下面我们学习if语句，输入下面的代码，确保能够正确运行。
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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."
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运行结果
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root@he-desktop:~/mystuff# python ex29.py
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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.
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加分练习
通过上面的练习，我们自己猜测一下if语句的作用，用自己的话回答下面的问题。
1. 你认为if对它下面的代码做了什么？
判断为True就执行它下面的代码，否则不执行。
2. 为什么if下面的代码要缩进4个空格？
为了表示这些代码属于if判断下包括的代码。
3. 如果不缩进会发生什么？
会提示一个缩进错误。
4. 你可以从第27节中拿一些布尔表达式来做if判断吗？
5. 改变people，dogs，cats变量的值，看看会发生什么？
答案：
1. if语句下面的代码是if的一个分支。就像书里的一个章节，你选择了这章就会跳到这里阅读。这个if语句就像是说：“如果布尔判断为True，就执行下面的代码，否则跳过这些代码”。
2. 用冒号结束一个语句就是要告诉python，我要开始一个新的代码段了。缩进4个空格就是说，这些代码是包含在这个代码段中的，和函数的使用一样。
3. 不缩进会报错，python规定冒号后面语句必须有缩进。
4. 可以，而且可以是复杂的语句。
5. 修改变量的值后，判断语句就会相应的变True或者False，然后输出不同的语句。
比较我的答案和你自己的答案，确保你能理解代码块这个概念，因为这个对于下面的练习非常重要。
输入下面的代码，运行它：
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people = 30
cars = 40
buses = 15
if cars > people:
print "We should take the cars."
elif cars < people:
print "We should not take the cars."
else:
print "We can't dicide."
if buses > cars:
print "That's too many buses."
elif buses < cars:
print "Maybe we could take the buses."
else:
print "We still can't decide."
if people > buses:
print "Alright, let's just take the buses."
else:
print "Fine, let's stay home then."
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运行结果
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root@he-desktop:~/mystuff# python ex30.py
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We should take the cars.
Maybe we could take the buses.
Alright, let's just take the buses.
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