How to perform an operation between two rows at a different index, repeatedly

I have an excel file that can calculate the last column with a formula. I am trying to replicate this into python code. How can I perform an operation to achieve such results?

Essentially, the last column is supposed to take, for example, row 2 and subtract itself by row 0 (because they share the same industry besides the month, which I am trying to compare the two by).

I tried using a for loop and take the index of row i then subtract row i‘s ‘No. of jobs’ by the row i-2‘s ‘No. of jobs’ and adding that result to the last column. Then, I would increment i by 1 to let the operation continue for the next row, but I was unsuccessful.

    Period       Industry      No. of jobs  Difference from prev. month
0   January      Farm          70200        N/A
1   January      Mining        4900         N/A
2   February     Farm          70100        -100
3   February     Mining        4850         -50
4   March        Farm          70200        100
5   March        Mining        4600         -250
6   April        Farm          70300        100
7   April        Mining        5200         600
8   May          Farm          70300        0
9   May          Mining        5300         100

Answer

Try as follows:

df['Difference from prev. month'] = df['No. of jobs'].groupby(df['Industry']).diff()

df['Difference from prev. month']

0      NaN
1      NaN
2   -100.0
3    -50.0
4    100.0
5   -250.0
6    100.0
7    600.0
8      0.0
9    100.0
Name: Difference from prev. month, dtype: float64

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Answer