Calculate new value based on decreasing value

Question

Problem:

What\'d I like to do is step-by-step reduce a value in a Series by a continuously decreasing base figure.

I\'m not sur

Answer 1

Your idea with cumsum and diff works. It doesn't look too complicated; not sure if there's an even shorter solution. First, we compute the cumulative sum, operate on that, and then go back (diff is kinda sorta the inverse function of cumsum).

import math

c = values.cumsum() - ALLOWANCE
# now we've got [-15, -5, 20, 50]
c[c < 0] = 0 # negative values don't make sense here

# (c - c.shift(1)) # <-- what I had first: diff by accident

# it is important that we don't fill with 0, in case that the first
# value is greater than ALLOWANCE
c.diff().fillna(math.max(0, values[0] - ALLOWANCE))