Python DataFrame: Cumulative sum and subraction of multiple columns?

Question

I have a df with name,marks and negative. I'm trying to do a cumulative sum and subract negative column.

    df

            name        marks       negative    

    0      marcus       20              0          
    1      marcus       30             10  
    2      marcus       0              20                 
    3      paul         10              0          
    4      bruno        50             50           
    5      bruno        20              0          
    6      lucy          0              5          





    Final df

            name        marks       negative    finsum

    0      marcus       20              0          20
    1      marcus       30             10          40           (20+30-10) - (finsum+marks-negative)
    2      marcus       0              20          20       
    3      paul         10              0          10
    4      bruno        50             50           0
    5      bruno        20              0          20
    6      lucy          0              5          -5

I have tried cumsum() for a single row, But how to add and subract with cumsum() or is there any other way?

Answer 1

Use GroupBy.cumsum:

df1 = df.groupby('name').cumsum()
df['fin'] = df1['marks'].sub(df1['negative'])

print (df)
     name  marks  negative  fin
0  marcus     20         0   20
1  marcus     30        10   40
2  marcus      0        20   20
3    paul     10         0   10
4   bruno     50        50    0
5   bruno     20         0   20
6    lucy      0         5   -5