使用条件 Pandas DataFrame 计算 If

Question

I have a data frame:

   A    B    C   
0  xx   No   1
1  xx   No   2
2  xx   Yes  3
3  xx   Yes  4
4  xx   No   1
5  xx   No   2
6  xx   No   3
7  xx   Yes  4
8  xx   No   1
9  xx   No   2

If want to create a column that counts the number of times a number appear in column C, when column B is equal to Yes

so the output would be

   A    B    C   countifs   
0  xx   No   1      
1  xx   No   2
2  xx   Yes  3      1
3  xx   Yes  4      2
4  xx   No   1
5  xx   No   2
6  xx   No   3
7  xx   Yes  4      2
8  xx   No   1
9  xx   No   2

Is this possible?

Answer 1

For count only True s values use GroupBy.transform with sum only for filtered rows by mask:

m = df['B'].eq('Yes')
df['countifs'] = m.groupby(df.loc[m, 'C']).transform('sum').fillna('')
print (df)
    A    B  C countifs
0  xx   No  1         
1  xx   No  2         
2  xx  Yes  3        1
3  xx  Yes  4        2
4  xx   No  1         
5  xx   No  2         
6  xx   No  3         
7  xx  Yes  4        2
8  xx   No  1         
9  xx   No  2         

Answer 2

Here's one approach:

y = df.B.eq('Yes')
df.loc[y, 'countifs'] = df[y].groupby('C').B.transform('count').values

print(df.fillna(''))

   A    B   C countifs
0  xx   No  1         
1  xx   No  2         
2  xx  Yes  3        1
3  xx  Yes  4        2
4  xx   No  1         
5  xx   No  2         
6  xx   No  3         
7  xx  Yes  4        2
8  xx   No  1         
9  xx   No  2         

Answer 3

Here is another approach, using numpy.where , .value_counts() , and .map() :

>>> col_C_cnts = df[df.B == 'Yes']['C'].value_counts()
>>> df['countifs'] = np.where(df.B == 'Yes', df['C'].map(col_C_cnts), pd.np.nan)
>>> print(df.fillna(''))
    A    B  C countifs
0  xx   No  1         
1  xx   No  2         
2  xx  Yes  3        1
3  xx  Yes  4        2
4  xx   No  1         
5  xx   No  2         
6  xx   No  3         
7  xx  Yes  4        2
8  xx   No  1         
9  xx   No  2