笨办法学Python 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。

#?-*-?coding:utf-8?-*-
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."

结果

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.

另外一种精简写法：

#?-*-?coding:utf-8?-*-
people?=?20
cats?=?30
dogs?=?15

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

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

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

elif?people?>?dogs:
????print?"The?world?is?dry!"

dogs?+=?5
if?people?>?dogs:
????print?"People?are?greater?than?or?equal?to?dogs."
elif?people?<?dogs:
????print?"People?are?less?than?or?equal?to?dogs."
else:
????print?"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？

????如果if判断成功则进行下面，否则退出判断。

2. 为什么 if 语句的下一行需要 4 个空格的缩进？

????和def一样

3. 如果不缩进，会发生什么事情？

????报错

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？

????判断结果不一样

常见问题回答

+= 是什么意思？

x += 1 和 x = 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。

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