Python中的if、else、elif语句用法简明讲解-Python Tutorial-php.cn

下面我们学习if语句，输入下面的代码，确保能够正确运行。

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

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：

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

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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Related labels：

else if python condition

source：php.cn

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