pandas new column counting id's existence in other dataframe

Question

This is my initial dataframe dfA :

idA	data
1	row
2	data
3	data

dataframe dfB

idC	idA
1	1
2	1
3	3
4	3
5	3
6	3

I am trying to count every id existence of dfA in dfB , and add to new column like this:

idA	data	count
1	row	2
2	data	0
3	data	4

how can i do this plz

Answer 1

Use groupby then count to get the count of idC in each group idA. At last, map the count Series to idA column of df1.

df1['count'] = df1['idA'].map(df2.groupby('idA')['idC'].count()).fillna(0).astype(int)

print(df1)

   idA  data  count
0    1   row    2
1    2  data    0
2    3  data    4