Count If with Condition Pandas DataFrame

fred.schwartz :

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?

jezrael :

For count only Trues 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