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

Problem's like this calls for itertools.groupby. Particularly groupby, groups consecutive values with same keys, where keys can be specified by user. Nevertheless, in this case, key is trivial enough as indexing a particular element of a list, so that should be a reason enough to use operator.itemgetter. Finally, you can wrap up either as a functional (using map/imap) or as a generator expression based on your taste and choice.

>>> from itertools import groupby, imap
>>> from operator import itemgetter
>>> lst=[[1, 0],[1, 2],[2, 9],[3, 0],[3, 8],[3, 1]]
>>> [[k, sum(imap(itemgetter(1), v))]
     for k, v in groupby(lst,key = itemgetter(0))]
[[1, 2], [2, 9], [3, 9]]