数组中第一或第二位的所有数字之和

Question

I have a 2d list, for example:

list1 = [[1,2],[3,4],[5,6],[7,8]] 

and I want to find the sum of all the numbers at the n'th place of every element.
For example if I want the answer for 0, I would calculate:

 my_sum = list1[0][0] + list1[1][0] + list1[2][0]  

or

my_sum = 0  
place = 0  
for i in range(len(list1)):  
    my_sum += list1[i][place]
return my_sum

Output: 16

Is there a more elegant way to do this? Or one that uses only one line of code?
I mean as fictional code for example:

fictional_function(list1,place) = 16

Answer 1

As a generalization if you want multiple indices (eg 0 and 1) you could use reduce combined with and element-wise sum something like this:

from functools import reduce


def fictional_function(lst, *places):
    s_places = set(places)

    def s(xs, ys):
        return [x + y for x, y in zip(xs, ys)]

    return [x for i, x in enumerate(reduce(s, lst)) if i in s_places]


list1 = [[1, 2], [3, 4], [5, 6], [7, 8]]
print(fictional_function(list1, 0))
print(fictional_function(list1, 0, 1))
print(fictional_function(list1, *[1, 0]))

Output

[16]
[16, 20]
[16, 20]

The idea is that the function s sums two list element-wise, for example:

s([1, 2], [3, 4])  # [4, 6]

and with reduce apply s to a list of lists, finally filter the result for the intended indices (places) only.

Answer 2

Since you are looking for a functional solution, consider operator.itemgetter :

from operator import itemgetter

L = [[1,2],[3,4],[5,6],[7,8]]

res = sum(map(itemgetter(0), L))  # 16

For performance and simpler syntax, you can use a 3rd party library such as NumPy:

import numpy as np

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

res = A[:, 0].sum()  # 16

Answer 3

list1 = [[1,2],[3,4],[5,6],[7,8]]
ind = 0
sum_ind = sum(list(zip(*list1))[ind])

The above can be even written as function taking list and the index as input and returns the sum of the common index. What we do in the above is first we get all the same indexes to individual lists using zip and then chooses which index one has to be summed and passes the same to sum function.