Python入门学习之字符串与比较运算符

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Python字符串
字符串或串(String)是由数字、字母、下划线组成的一串字符。
一般记为 :

s="a1a2···an"(n>=0)

它是编程语言中表示文本的数据类型。
python的字串列表有2种取值顺序:

从左到右索引默认0开始的，最大范围是字符串长度少1
从右到左索引默认-1开始的，最大范围是字符串开头
如果你的实要取得一段子串的话，可以用到变量[头下标:尾下标]，就可以截取相应的字符串，其中下标是从0开始算起，可以是正数或负数，下标可以为空表示取到头或尾。

比如:

s = 'ilovepython'

s[1:5]的结果是love。
当使用以冒号分隔的字符串，python返回一个新的对象，结果包含了以这对偏移标识的连续的内容，左边的开始是包含了下边界。
上面的结果包含了s[1]的值l，而取到的最大范围不包括上边界，就是s[5]的值p。
加号（+）是字符串连接运算符，星号（*）是重复操作。如下实例：

#!/usr/bin/python
# -*- coding: UTF-8 -*-

str = 'Hello World!'

print str # 输出完整字符串
print str[0] # 输出字符串中的第一个字符
print str[2:5] # 输出字符串中第三个至第五个之间的字符串
print str[2:] # 输出从第三个字符开始的字符串
print str * 2 # 输出字符串两次
print str + "TEST" # 输出连接的字符串

以上实例输出结果：

Hello World!
H
llo
llo World!
Hello World!Hello World!
Hello World!TEST

Python比较运算符
以下假设变量a为10，变量b为20：

以下实例演示了Python所有比较运算符的操作：

#!/usr/bin/python

a = 21
b = 10
c = 0

if ( a == b ):
  print "Line 1 - a is equal to b"
else:
  print "Line 1 - a is not equal to b"

if ( a != b ):
  print "Line 2 - a is not equal to b"
else:
  print "Line 2 - a is equal to b"

if ( a <> b ):
  print "Line 3 - a is not equal to b"
else:
  print "Line 3 - a is equal to b"

if ( a < b ):
  print "Line 4 - a is less than b" 
else:
  print "Line 4 - a is not less than b"

if ( a > b ):
  print "Line 5 - a is greater than b"
else:
  print "Line 5 - a is not greater than b"

a = 5;
b = 20;
if ( a <= b ):
  print "Line 6 - a is either less than or equal to b"
else:
  print "Line 6 - a is neither less than nor equal to b"

if ( b >= a ):
  print "Line 7 - b is either greater than or equal to b"
else:
  print "Line 7 - b is neither greater than nor equal to b"

以上实例输出结果：

Line 1 - a is not equal to b
Line 2 - a is not equal to b
Line 3 - a is not equal to b
Line 4 - a is not less than b
Line 5 - a is greater than b
Line 6 - a is either less than or equal to b
Line 7 - b is either greater than or equal to b

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