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 put the key in a dictionary and store the counts in the values. Like this:

#!/usr/bin/python
# -*- coding: utf-8 -*-

a = [[1, 0],[1, 2],[2, 9],[3, 0],[3, 8],[3, 1]]
res = {}
for i in a:
    if i[0] in res:
        res[i[0]] += i[1]
    else:
        res[i[0]] = i[1]

print res

OUTPUT:

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

This output is in dictionary format. You can turn it to list format as you like.