Summing values in nested list when item changes

Question

I have some problem which has stumped my beginner-knowledge of python and I hope someone out there can point me in the correct direction.

I generated a nested list, each

Answer 1

Here's a variation on m170897017's technique:

a = [[1, 0],[1, 2],[2, 9],[3, 0],[3, 8],[3, 1]]

result = {}
for day, val in a:
    if day not in result:
       result[day] = 0
    result[day] += val

print result

#Convert back into a list
print [list(t) for t in result.items()]

output

{1: 2, 2: 9, 3: 9}
[[1, 2], [2, 9], [3, 9]]

If you're using Python 2.7 or later, you could also use a Counter.

Another possibility is to use a defaultdict, which has been available since Python 2.5.

from collections import defaultdict

a = [[1, 0],[1, 2],[2, 9],[3, 0],[3, 8],[3, 1]]

result = defaultdict(int)
for day, val in a:
    result[day] += val

print [list(t) for t in result.items()]

output

[[1, 2], [2, 9], [3, 9]]