Loop on Python: Index extension

You can try this. It will take list element from 0 to last-1

for i in range(0,len(A)-1):
    A1 = (A['wap'][i]-A['wap'][i+1]) + (A['wap'][i+1])

you didn't mentioned , what you would add or subtract while its the last element . if you don't add or subtract anything to the last element than you could easily get away with a if statement

for i in range(len(A)):

if (i<len(A)):
    A1 =  (A[i]-A['wap'][i+1]) + (A['wap'][i+1])
else  
    A=A['wap'][i]

if you want to loop back and add or subtract the very fast element to the last element use :

for i in range(len(A)):

    A1 =  (A['wap'][i]-A['wap'][i%len(A)+1]) + (A['wap'][i%len(A)+1])

this will add and subtract A['wap'][0] to the very last element .

but you should not use for i in range(len(A)-1) because then you are always ignoring the last element of the array .

import pandas as pd // imports pandas package to create datsfranme  
from IPython.display import display //  import display to display your dataframe 
import numpy as np

create your dataframe with 5 columns and 5 columns

A = pd.DataFrame({"index":[1,2,3,4,5], "date
from":["1/2/2017","2/2/2017","3/2/2017","4/2/2017","5/2/2017"],
              "date until":  
["1/3/2017","2/3/2017","3/3/2017","4/3/2017","5/3/2017"],
 "product":["a","b","c","d","e"] , 
                  "wap":[100,200,300,400,500] })

create the new column you need

 A["wap_update"]=np.nan

display your dataframe

 display(A.head())

 for i in range(len(A)):

    if (i<len(A)-1):

    A.at[i,"wap_update"] =  (A.iloc[i]['wap'] )+ (A.iloc[i+1]['wap'])
else : 

    A.at[i,"wap_update"]=A.iloc[i]['wap']

display(A)

A.iloc[i]['wap'] will get values at i row and "wap" column A.at[i,"wap_update"] will set the value at at i row and "wap" column