Python入门学习之字符串与比较运算符

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