Python: Sum part of a list

Question

Given a list, how can I find sums of part of the list? For example:

a = [2,5,4,6,8]
sum(a) = 25

I want to find out if a part of the list sums up to a certain number.
The list part should have a certain length. This is my goal:

ex_list = [2,5,4,6,8]
dif_list = partsum(ex_list, 3)
print(dif_list) ==> [11, 13, 15, 12, 14, 16, 15, 17, 19, 18]

Each element in 'dif_list' is given by taking 3 numbers out of 'ex_list' and summing them up, i.e. 2+5+4 = 11, 2+5+6 = 13, 2+5+8 = 15, 2+4+6 = 12, etc.
Also for reference:

ex_list = [2,5,4,6,8]
another_list = partsum(ex_list, 4)
print(another_list) ==> [17, 19, 21, 20, 23]

Because 2+5+4+6 = 17, 2+5+4+8 = 19, etc.
Basically, partsum(thing, num) will take 'num' items of 'thing', sum them up, and append it to a new list. All help is appreciated, thanks!

So you want all combinations with num elements and their sum? — timgeb
– timgeb, Commented Dec 30, 2017 at 20:50
If you are trying to find partitions, using combinations is the least efficient way. — user2285236
– user2285236, Commented Dec 30, 2017 at 20:53

Adam Smith · Accepted Answer · 2017-12-30 20:48:31Z

3

You want itertools.combinations.

import itertools

lst = [2,5,4,6,8]
combos = itertools.combinations(lst, 3)
# combos is equivalent to
# [ [2, 5, 4], [2, 5, 6], [2, 5, 8],
#   [2, 4, 6], [2, 4, 8], [2, 6, 8],
#   [5, 4, 6], [5, 4, 8], [5, 6, 8],
#   [4, 6, 8] ]
result = [sum(combo) for combo in combos]

answered Dec 30, 2017 at 20:48

Adam Smith

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8 Comments

hansaplast Over a year ago

and if you want to only have distinct sums change the last line into set([sum(combo) for combo in combos])

Stefan Pochmann Over a year ago

@hansaplast Why not a set comprehension?

Christopher Marley Over a year ago

@AdamSmith Thanks! And will this work if there are less than 3 items in a list 'lst' for combinations(lst,3)? (Or I can find out eventually...)

Christopher Marley Over a year ago

@adamsmith The program I'm using can't import itertools

Adam Smith Over a year ago

@ChristopherMarley that's fine, write your own version then. I'm not sure why you wouldn't be able to import it though, it's part of the stdlib LOL!

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