退出双层循环：
方式1：try--except

try:for i in range(5):for j in range(5):if i==3 and j ==3:raise
except:print(1)
pass

方式2：函数实现的return

def fun():for i in range(5):for j in range(5):print(i,j)if i==3 and j ==3:return True
fun()

方式3：多层break

for i in range(5):for j in range(5):for k in range(5):if i == j == k == 3:breakelse:print (i, '----', j, '----', k)else: continuebreakelse: continuebreak

习题12：输入3个数字，达到3个数字求和，结束程序

result = 0
for i in range(3):number = input("please input number: ")result += int(number)
print(result)
习题13、 用户输入不同的数据，当输入的数据达到3个数字的时候，求和结束程序。（数字可以是整数）
提示：判断是否整数的方法，isdigit()
遍历所有的输入数据，判断是否在0-9的字符串范围内
方式1：
#coding=utf-8
result = 0
count = 0
while True:s = input("please input the number: ")for v in s:if v not in "0123456789":#如果不是数字跳出当前循环breakelse:count+=1result += int(s)if count ==3:break
print(result)

方式2：先定义一个判断数字的函数

#encoding=utf-8
def is_int(num):for n in num:if n not in "0123456789":return False
return Trueresult = 0
number_count = 0
while True:s = input("please input the number: ")if is_int(s):result += int(s)number_count += 1  if number_count == 3:break
print(result)

方式3：利用isdigit()函数

result1 = 0
count1=0
while True:s = input("please input the number: ")if s.isdigit():count1+=1result1 += int(s)if count1 ==3:break
print(result1)

习题14：用嵌套列表的方式，遍历输出一个矩阵
方式1：

l = [[1,2,3],[4,5,6],[7,8,9]
]
for i in l:for j in i:print(j,end=" ")print()

方式2：

for i in range(len(l)):for j in range(len(l[i])):print(l[i][j],end = " ")print()

习题15：嵌套列表的正、反对角线之和
正对角线之和

l = [[1,2,3], [4,5,6], [7,8,9]  ]
rusult = 0
for i in range(len(l)):for j in range(len(l[i])):if i==j:rusult += l[i][j]
print(rusult)

反对角线之和

rusult = 0
for i in range(len(l)):for j in range(len(l[i])):if (i+j)==2:rusult += l[i][j]
print(rusult)

习题16：求以下矩阵四边元素之和
l = [
[1,2,3,4,5],
[1,2,3,4,5],
[1,2,3,4,5],
[1,2,3,4,5],
[1,2,3,4,5]
]

方法1：

1、第1行和第5行所有元素求和
2、其他行 只要第1列和第5列求和

rusult = 0for i in range(len(l)):for j in range(len(l[i])):if i == 0 or i == 4:rusult += l[i][j]else:if j==0 or j==4:rusult += l[i][j]
print(rusult)

方法2：所有元素之和，减去中间矩阵之和

l = [[1,2,3,4,5], [1,2,3,4,5], [1,2,3,4,5],[1,2,3,4,5],[1,2,3,4,5]
]matrix_element_sum = 0
sub_matrix_element_sum = 0for i in range(len(l)):for j in range(len(l[i])):matrix_element_sum += l[i][j]result_mid = 0for i in range(len(l)):for j in range(len(l[i])):if i == 0 or i ==4:continueelse:if j !=0 and j!=4:sub_matrix_element_sum += l[i][j]
print(matrix_element_sum - sub_matrix_element_sum)