条件判断在程序的重要组成部分，一段智能化的程序中必然包含有条件判断语句。
条件判断语句格式：
if 表达式1:
代码1
elif 表达式2:
代码2
else:
代码3
如果表达式1为真，则执行代码1；
如果表达式1为假且表达式2为真，则执行代码2；
如果表达式1和表达式2都为假则执行代码3.
条件语句可以嵌套使用，以便进行更智能化的判断。
注意语法格式：
1.表达式后跟冒号
2.缩进
教材代码
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 is 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."
这是个文字游戏程序，条件语句进行了2层嵌套。
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