Python - Summing elements on list of lists based on first element of inner lists

Question

I have a list

[[0.5, 2], [0.5, 5], [2, 3], [2, 6], [2, 0.6], [7, 1]]

I require the output from summing the second element in each sublist f

Answer 1

You can get away with sorting and itertools.groupby:

from operator import itemgetter
from itertools import groupby

data = [[0.5, 2], [0.5, 5], [2, 3], [2, 6], [2, 0.6], [7, 1]]

key = itemgetter(0)
data.sort(key=key)  # Use data = sorted(data, key=key) to avoid clobbering
result = [[k, sum(group)] for k, group in groupby(data, key)]

This will not preserve the original order of the keys.