试图将 countif function 从 excel 复制到 python

Question

I'm trying to get the below required output in python

Input:

S.No      Name         Marks     
1        Jack           85       
2        Ram            75       
3        Jack           86
4        Ann            90

Required Output:

S.No      Name         Marks     Count     
1        Jack           85       2
2        Ram            75       1
3        Jack           86       2
4        Ann            90       1

i tried the below code to add the count column which is similar to excel workings using countif function '''

data['count']=data['Name'].value_counts()

''' this code gives the below output

S.No      Name         Marks     Count     
1        Jack           85       NaN
2        Ram            75       NaN
3        Jack           86       NaN
4        Ann            90       NaN

I'm not sure what is missing, Can someone help me in this? Really appreciate your help.

Answer 1

You can get what you need with groupby and transform :

df['count'] = df.groupby('Name')['Name'].transform('size')

which prints:

   S.No  Name  Marks  count
0     1  Jack     85      2
1     2   Ram     75      1
2     3  Jack     86      2
3     4   Ann     90      1

Answer 2

Try this:

df.merge(df.groupby('Name').count(), on='Name').loc[:,:'S.No_y'].rename(columns={'S.No_x':'S.No', 'Marks_x':'Marks', 'S.No_y':'Count'}).set_index('S.No').sort_index()

      Name  Marks  Count
S.No
1     Jack     85      2
2      Ram     75      1
3     Jack     86      2
4      Ann     90      1

Answer 3

Try this:

import pandas as pd
import numpy as np

df =\
pd.DataFrame([["1","2","3","4"],
              ["Jack","Ram","Jack","Ann"],
              [85,75,86,90]] ).T  

df.columns = ["S.No","Name","Marks"]  
df.set_index("S.No", inplace=True) 
names_count = df.groupby("Name").size().rename("Count")
df = df.merge(names_count, on="Name", right_index=True)

print(df)