Inserting new rows in pandas data frame at specific indices

Question

I have a following data frame df with two columns \"identifier\", \"values\" and \"subid\":

     identifier   values    subid
0      1              


        

Answer 1

subtract where the prior row is different than the current row

# edit in place
df['values'] -= df.identifier.ne(df.identifier.shift().bfill())
df

    identifier  values
0            1     101
1            1     102
2            1     103
3            1     104
4            1     105
5            2     105
6            2     107
7            2     108
8            2     109
9            2     110
10           3     110
11           3     112
12           3     113

Or

# new dataframe
df.assign(values=df['values'] - df.identifier.ne(df.identifier.shift().bfill()))

    identifier  values
0            1     101
1            1     102
2            1     103
3            1     104
4            1     105
5            2     105
6            2     107
7            2     108
8            2     109
9            2     110
10           3     110
11           3     112
12           3     113