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?
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))
.