How to calculate inverse cumsum in pandas

Question

I am trying to find a way to calculate an inverse cumsum for pandas. This means applying cumsum but from bottom to top. The problem I\'m facing is, I\'m trying to f

Answer 1

Solution

Whichever column you want to apply cumsum to you have two options:
1. Order descending a copy of that column by index, followed by cumsum and then order ascending by index. Finally assign it back to the data frame column.

2. Use numpy:

import numpy as np

array = df.column_data.to_numpy()    
array = np.flip(array)  # to flip the order 
array = numpy.cumsum(array)    
array =  numpy.flip(array)  # to flip back to original order    
df.column_data_cumsum = array