    2.6    B   4.6      x/y  500

In 0.13 another way to do this is to use cumcount:

df[g.cumcount() == n - 1]

...which is significantly faster.

In [21]: %timeit g.nth(1).dropna()
100 loops, best of 3: 11.3 ms per loop

In [22]: %timeit df[g.cumcount() == 1]
1000 loops, best of 3: 286 µs per loop