n = 10
a = [[1, 2, 3],
[4, 5, 6],
[7, 8, 9]]
s_1 = {i for i in range(n)}
s_2 = {i for i in range(n)}
s = 0
i = 0
for b in a:
    if i in s_1:
        j = 0
        for c in b:
            if j in s_2:
                s = s + a[i][j]
            j = j + 1
    i = i + 1
print(s)

from random import randint
n = 10
A = [[1, 1, 1, 1, 1],
     [2, 2, 2, 2, 2],
     [3, 3, 3, 3, 3],
     [4, 4, 4, 4, 4],
     [5, 5, 5, 5, 5]]
rows = (randint(0, len(A) - 1) for _ in range(n))
cols = (randint(0, len(A[0]) - 1) for _ in range(n))
print(sum(A[i][j] for i, j in zip(rows, cols)))

import numpy as np
a=np.array([[100*i+j for i in range(5)] for j in range(5)])
>>> a[[1,3,4]][:,[0,1]].sum()
316

n = 10
a = [[1, 2, 3],
[4, 5, 6],
[7, 8, 9]]
s_1 = {i for i in range(n)}
s_2 = {i for i in range(n)}

print(A.sum())

print(A.sum())

# -*-coding:utf-8-*-
a = [[1, 1, 1, 1, 1],
     [2, 2, 2, 2, 2],
     [3, 3, 3, 3, 3],
     [4, 4, 4, 4, 4],
     [5, 5, 5, 5, 5]]
cols = {1, 2, 3}
rows = {0, 2, 4}
s = 0
for row_num, row in enumerate(a):
    for col_num, item in enumerate(row):
        if row_num in rows and col_num in cols:
            s += item
            
print(s)