How can I sum every n array values and place the result into a new array? [duplicate]

Answer 1

Cut the array to the correct length first then do a reshape.

import numpy as np

N = 3
a = np.array([1,2,3,4,5,6,7,8,1,2,3,4,5,6,7,8])


# first cut it so that lenght of a % N is zero
rest = a.shape[0]%N
a = a[:-rest]


assert a.shape[0]%N == 0


# do the reshape
a_RS = a.reshape(-1,N)
print(a_RS)

>> [[1 2 3]
    [4 5 6]
    [7 8 1]
    [2 3 4]
    [5 6 7]]

then you can simply add it up:

print(np.sum(a_RS,axis=1))

>> [ 6 15 16  9 18]

Answer 2

You can use a list comprehension do this:

ls = [1,2,3,4,5,6,7,8,1,2,3,4,5,6,7,8]
res = [sum(ls[i:i+3]) for i in range(0, len(ls), 3)]

[6, 15, 16, 9, 18, 8]

This will result in all the numbers being included in the resulting sum. If you don't want this to happen, then you can just check for it and replace the last sum with whatever value you want:

if (len(ls)%3) != 0:
    res[-1] = 'x' 

[6, 15, 16, 9, 18, 'x']

Or remove it entirely:

if (len(ls)%3) != 0:
    res[:] = res[:-1] 

[6, 15, 16, 9, 18]

Answer 3

Why don't you just simply use a list comprehension? E.g.

my_list = [1,2,3,4,5,6,7,8,1,2,3,4,5,6,7,8]
len_list = len(my_list) - len(my_list) % 3  # ignore end of list, s.t., only tuples of three are considered
[my_list[i] + my_list[i+1] + my_list[i+2] for i in range(0, len_list, 3)]