How to find the cumulative sum of numbers in a list?

Question

time_interval = [4, 6, 12]

I want to sum up the numbers like [4, 4+6, 4+6+12] in order to get the list t = [4, 10, 22].

Answer 1

A pure python oneliner for cumulative sum:

cumsum = lambda X: X[:1] + cumsum([X[0]+X[1]] + X[2:]) if X[1:] else X

This is a recursive version inspired by recursive cumulative sums. Some explanations:

1. The first term X[:1] is a list containing the previous element and is almost the same as [X[0]] (which would complain for empty lists).
2. The recursive cumsum call in the second term processes the current element [1] and remaining list whose length will be reduced by one.
3. if X[1:] is shorter for if len(X)>1.

Test:

cumsum([4,6,12])
#[4, 10, 22]

cumsum([])
#[]

And simular for cumulative product:

cumprod = lambda X: X[:1] + cumprod([X[0]*X[1]] + X[2:]) if X[1:] else X

Test:

cumprod([4,6,12])
#[4, 24, 288]