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

You can try using defaultdict ...

from collections import defaultdict

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

d = defaultdict(int)
for k,v in dat: d[k] += v