Pandas: Groupby count as column value

Question

I have a pandas dataframe that looks like this:

I would like to generate counts instances of 'x' (regardless of whether they're unique, or not) per 'id'. The result would be insert as a column labeled 'x_count' as shown below:

Any tips would be helpful.

Answer 1

Simply a groupby with transform count

df['x_count'] = df.groupby('id')['x'].transform('count')

If you also want to count the NaN , use `size'

df['x_count'] = df.groupby('id')['x'].transform('size')

Answer 2

Try .value_counts with .map

df['x_count'] = df['id'].map(df.value_counts('id'))