def two_dimensionalArray(self):
 '二维数组实现求三阶矩阵的对角线元素之和'
 sum = 0
 matrix = [[0, 1, 0], [0, 21, 0], [0, 12, 0]]
 matrix2 = [[0 for i in range(3)] for i in range(3)]
 matrix2[0][0] = 123
 matrix2[1][1] = 123
 matrix2[2][2] = 123
 for i in range(3):
  sum += matrix2[i][i]
 print(sum)