# 习题 29: 如果(if)¶

 ``` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29``` ```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 or equal to dogs.
People are less than or equal to dogs.
People are dogs.
\$
```

## 加分习题¶

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 27 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？

## 常见问题回答¶

+= 是什么意思？

x += 1x = x + 1 一样，只不过可以少打几个字母。你可以把它叫做加值符。一样的，你后面还会学到 -= 以及很多别的表达式。