if 条件1: 
　　代码块1
elif 条件2:
　　代码块2
else:
　　代码块3

age = 3
if age >= 18:
    print(‘adult‘)
elif age >= 6:
    print(‘teenager‘)
else:
    print(‘kid‘)


if a == 9:
    pass
elif a != 9:
    pass 
else
    pass

if判断条件还可以简写，比如写：
if x:
    print(‘True‘)

# -*- coding:utf-8 -*-

‘‘‘
input()返回的数据类型是str，str不能直接和整数比较，可以进行类型转换。
‘‘‘
a = input("请输入：")
age = int(a)
if age >= 20:
    print("已成年")
elif age > 50:
    print("中年")
else:
    print("其他")

if age < 20:
    print("未成年")
elif age < 0:
    print("输入有误：")
else:
    print("age=%d" % (age))

关键字：
while ... :

while ... : 代码块1  [else: 代码块2]

for ... in ... :

continue                 # 跳出当前循环

break                      # 跳出整个循环

# -*- coding:utf-8 -*-
‘‘‘
计算1~100的累计和
‘‘‘
i = 1
sum = 0
while i<=100:
    sum += i
    i += 1

print("1~100的累积和为：%09d"%(sum))

count = 1
while count < 10:
    continue    # 跳出当前循环
    print(123)  # 永远不会执行
print(‘end‘)    

count = 1
while count < 10:
    print(count)     # 输出 1 2 3 ... 9
    count = count + 1
else:   
    print(‘else‘)    #当count=10时，跳出循环，执行一次else
print(‘...‘)    

m = 1
while m <= 9:
    n = 1
    while n <= m:
        print("%d * %d = %d" % (n, m, m * n), end="\t")
        if m == n:
            print()
        n += 1
    m += 1

#1、使用while循环输入 1 2 3 4 5 6   8 9 10
n = 1
while n < 11:
    if n == 7:
        pass
    else:
        print(n)
    n = n + 1    
    
#2、求1-100的所有数的和
n = 1 
sum = 0
while n < 101:
    sum = sum + n
    n = n + 1
print(sum)    

#3、输出1-100内所有奇数
n = 1
while n < 101:
    tmp = n % 2
    if tmp == 0:
        pass
    else:
        print(n)
    n = n + 1        

# 4、求1-2+3-4+5-6...99的所有数的和
n = 1
s = 0      
while n < 100:
    tmp = n % 2
    if tmp == 0:
        s = s - n
    else:
        s = s + n
    n = n + 1
print(s)    

count  = 0
while count < 3:
    user = input(‘>>>‘)
    pwd = input(‘>>>‘)
    if user == ‘alex‘ and pwd == ‘123‘:
        print(‘欢迎登录‘)
        ...
        break
    else:
       print(‘用户名或者密码错误‘)
       count = count + 1  

for  临时变量  in  列表或者字符串等:
    循环满足条件时执行的代码
else:
    循环不满足条件时执行的代码

#!/usr/bin/env python
# -*- coding:utf-8 -*-

for a in range(1,101):
    if a % 2 == 0:
        continue  # 跳出当前循环
    if a > 50:
        break  # 跳出整个for循环
    print(a)
    
    
for c in "abcd":
    print(c,end="\t")    #  a    b    c    d