if 及循环语句

 1. if 语句

• if语句支持嵌套
• 当代码块不想执行任何内容时，需用pass占位

if 条件:
　　代码块
else:
　　代码块

if 条件:
　　代码块
elif 条件:
　　代码块
else:
　　代码块

if 1==1:
    if 2==2:
        print('Hello')
    else:
        print('welcome')
else:
    print('bye')

if 1==1:
    pass
else:
    print('hello')

 2. while循环语句

# 死循环
import time

while 1==1:
    print('OK',time.time())
#--------------------------------------------

import time

count = 0

while count < 10:
    print('OK',time.time())
    count += 1
print(123)
#--------------------------------------------

count = 0
while count < 10:
    print(count)
    count += 1
else:
    print('else')
print('...')

 

# 使用while循环输入1 2 3 4 5 6    8 9 10

count = 1
while count < 11:
    if count != 7:
        print(count,end=' ')
        count += 1
    else:
        print(' ',end=' ')
        count += 1
# -------------------------------------------------

# 求1-100的所有数的和

s = 0
count = 0
while count < 100:
    count +=1
    s += count
print(s)
# -------------------------------------------------

# 输出1-100内的所有奇数

count = 1
while count <= 100:
    if count%2 != 0:
        print(count, end=' ')
    count += 1
# -------------------------------------------------

#输出1-100内的所有偶数

count = 1
while count <= 100:
    if count%2 ==0:
        print(count, end=' ')
    count += 1
# -------------------------------------------------

#求1-2+3-4+5...99的所有数的和

count = 0
s1 = 0
s2 = 0
while count < 99:
    count += 1
    if count%2 !=0:
        s1 += count
    else:
        s2 += count
print(s1-s2)

# 即就是：
count = 1
s = 0
while count < 100:
    if count%2 !=0:
        s += count
    else:
        s -= count
    count += 1
print(s)
# -------------------------------------------------

#用户登录（三次机会重试）

count = 0
while count < 3:
    user = input('请输入用户名：')
    pwd = input('请输入密码：')
    if user == 'root' and pwd == 'root':
        print('欢迎登录')
        break
    else:
        if count != 2:
            print('用户名或密码输入有误，请重新输入，若连续输入错误3次，今日将锁定账户')
        else:
            print('连续输入错误3次，账户已锁定')
    count += 1

3. for循环语句

for 变量名 in 字符串
    代码块

4. break和continue

• continue，终止当前循环，开始下一次循环
• break，终止所有循环

count = 0
while count < 10:
#当count等于7时，执行continue终止当前循环，不再执行后面的print和count+=1，直接回到while开头开始下一次循环
    if count == 7:
        count += 1
        continue
    print(count)
    count += 1
# -------------------------------------------------

count = 0
while count < 10:
    count += 1
    print(count)
    break
    print(111)
print('end')
#结果为1和end，遇到break终止所有循环，不会执行print(111)，直接跳出循环向下继续执行
# -------------------------------------------------

test = '朝辞白帝彩云间'

for item in test:
    print(item)
    break
# 朝

for item in test:
    continue
    print(item)
# 什么也不输出