Pandas minus same row value with next row value

How can I calculate the needed value field,

for 1st row it takes the value field as it is,

for 2nd row it takes 2nd row needed value – 3rd row Agg value

for 3rd row, 3rd row needed value -4th row agg

>Solution :

use cumsum to get cumulative sum of Agg, then subtract Value:

first_val = float(df['Value'].iloc[0])
df['Needed'] = first_val - df['Agg'].iloc[1:].cumsum()
df['Needed'] = df['Needed'].fillna(first_val)

Output:

        Time    Agg   Value        Needed Value  Needed
0  10:55:00   178.0  322.0                  322   322.0
1  11:00:00   354.0      -         (322-354)-32   -32.0
2  11:05:00   354.0      -        (-32-354)-386  -386.0
3  11:10:00   354.0      -   (-386-next Agg nu)  -740.0