Subtraction/Addition from seperate rows/columns

Question

I have a dataframe like this:

Day    Diff
137    0
185    48
249    64
139   -110

In the column Diff whenever a negative value is encountered I want to subtract 365 from the value in Day from the previous row and then add that value to the Day value in the current row of the negative number. For example, in this scenario when -110 is encountered I want to do 365-249 (249 is from Day in previous row) and then add 139. So 365-249 = 116 and 116 + 139 = 255. Therefore -110 would be replaced with 255.

My desired output then is:

Day    Diff
137    0
185    48
249    64
139    255

Answer 1

you can do it this way:

In [32]: df.loc[df.Diff < 0, 'Diff'] = 365 + df.Day - df.shift().loc[df.Diff < 0, 'Day']

In [33]: df
Out[33]:
   Day   Diff
0  137    0.0
1  185   48.0
2  249   64.0
3  139  255.0