Conditional counting within groups

Question

I wanted to do conditional counting after groupby; for example, group by values of column A, and then count within each group how often value 5

Answer 1

Select all rows where B equals 5, and then apply groupby/size:

In [43]: df.loc[df['B']==5].groupby('A').size()
Out[43]: 
A
0    1
4    2
dtype: int64

Alternatively, you could use groupby/agg with a custom function:

In [44]: df.groupby('A')['B'].agg(lambda ser: (ser==5).sum())
Out[44]: 
A
0    1
4    2
Name: B, dtype: int64

Note that generally speaking, using agg with a custom function will be slower than using groupby with a builtin method such as size. So prefer the first option over the second.

In [45]: %timeit df.groupby('A')['B'].agg(lambda ser: (ser==5).sum())
1000 loops, best of 3: 927 µs per loop

In [46]: %timeit df.loc[df['B']==5].groupby('A').size()
1000 loops, best of 3: 649 µs per loop

---

To include A values where the size is zero, you could reindex the result:

import pandas as pd
df = pd.DataFrame({'A': [0, 4, 0, 4, 4, 6], 'B': [5, 10, 10, 5, 5, 10]})
result = df.loc[df['B'] == 5].groupby('A').size()
result = result.reindex(df['A'].unique())

yields

A
0    1.0
4    2.0
6    NaN
dtype: float64