习题 31: 作出决定

这本书的上半部分你打印了一些东西，而且调用了函数，不过一切都是直线式进行的。你的脚本从最上面一行开始，一路运行到结束，但其中并没有决定程序流向的分支点。现在你已经学了 if, else, 和 elif ，你就可以开始创建包含条件判断的脚本了。

上一个脚本中你写了一系列的简单提问测试。这节的脚本中，你将需要向用户提问，依据用户的答案来做出决定。把脚本写下来，多多鼓捣一阵子，看看它的工作原理是什么。

print "You enter a dark room with two doors.  Do you go through door #1 or door #2?"

door = raw_input("> ")

if door == "1":
    print "There's a giant bear here eating a cheese cake.  What do you do?"
    print "1. Take the cake."
    print "2. Scream at the bear."

    bear = raw_input("> ")

    if bear == "1":
        print "The bear eats your face off.  Good job!"
    elif bear == "2":
        print "The bear eats your legs off.  Good job!"
    else:
        print "Well, doing %s is probably better.  Bear runs away." % bear

elif door == "2":
    print "You stare into the endless abyss at Cthulhu's retina."
    print "1. Blueberries."
    print "2. Yellow jacket clothespins."
    print "3. Understanding revolvers yelling melodies."

    insanity = raw_input("> ")
    
    if insanity == "1" or insanity == "2":
        print "Your body survives powered by a mind of jello.  Good job!"
    else:
        print "The insanity rots your eyes into a pool of muck.  Good job!"

else:
    print "You stumble around and fall on a knife and die.  Good job!"

这里的重点是你可以在“if 语句”内部再放一个“if 语句”。这是一个很强大的功能，可以用来创建嵌套(nested)的决定，其中的一个分支将引向另一个分支的子分支。

你需要理解 if 语句 包含 if 语句 的概念。做一下加分习题，这样你会确信自己真正理解了它们。

你应该看到的结果

我在玩一个小冒险游戏，我玩的水平不怎么好：

$ python ex31.py 
You enter a dark room with two doors.  Do you go through door #1 or door #2?
> 1
There's a giant bear here eating a cheese cake.  What do you do?
1. Take the cake.
2. Scream at the bear.
> 2
The bear eats your legs off.  Good job!

$ python ex31.py 
You enter a dark room with two doors.  Do you go through door #1 or door #2?
> 1
There's a giant bear here eating a cheese cake.  What do you do?
1. Take the cake.
2. Scream at the bear.
> 1
The bear eats your face off.  Good job!

$ python ex31.py 
You enter a dark room with two doors.  Do you go through door #1 or door #2?
> 2
You stare into the endless abyss at Cthuhlu's retina.
1. Blueberries.
2. Yellow jacket clothespins.
3. Understanding revolvers yelling melodies.
> 1
Your body survives powered by a mind of jello.  Good job!

$ python ex31.py 
You enter a dark room with two doors.  Do you go through door #1 or door #2?
> 2
You stare into the endless abyss at Cthuhlu's retina.
1. Blueberries.
2. Yellow jacket clothespins.
3. Understanding revolvers yelling melodies.
> 3
The insanity rots your eyes into a pool of muck.  Good job!

$ python ex31.py 
You enter a dark room with two doors.  Do you go through door #1 or door #2?
> stuff
You stumble around and fall on a knife and die.  Good job!

$ python ex31.py 
You enter a dark room with two doors.  Do you go through door #1 or door #2?
> 1
There's a giant bear here eating a cheese cake.  What do you do?
1. Take the cake.
2. Scream at the bear.
> apples
Well, doing apples is probably better.  Bear runs away.

加分习题

为游戏添加新的部分，改变玩家做决定的位置。尽自己的能力扩展这个游戏，不过别把游戏弄得太怪异了。

常见问题回答

可以用多个 if/else 来取代 elif 吗？

有时候可以，不过这也取决于额 if/else 是怎样写的，而且这样一来 python 就需要去检查每一处 if/else，而不是像 if/elif/else 一样，只要检查到第一个 True 就可以停下来了。试着写些代码看两者有何不同。

怎样判断一个数字处于某个值域中？

两个办法：经典语法是使用 1 < x < 10，或者用 x in range(1, 10) 也可以。

怎样用 if/elif/else 区块实现四个以上的条件判断？

简单，多写几个 elif 区块就可以了。