这本书的上半部分，你印出了一些东西，并且呼叫了函式，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式流向的分支点。现在你已经学会了 if、else和 elsif，你就可以开始建立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，你将需要向使用者提问，依据使用者的答案来做出决定。把脚本写下来，多多捣鼓一阵子，看看他的运作原理是什么。
def prompt
print "> "
end
puts "You enter a dark room with two doors. Do you go through door #1 or door #2?"
prompt; door = gets.chomp
if door == "1"
puts "There's a giant bear here eating a cheese cake. What do you do?"
puts "1\. Take the cake."
puts "2\. Scream at the bear."
prompt; bear = gets.chomp
if bear == "1"
puts "The bear eats your face off. Good job!"
elsif bear == "2"
puts "The bear eats your legs off. Good job!"
else
puts "Well, doing #{bear} is probably better. Bear runs away."
end
elsif door == "2"
puts "You stare into the endless abyss at Cthuhlu's retina."
puts "1\. Blueberries."
puts "2\. Yellow jacket clothespins."
puts "3\. Understanding revolvers yelling melodies."
prompt; insanity = gets.chomp
if insanity == "1" or insanity == "2"
puts "Your body survives powered by a mind of jello. Good job!"
else
puts "The insanity rots your eyes into a pool of muck. Good job!"
end
else
puts "You stumble around and fall on a knife and die. Good job!"
end这里的重点是你可以在 if 语句中内部再放一个 if 语句。这是一个很强大的功能，可以用来建立“巢状(nested)”的决定（decision)。
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。
我在玩一个小冒险游戏。我的水准不怎么样。
$ ruby ex31.rb
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!
$ ruby ex31.rb
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!
$ ruby ex31.rb
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!
$ ruby ex31.rb
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!
$ ruby ex31.rb
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!
$ ruby ex31.rb
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.为游戏添加新的部分，改变玩家做决定的位置。尽自己能力扩充这个游戏，不过别把游戏弄得太诡异了。