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 statement string = raw_input() if string == "name": print "your input\'s string is %s" % string else: print "your input\'s string is %s" % string print "please input again" ~

执行结果：

[root@python ~]# python iftest.py name your input's string is name [root@python ~]# python iftest.py python your input's string is python please 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-8 number = 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 3 Traceback (most recent call last): File "eliftest.py", line 6, in <module> print "your input's is %d" % number TypeError: %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-8 print "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: 4 your input's is 4 number more than 3 [root@python ~]# python eliftest.py Please input a number: 3 your input's is 3 number more than 3 [root@python ~]# python eliftest.py Please input a number: 2 your input's is 2 number 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-8 print "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: 4 your input's is 4 number more than 3 [root@python ~]# python eliftest.py Please input a number: 3 your input's is 3 number is qual 3 [root@python ~]# python eliftest.py Please input a number: 2 your input's is 2 number 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-8 print "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: 4 your input's is 4 number more than 3 [root@python ~]# python eliftest.py Please input a number: 3 your input's is 3 number is qual 3 [root@python ~]# python eliftest.py Please input a number: 2 your input's is 2 number 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。