Sum tuples if identical values

Question

Here's my list of tuples:

regions = [(23.4, 12, 12341234), 
           (342.23, 19, 12341234), 
           (4312.3, 12, 12551234), 
           (234.2, 12, 12341234)]

I'm trying to sum the first index value in a list of tuples where the values at indices 1 and 2 are identical. Note that regions[0] and regions[3] have the same values at indices 1 and 2.

My desired list is:

result = [(257.6, 12, 12341234), 
          (342.23, 19, 12341234), 
          (4312.3, 12, 12551234)]

I realize that I probably need to save it as a dictionary first, probably with the second value as the first key and the third value as the second key and summing if it already exists. Just wondering if there's a better or easier way: I'm thinking maybe using some intersect function.

Does the ordering of tuples in the result matter?

NPE
– NPE

2013-04-11 16:05:03 +00:00
Commented Apr 11, 2013 at 16:05 — NPE
– NPE, Commented Apr 11, 2013 at 16:05

greole · Accepted Answer · 2013-12-29 23:01:16Z

6

from collections import defaultdict

sums = defaultdict(float)
for c, a, b in regions:
    sums[a, b] += c
result = [(csum, a, b) for (a, b), csum in sums.iteritems()]

There isn't a built-in function to do this; it's far too specialized a task.

edited Dec 29, 2013 at 23:01

greole

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answered Apr 11, 2013 at 16:05

Amber

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4 Comments

Tim Pietzcker Over a year ago

Beat me to it - and more completely :)

bozdoz Over a year ago

and is .iteritems() better/different than .items()?

William Over a year ago

.iteritems() is for Python 2 and I believe returns a generator. In Python 3 .iteritems() does not exist. @bozdoz

jfs Over a year ago

you could drop (). A tuple is created by a comma (,): for a, b, c in regions: results[b,c] += a. You could also unpack in the for-loop: result = [(a, b, c) for (b, c), a in sums.items()]. Or result = [(a,) + t for t, a in sums.items()]

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