Python条件控制与循环语句

1. 条件控制

# if-elif-else结构
age = 12

if age < 4:
    price = 0
elif age < 18:
    price = 5
else:
    price = 10

print("Your admission cost is $" + str(price) + ".")

# Your admission cost is $5.

可以使用多个elif代码块，也可以省略else代码块。

1.1 使用if语句处理列表

# 确定列表不是空的
requested_toppings = []

if requested_toppings:
    for requested_topping in requested_toppings:
        print("Adding " + requested_topping + ".")
    print("\nFinished making your pizza!")
else:
    print("Are you sure you want to plain pizza?")
    
# Are you sure you want to plain pizza?

# 使用多个列表
available_toppings = ['mushrooms', 'olives', 'green peppers', 'pepperoni', 'pineapple', 'extra cheese']

requested_toppings = ['mushrooms', 'french fries', 'extra cheese']

for requested_topping in requested_toppings:
    if requested_topping in available_toppings:
        print("Adding " + requested_topping + ".")
    else:
        print("Sorry, we don't have " + requested_topping + ".")

print("\nFinished making your pizza!")
# Adding mushrooms.
# Sorry, we don't have french fries.
# Adding extra cheese.
# 
# Finished making your pizza!

2. 循环语句

n = 100
 
sum = 0
counter = 1
while counter <= n:
    sum = sum + counter
    counter += 1
 
print("1 到 %d 之和为: %d" % (n,sum))

# 1 到 100 之和为: 5050

2.1 while 循环使用 else 语句

count = 0
while count < 5:
   print (count, " 小于 5")
   count = count + 1
else:
   print (count, " 大于或等于 5")

# 0  小于 5
# 1  小于 5
# 2  小于 5
# 3  小于 5
# 4  小于 5
# 5  大于或等于 5  

如果你的while循环体中只有一条语句，你可以将该语句与while写在同一行中。

while(True): print('Hello!')

注意：以上的无限循环你可以使用 CTRL+C 来中断循环。

2.2 for语句

languages = ["C", "C++", "Perl", "Python"]
for x in languages:
    print (x)

# C
# C++
# Perl
# Python    

2.3 break语句

n = 1
while n <= 10:
    if n > 5: # 当n = 6时，条件满足，执行break语句
        break # break语句会结束当前循环
    print(n)
    n = n + 1
print('END')

# 1
# 2
# 3
# 4
# 5
# END

2.4 continue语句

n = 0
while n < 10:
    n = n + 1
    if n % 2 == 0: # 如果n是偶数，执行continue语句
        continue # continue语句会直接继续下一轮循环，后续的print()语句不会执行
    print(n)

# 1
# 3
# 5
# 7
# 9

参考资料：

• Python3 教程 | 菜鸟教程
• Python教程 - 廖雪峰的官方网站
• 《Python编程从入门到实践》——【美】Eric Matthes 著