'''求一个3*3矩阵主对角线元素之和。'''
a = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
c = 0
for b in range(0, 3):
c += a[b][b]
c += a[b][-b]
print(c)
'''求一个3*3矩阵主对角线元素之和。'''
a = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
c = 0
for b in range(0, 3):
c += a[b][b]
c += a[b][-b]
print(c)