习题 29: 如果(if)

下面是你要写的作业，这段向你介绍了“if语句”。把这段输入进去，让它能正确执行。然后我们看看你是否有所收获。

people = 20
cats = 30
dogs = 15


if people < cats:
    print "Too many cats! The world is doomed!"

if people > cats:
    print "Not many cats! The world is saved!"

if people < dogs:
    print "The world is drooled on!"

if people > dogs:
    print "The world is dry!"


dogs += 5

if people >= dogs:
    print "People are greater than or equal to dogs."

if people <= dogs:
    print "People are less than or equal to dogs."


if people == dogs:
    print "People are dogs."

你应该看到的结果

$ python ex29.py 
Too many cats! The world is doomed!
The world is dry!
People are greater than equal to dogs.
People are less than equal to dogs.
People are dogs.
$

加分习题

猜猜“if语句”是什么，它有什么用处。在做下一道习题前，试着用自己的话回答下面的问题:

1. 你认为 if 对于它下一行的代码做了什么？
2. 为什么 if 语句的下一行需要 4 个空格的缩进？
3. 如果不缩进，会发生什么事情？
4. 把习题 26 中的其它布尔表达式放到``if语句``中会不会也可以运行呢？试一下。
5. 如果把变量 people, cats, 和 dogs 的初始值改掉，会发生什么事情？