ex29-30 if,elif and else.

1、if 的作用在于为代码创建了一个所谓的“分支”。if 语句告诉脚本：如果这个布尔表达式为真，就运行接下来的代码。

2、行尾的冒号作用在于告诉python 接下来要创建一个新的代码块，这和创建函数是一个道理。

3、在pyhton 的运行规则里，只要一行一（：）结尾，接下来的内容就应该有缩进。

4、如果python遇到多个elif块都是True,那么只有第一个为True的块会运行。

下边是ex29的代码。

#-*- coding: UTF-8 -*- 
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."
    
#在if 语句中加入布尔值
if False or True:
    print "people are dogs."
    
if False and True:
    print "people are dogs."

下面是ex30的代码：

#-*- coding: UTF-8 -*-
people = 30
cars = 40
buses = 15

if cars > people:
    print "we should take cars."
elif cars < people:
    print "We should not take cars."
else:
    print "We can't decide."
    
if buses > cars:
    print "That's too many buses."
elif buses < cars:
    print "maybe we should take 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."

#稍微复杂一点的布尔表达式
if people > cars and cars <buses:
    print "YOhooooo"
else:
    print "Fine,let's stop here."