Python pandas: add a column to my data framework that respects a variable

Question

I have a dataframe 'gt' like this:

org group org1 1 org2 1 org3 2 org4 3 org5 3 org6 3 

and I would like to add a “count” column to the gt dataframe to count the number of group members, expected results as follows:

 org group count org1 1 2 org2 1 2 org3 2 1 org4 3 3 org5 3 3 org6 3 3 

I know how to do this on one element of the group, but I don’t know how to make the counter repeat for all elements of the group, here is the code I used:

 gtcounts = gt.groupby('group').count() 

Does anyone help?

Answer 1

Call transform , this will return a series aligned with the original df:

 In [223]: df['count'] = df.groupby('group')['group'].transform('count') df Out[223]: org group count 0 org1 1 2 1 org2 1 2 2 org3 2 1 3 org4 3 3 4 org5 3 3 5 org6 3 3