a = [[1,2,3],[4,1],[2,2,6],[3]]
the sum of a would print 24
I know for a single list such as [1,2,3] i can just do sum(a) but I'm struggling with a nested list.
Thank you.
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2 Answers
This does not create a new temporary list:
sum(sum(b) for b in a)
3 Comments
Steven Rumbalski
+1. This just as clear as
sum(itertools.chain(*a)). However, I'm not sure why someone would simply rule out itertools.
Jack
Thanks for this, but i'm very new could you possibly explain what b is standing for? I just cannot wrap my head around it, maybe i need some sleep.
Steven Rumbalski
@jack:
b is one of your sublists. To see how this works use a for-loop: for b in a:print b.sum(sum(a, []))
The inner sum combines all the individual lists into one list. The outer sum then finds the actual sum.
For lists that are nested more than one deep, you can use this:
def recursiveSum(data):
try:
total = 0
for item in data:
total += recursiveSum(item)
return total
except TypeError: #data isn't iterable, it's probably a number
return data
print recursiveSum([[[[1,2,3],[4,1]],[2,2,6]],3])
output:
24
1 Comment
Steven Rumbalski
This has the potential to horribly inefficient. Each time a list is added a new and ever larger list is created.