how do I perform the before calculation in dataframe

Question

Hi Assuming I have a df

x=[.12,.02,.05,.04]
y=[1,1,1,1]
df=pd.DataFrame(dict(x=x,y=y))

but I want to change values under column 'y' in way that first row is 100, 2nd row is 102 (i.e. 100*.02) third row is 107.1 (102*1.05) and so on.

How do I do that? (I think it should be done using shift function?)

Answer 1

I believe you need cumprod(). It requiered a bit of a hardcoding for the first number, afterwards it seems fine.

import pandas as pd
import numpy as np
x=[.12,.02,.05,.04]
y=[1,1,1,1]
df=pd.DataFrame(dict(x=x,y=y))

df['x'] = df['x'] + 1 #Add 1 to make the cumprod() easier
df['y'].loc[0] = 100 #Force the first value to be 100
df['y'].loc[1:] = df['x'].loc[1:].cumprod() * df['y']
df['x'] = df['x'] - 1 #Substract 1 to keep original values

Output:

      x          y
0  0.12  100.00000
1  0.02    1.02000
2  0.05    1.07100
3  0.04    1.11384