Calculate difference of two rows based on another columns entries

Question

I have data frame like this:

Column A	Column B
zu	2.5
ab	1
fi	0
zu	2
ab	0.5
fi	0
uv	1
zu	2
ab	1

I need two calculate the difference between "zu" and "ab" and position it on a new column

Column A	Column B	diff
zu	2.5	1.5
ab	1	0
fi	0	0
zu	2	1.5
ab	0.5	0
fi	0	0
uv	1	0
zu	2	1
ab	1	0

I tried the diff() and shift() Funktion but that didn't work. Do you have any suggestions?

Answer 1

Example Code

data = {'Column A': {0: 'zu', 1: 'ab', 2: 'fi', 3: 'zu', 4: 'ab', 5: 'fi', 6: 'uv', 7: 'zu', 8: 'ab'},
        'Column B': {0: 2.5, 1: 1.0, 2: 0.0, 3: 2.0, 4: 0.5, 5: 0.0, 6: 1.0, 7: 2.0, 8: 1.0}}
df = pd.DataFrame(data)

---

Code

s = df['Column B'].diff(-1).where(df['Column A'].eq('zu'), 0)

s

0    1.5
1    0.0
2    0.0
3    1.5
4    0.0
5    0.0
6    0.0
7    1.0
8    0.0

make s to diff column

df.assign(diff=s)

output

 Column A   Column B    diff
0   zu      2.5         1.5
1   ab      1.0         0.0
2   fi      0.0         0.0
3   zu      2.0         1.5
4   ab      0.5         0.0
5   fi      0.0         0.0
6   uv      1.0         0.0
7   zu      2.0         1.0
8   ab      1.0         0.0