Subtract rows in a grouped Dataframe

Question

I have a pandas dataframe and i need to subtract rows if they have the same group.

Input Dataframe:

A	B	Value
A1	B1	10.0
A1	B1	5.0
A1	B2	5.0
A2	B1	3.0
A2	B1	5.0
A2	B2	1.0

Expected Dataframe:

A	B	Value
A1	B1	5.0
A1	B2	5.0
A2	B1	-2.0
A2	B2	1.0

Logic: For example the first and second rows of the dataframe are in group A1 and B1 so the value must be 10.0 - 5.0 = 5.0. 4º and 5º rows have the same group as well so the value must be 3.0 - 5.0 = -2.0.

Only subtract rows if they have same A and B value.

Thank you!

Answer 1

Try:

subtract = lambda x: x.iloc[0] - (x.iloc[1] if len(x) == 2 else 0)
out = df.groupby(['A', 'B'])['Value'].apply(subtract).reset_index()
print(out)

# Output:
    A   B  Value
0  A1  B1    5.0
1  A1  B2    5.0
2  A2  B1   -2.0
3  A2  B2    1.0