List comprehension for running total

user_366309 0
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I want to get a running total from a list of numbers.

For demo purposes, I start with a sequential list of numbers using range

a = range(20)

runningTotal = []
for n in range(len(a)):
    new = runningTotal[n-1] + a[n] if n > 0 else a[n]
    runningTotal.append(new)

# This one is a syntax error
# runningTotal = [a[n] for n in range(len(a)) if n == 0 else runningTotal[n-1] + a[n]]

for i in zip(a, runningTotal):
    print "{0:>3}{1:>5}".format(*i)

yields

  0    0
  1    1
  2    3
  3    6
  4   10
  5   15
  6   21
  7   28
  8   36
  9   45
 10   55
 11   66
 12   78
 13   91
 14  105
 15  120
 16  136
 17  153
 18  171
 19  190

As you can see, I initialize an empty list [], then append() in each loop iteration. Is there a more elegant way to this, like a list comprehension?

cumulative-sum list-comprehension python
13 Answers

Starting Python 3.8, and the introduction of assignment expressions (PEP 572) (:= operator), we can use and increment a variable within a list comprehension:

# items = range(7)
total = 0
[(x, total := total + x) for x in items]
# [(0, 0), (1, 1), (2, 3), (3, 6), (4, 10), (5, 15), (6, 21)]

This:

  • Initializes a variable total to 0 which symbolizes the running sum
  • For each item, this both:
    • increments total by the current looped item (total := total + x) via an assignment expression
    • and at the same time returns the new value of total as part of the produced mapped tuple
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