Iterate over rows and subtract values in pandas df

Question

I have the following table:

ID	Qty_1	Qty_2
A	1	10
A	2	0
A	3	0
B	3	29
B	2	0
B	1	0

I want to iterate based on the ID, and subtract Qty_2 - Qty_1 and update the next row with that result.

The result would be:

ID	Qty_1	Qty_2
A	1	10
A	2	8
A	3	5
B	3	29
B	2	27
B	1	26

Ideally, I would also like to start by subtracting the first row end a new ID appears and only after that start the loop:

ID	Qty_1	Qty_2
A	1	9
A	2	7
A	3	4
B	3	26
B	2	24
B	1	23

Each of the solutions is ok! Thank you!

Answer 1

First compute the difference between 'Qty_1' and 'Qty_2' row by row, then group by 'ID' and compute cumulative sum:

df['Qty_2'] = df.assign(Qty_2=df['Qty_2'].sub(df['Qty_1'])) \
                .groupby('ID')['Qty_2'].cumsum()
print(df)

# Output:
  ID  Qty_1  Qty_2
0  A      1      9
1  A      2      7
2  A      3      4
3  B      3     26
4  B      2     24
5  B      1     23

Setup:

data = {'ID': ['A', 'A', 'A', 'B', 'B', 'B'],
        'Qty_1': [1, 2, 3, 3, 2, 1],
        'Qty_2': [10, 0, 0, 29, 0, 0]}
df = pd.DataFrame(data)