Python problem 4 of 30

Running total

Python
Easy

Write running_total(nums) that returns a new list where each item is the sum of all items up to and including that position.

running_total([1, 2, 3, 4])   # [1, 3, 6, 10]
running_total([5, -2, 7])     # [5, 3, 10]
running_total([])             # []

Show hint

Keep one variable holding the total so far, and append it after each step.

Show solution

def running_total(nums):
    out = []
    total = 0
    for n in nums:
        total += n
        out.append(total)
    return out

This is the accumulator pattern: one pass, O(n) time. The standard library has it built in as list(itertools.accumulate(nums)). Running totals (also called prefix sums) are a key trick for answering "sum of a range" questions quickly.

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