How do I sum the first value in each tuple in a list of tuples in python?

Question

Hi,

I have a list of tuples (always pairs) like this:

[(0, 1), (2, 3), (5, 7), (2, 1)]

I'd like to find the sum of the first items in each pair, i.e.:

0 + 2 + 5 + 2

How can I do this in python? At the moment I'm iterating through the list:

sum = 0
for pair in list_of_pairs:
   sum += pair[0]

but I have a feeling there must be a more pythonic way.

Thanks,

Ben

Answer 1

sum(pair[0] for pair in list_of_pairs)

Answer 2

sum(i for i, j in list_of_pairs)

will do too.

Answer 3

If you have a very large list or a generator that produces a large number of pairs you might want to use a generator based approach. For fun I use itemgetter() and imap(), too. A simple generator based approach might be enough, though.

import operator
import itertools
idx0 = operator.itemgetter(0)
list_of_pairs = [(0, 1), (2, 3), (5, 7), (2, 1)]
sum(itertools.imap(idx0, list_of_pairs)

Edit: itertools.imap() is available in Python 2.3. So you can use a generator based approach there, too.

Answer 4

Obscure (but fun) answer:

>>> sum(zip(*list_of_pairs)[0])
9

Or when zip's are iterables only this should work:

>>> sum(zip(*list_of_pairs).__next__())
9