Pyhton学习之条件语句

条件 语句

条件语句是编程语言中重要的一部分，程序在处理的时候先检测条件，如果符合则执行语句块儿，如果不符合则不执行语句块。

python中的条件结构

if condition :
statement

condition 代表条件 如果条件成立，则执行statement语句块儿，如果条件不成立，则不执行语句块儿。
规则
在Python中，规定
·条件的结束以一个冒号“:”结束
·语句块儿必须以4个空格来进行缩进（一般是4个空格），python这样做就是为了每个编程人员养成良好的编程习惯。
·对于条件语句和后面将要学习的循环语句，最好使用文本编辑器，因为一旦在命令行直接输入，一旦错误，修改起来将会很麻烦

单条件结构也叫单分支语句

if condition**：**
statement

需求，假设a = 10,只有当a > 3 的时候，那么print a,

>>> a = 10>>> if a > 3:...     print a... 10>>> 

需求，假设string = name，只有当string == “name的时候输出string，否则输出“string is not name”

>>> string = "name">>> if string == "name":...     print string... else:...     print "string is not equal name"... name>>> if string == "age":...     print string... else:...     print "string is not equal name"... string is not equal name>>> 

我们这样做有点太古板了，就是每次我们都是先定义好的变量，然后在让其去执行，那么现在试试人机交互模式
先来看看帮助

help(raw_input)
Help on built-in function raw_input in module builtin:

raw_input(…)
raw_input([prompt]) -> string

Read a string from standard input.  The trailing newline is stripped.If the user hits EOF (Unix: Ctl-D, Windows: Ctl-Z+Return), raise EOFError.On Unix, GNU readline is used if enabled.  The prompt string, if given,is printed without a trailing newline before reading.

(END)
解释：从标准输入设备读取一个字符串。并且换行，如果用户触发EOF(unix ctl+D, windows:ctl-Z+Return)将会引发EOFError。
其余的就不解释了，反正就是从标准输入设备读取一个字符串

>>> string = raw_input()name>>> string'name'>>> string = raw_input()xiaohuafen>>> string'xiaohuafen'>>> 

那么在条件中我们应该怎么写。这里我使用文本编辑器vi，因为我的操作系统是Linux，所以使用vi编辑器，当然也可以在windows下使用sublime，editplus，notpad等等，但是不要使用txt文本编辑，因为可能会引发字符编码的错误。

#!/usr/bin/env python#coding:utf-8#anthor:xiaohuafen#description:this is test if condition statementstring = raw_input()if string == "name":    print "your input\'s string is %s" % stringelse:    print "your input\'s string is %s" % string    print "please input again"~                                

执行结果：

[root@python ~]# python iftest.py nameyour input's string is name[root@python ~]# python iftest.py pythonyour input's string is pythonplease input again[root@python ~]# 

多条件结构语句，也叫多分支语句

语法格式：

if condition:
statement
elif condition1:
statement1
elif condition2:
statemnet2
else
statement4

需求：假设从键盘接收一个数字number,如果number > 3 那么输出 number more than 3,并且输出Number，如果number == 3,那么输出number is equal 3,并输出Number，如果number<3那么输出number less than 3，并且输出Number，否则输出，you are computer

#!/usr/bin/env python#coding:utf-8number = raw_input()if number > 3:    print "your input's is %d" % number    print "number more than 3"elif number == 3:    print "your input's is %d" % nubmer    print "number is qual 3"elif number < 3:    print "your input's is %d" % number    print "number less than 3"else:    print "you are computer,please try again"~                        

执行结果：

[root@python ~]# python eliftest.py 3Traceback (most recent call last):  File "eliftest.py", line 6, in <module>    print "your input's is %d" % numberTypeError: %d format: a number is required, not str

我们发现报错了，提示说格式类型错误，请求的是一个数字，而不是字符串
我们之前说到了raw_input( ）函数，它只接受字符串
修改方法，进行类型转换或者修改输出属性
方法一，修改输出属性

#!/usr/bin/env python#coding:utf-8print "Please input a number: "number = raw_input()if number > 3:    print "your input's is %s" % number    print "number more than 3"elif number == 3:    print "your input's is %s" % number    print "number is qual 3"elif number < 3:    print "your input's is %s" % number    print "number less than 3"else:    print "you are computer,please try again"~        

将%d修改为%s
执行输出结果：

[root@python ~]# python eliftest.py Please input a number: 4your input's is 4number more than 3[root@python ~]# python eliftest.py Please input a number: 3your input's is 3number more than 3[root@python ~]# python eliftest.py Please input a number: 2your input's is 2number more than 3[root@python ~]# 

发现都只执行第一个条件，我们的程序执行有问题
方法二：修改条件，将条件修改为字符串

#!/usr/bin/env python#coding:utf-8print "Please input a number: "number = raw_input()if number > "3":    print "your input's is %s" % number    print "number more than 3"elif number == "3":    print "your input's is %s" % nubmer    print "number is qual 3"elif number < "3":    print "your input's is %s" % number    print "number less than 3"else:    print "you are computer,please try again"~                                            

执行结果：

[root@python ~]# python eliftest.py Please input a number: 4your input's is 4number more than 3[root@python ~]# python eliftest.py Please input a number: 3your input's is 3number is qual 3[root@python ~]# python eliftest.py Please input a number: 2your input's is 2number less than 3[root@python ~]# 

发现可以执行，但是number被当做字符串来执行
方法三，进行类型转换，将raw_input( )转换为整型

#!/usr/bin/env python#coding:utf-8print "Please input a number: "number = int(raw_input())if number > 3:    print "your input's is %s" % number    print "number more than 3"elif number == 3:    print "your input's is %s" % number    print "number is qual 3"elif number < 3:    print "your input's is %s" % number    print "number less than 3"else:    print "you are computer,please try again"

执行输出结果：

[root@python ~]# python eliftest.py Please input a number: 4your input's is 4number more than 3[root@python ~]# python eliftest.py Please input a number: 3your input's is 3number is qual 3[root@python ~]# python eliftest.py Please input a number: 2your input's is 2number less than 3

三元操作符

三元操作符在其它编程语言中也叫做三目运算符如java中，它们表示的都是一样的，是一种简单的条件结构
语句结构
statement if condition else statement

>>> a = 10>>> b = 3>>> string = "python" if a > b else "Java">>> string'python'>>> string = "python" if a < b else "Java">>> string'Java'>>> 

解释，如果condition为true，则执行string = “python”,如果condition 为假，则执行 string = “Java”
关于条件分支结构，就学习到这里，接下来要学习的是多种循环结构。