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].

I tried the following:

t1 = time_interval[0] t2 = time_interval[1] + t1 t3 = time_interval[2] + t2 print(t1, t2, t3)  # -> 4 10 22 

Answer 1

If you're doing much numerical work with arrays like this, I'd suggest numpy, which comes with a cumulative sum function cumsum:

import numpy as np  a = [4,6,12]  np.cumsum(a) #array([4, 10, 22]) 

Numpy is often faster than pure python for this kind of thing, see in comparison to @Ashwini's accumu:

In [136]: timeit list(accumu(range(1000))) 10000 loops, best of 3: 161 us per loop  In [137]: timeit list(accumu(xrange(1000))) 10000 loops, best of 3: 147 us per loop  In [138]: timeit np.cumsum(np.arange(1000)) 100000 loops, best of 3: 10.1 us per loop 

But of course if it's the only place you'll use numpy, it might not be worth having a dependence on it.

Answer 2

In Python 2 you can define your own generator function like this:

def accumu(lis):     total = 0     for x in lis:         total += x         yield total  In [4]: list(accumu([4,6,12])) Out[4]: [4, 10, 22] 

And in Python 3.2+ you can use itertools.accumulate():

In [1]: lis = [4,6,12]  In [2]: from itertools import accumulate  In [3]: list(accumulate(lis)) Out[3]: [4, 10, 22]