How to locate and replace values in dataframe based on some criteria

Question

I would like to locate all places when in Col2 there is a change in value (for ex. change from A to C) and then modify value from Col1 (corresponding to row when the change happens, so when A -> C then it will be value in the same row as C) by dividing subtraction current value and previous value by two (in this example will be 1 + (1.5-1)/2 = 1.25.

Output table is result of replacing all that occurrences in whole table

How I can achieve that ?

Col1	Col2
1	A
1.5	C
2.0	A
2.5	A
3.0	D
3.5	D

OUTPUT:

Col1	Col2
1	A
1.25	C
1.75	A
2.5	A
2.75	D
3.5	D

Answer 1

Use np.where and series holding values of your formula

solution = df.Col1.shift() + ((df.Col1 - df.Col1.shift()) / 2)
df['Col1'] = np.where(~df.Col2.eq(df.Col2.shift()), solution.fillna(df.Col1), df.Col1)