Adding certain elements of list

Question

I want to add elements in a list every ten elements. For example:

a = [5, 31, 16, 31, 19, 5, 25, 34, 8, 13, 17, 17, 43, 9, 29, 41, 8, 24,
48, 1, 28, 20, 37, 40, 32, 35, 9, 36, 17, 46, 10, 30, 49, 28, 2, 3, 8,
11, 36, 20, 7, 24, 29, 15, 0, 4, 35, 11, 42, 7, 28, 40, 31, 45, 6, 45,
15, 27, 39,  6]

So I want to create a new list with the sum of every 10 elements, such as:

new = [187, 237, 300, 197, 174, 282]

Where the first entry corresponds to the add up of the first 10 numbers:

x = sum(5, 31, 16, 31, 19, 5, 25, 34, 8, 13)
x = 187

The second one to the 10 numbers in the range 10-19:

y = sum(17, 17, 43, 9, 29, 41, 8, 24, 48, 1)
y = 237

And so on; is there an efficient way to do this?

Possible duplicate of [How do you split a list into evenly sized chunks in Python?](http://stackoverflow.com/questions/312443/how-do-you-split-a-list-into-evenly-sized-chunks-in-python) — Mureinik, Jul 24 '16 at 16:06

Colonel Beauvel · Accepted Answer · 2016-07-24T16:50:41.860

3

In [25]: map(sum, zip(*[iter(a)]*10))
Out[25]: [187, 237, 300, 197, 174, 282]

edited Jul 24 '16 at 16:50

answered Jul 24 '16 at 16:08

Colonel Beauvel

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Well done. Where, or how, did you learn about the `zip(*[iter(a)]*10)`? – Jossie Calderon Jul 24 '16 at 16:27

Moses Koledoye · Answer 2 · 2016-07-24T16:10:17.500

2

Use map on an iterator of the list:

>>> it = iter(a)
>>> map(lambda *x: sum(x), *(it,)*10)
[187, 237, 300, 197, 174, 282]

Create an iterator for your list. Pass the items to map in 10s using the iterator and use map to return the sum of the passed parameters.

Python 3.x will require an explicit list call on map

edited Jul 24 '16 at 16:10

answered Jul 24 '16 at 16:04

Moses Koledoye

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score 1 · Answer 3 · answered Jul 24 '16 at 16:08

You can use nested comprehensions with a list iterator:

i= iter(a)
s= [sum(next(i) for _ in range(10)) for _ in range(len(a)//10)]
print s

Note that this will silently ignore any leftover values:

a= [1]*11 #<- list has 11 elements
i= iter(a)
s= [sum(next(i) for _ in range(10)) for _ in range(len(a)//10)]
print s
# output: [10]

score 1 · Answer 4 · answered Jul 24 '16 at 17:18

1

How about list comprehension, like this one:

>>> step = 10
>>> 
>>> [sum(a[x:x+step]) for x in range(0, len(a), step)]
[187, 237, 300, 197, 174, 282]

answered Jul 24 '16 at 17:18

Iron Fist

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Adding certain elements of list

4 Answers4