how to create a counter for each duplicated row in a dataframe-CodePudding

I created a dataframe that contains only duplicated rows using the duplicated() method. my question is probably quite easy. i'd like to add a count column to the right and for each row count how many duplicated appearances of it are inside the duplicated df. I thought about creating a groupby of each column but that didn't really work.

something like df.groupby([*all columns*]).count()

this is how the df looks like :

thanks!

CodePudding user response：

You can try .groupby() .transform() size:

df['count'] = df.groupby(df.columns.tolist(), dropna=False)[df.columns[0]].transform('size')

As your data contains NaN, we have to use parameter dropna=False in .groupby() in order to get a complete list of count including rows with NaN values. Otherwise, rows with NaN values will be skipped and excluded from count.

Demo

Data Input

print(df)

  Col1  Col2 Col3 Col4
0  ABC   123  XYZ  NaN       # group #1 of 3
1  ABC   123  XYZ  NaN       # group #1 of 3
2  ABC   678  PQR  def       # group #2 of 1
3  MNO   890  EFG  abc       # group #3 of 4 
4  MNO   890  EFG  abc       # group #3 of 4 
5  CDE   234  567  xyz       # group #4 of 2 
6  ABC   123  XYZ  NaN       # group #1 of 3
7  CDE   234  567  xyz       # group #4 of 2 
8  MNO   890  EFG  abc       # group #3 of 4 
9  MNO   890  EFG  abc       # group #3 of 4

Output

print(df)

  Col1  Col2 Col3 Col4  count
0  ABC   123  XYZ  NaN      3           
1  ABC   123  XYZ  NaN      3
2  ABC   678  PQR  def      1
3  MNO   890  EFG  abc      4
4  MNO   890  EFG  abc      4
5  CDE   234  567  xyz      2
6  ABC   123  XYZ  NaN      3
7  CDE   234  567  xyz      2
8  MNO   890  EFG  abc      4
9  MNO   890  EFG  abc      4

Edit

If you get memory problem by using the .groupby() solution, we can go for using the .value_counts() solution by getting the count by .value_counts(), followed by merging with the original dataframe by .merge(), as follows:

df_count = df.value_counts(dropna=False).reset_index(name='count')  

df_out = df.merge(df_count, how='left')    # left join to keep the original row sequence order of df

Result:

print(df_count)

  Col1  Col2 Col3 Col4  count
0  MNO   890  EFG  abc      4
1  ABC   123  XYZ  NaN      3
2  CDE   234  567  xyz      2
3  ABC   678  PQR  def      1


print(df_out)

  Col1  Col2 Col3 Col4  count
0  ABC   123  XYZ  NaN      3
1  ABC   123  XYZ  NaN      3
2  ABC   678  PQR  def      1
3  MNO   890  EFG  abc      4
4  MNO   890  EFG  abc      4
5  CDE   234  567  xyz      2
6  ABC   123  XYZ  NaN      3
7  CDE   234  567  xyz      2
8  MNO   890  EFG  abc      4
9  MNO   890  EFG  abc      4