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people = 20
cats = 20
dogs = 15


if people < cats:
   print "Too many cats! The world os 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."

习题重点

1. if语句的功能是处理到True就打印出下面四格缩进的内容

附加练习

1. 你认为if语句对它的下一行代码做了什么？ 答： if的意思就是遇见True就将下面的内容打印下来。
2. 为什么if语句的下一行需要4个空格的缩进。答：就像函数那样，像一个独立的小脚本但性质不一样了。
3. 如果不缩进会发生什么事情？答：**python会直接打印那些字符串，并且不管**if语句的命令了。
4. 把习题27中的其他布尔表达式放到if语句中会不会也可以运行呢？试一下！

people = 20
cats = 30
dogs = 15


if people == cats:
   print "Too many cats! The world os doomed!"

if people == cats:
   print "Not many cats! The world is saved!"

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

if people or 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."

注：+=为“递增”运算。