List comprehension for running total

I'm not sure about 'elegant', but I think the following is much simpler and more intuitive (at the cost of an extra variable):

a = range(20)

runningTotal = []

total = 0
for n in a:
  total += n
  runningTotal.append(total)

The functional way to do the same thing is:

a = range(20)
runningTotal = reduce(lambda x, y: x+[x[-1]+y], a, [0])[1:]

...but that's much less readable/maintainable, etc.

@Omnifarous suggests this should be improved to:

a = range(20)
runningTotal = reduce(lambda l, v: (l.append(l[-1] + v) or l), a, [0])

...but I still find that less immediately comprehensible than my initial suggestion.

Remember the words of Kernighan: "Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it."

If you can use numpy, it has a built-in function named cumsum that does this.

import numpy as np
tot = np.cumsum(a)  # returns a np.ndarray
tot = list(tot)     # if you prefer a list

A list comprehension has no good (clean, portable) way to refer to the very list it's building. One good and elegant approach might be to do the job in a generator:

def running_sum(a):
  tot = 0
  for item in a:
    tot += item
    yield tot

to get this as a list instead, of course, use list(running_sum(a)).