跟老齐学Python之玩转字符串(3)
字符串就是一个话题中心。
给字符串编号
在很多很多情况下，我们都要对字符串中的每个字符进行操作（具体看后面的内容），要准确进行操作，必须做的一个工作就是把字符进行编号。比如一个班里面有50名学生，如果这些学生都有学号，老师操作他们将简化很多。比如不用专门找每个人名字，直接通过学号知道谁有没有交作业。
在python中按照这样的顺序对字符串进行编号：从左边第一个开始是0号，向下依次按照整数增加，为1、2...，直到最后一个，在这个过程中，所有字符，包括空格，都进行变好。例如：
Hello,wor ld
对于这个字符串，从左向右的变好依次是：
0 1 2 3 4 5 6 7 8 9 10 11 H e l l o , w o r l d
在班级了，老师只要喊出学生的学号，自动有对应的学生站起来。在python里面如何把某个编号所对应的字符调出来呢？看代码：
>>> a = "Hello,wor ld" >>> len(a) #字符串的长度是12,说明公有12个字符，最后一个字符编号是11 12 >>> a[0] 'H' >>> a[3] 'l' >>> a[9] ' ' >>> a[11] 'd' >>> a[5] ','
特别说明，编号是从左边开始，第一个是0。
能不能从右边开始编号呢？可以。这么人见人爱的python难道这点小要求都不满足吗？
>>> a[-1] 'd' >>> a[11] 'd' >>> a[-12] 'H' >>> a[-3] ' '
看到了吗？如果从右边开始，第一个编号是-1,这样就跟从左边区分开了。也就是a[-1]和a[11]是指向同一个字符。
不管从左边开始还是从右边开始，都能准确找到某个字符。看官喜欢从哪边开始就从哪边开始，或者根据实际使用情况，需要从哪边开始就从哪边开始。
字符串截取
有了编号，不仅仅能够找出某个字符，还能在字符串中取出一部分来。比如，从“hello,wor ld”里面取出“llo”。可以这样操作
>>> a[2:5] 'llo'
这就是截取字符串的一部分，注意：所截取部分的第一个字符（l）对应的编号是(2)，从这里开始；结束的字符是(o)，对应编号是（4），但是结束的编号要增加1,不能是4,而是5.这样截取到的就是上面所要求的了。
试一试，怎么截取到",wor"
也就是说，截取a[n,m]，其中n 有几个比较特殊的 去掉字符串两头的空格 这个功能，在让用户输入一些信息的时候非常有用。有的朋友喜欢输入结束的时候敲击空格，比如让他输入自己的名字，输完了，他来个空格。有的则喜欢先加一个空格，总做的输入的第一个字前面应该空两个格。 好吧，这些空格是没用的。python考虑到有不少人可能有这个习惯，因此就帮助程序员把这些空格去掉。 方法是： S.strip() 去掉字符串的左右空格 练习 学编程，必须做练习，通过练习熟悉各种情况下的使用。 下面共同做一个练习：输入用户名，计算机自动向这个用户打招呼。代码如下： 这段代码中的raw_input()的含义，就是要用户输入内容，所输入的内容是一个字符串。 其实，上面这段代码存在这改进的地方，比如，如果用户输入的是小写，是不是要将名字的首字母变成大写呢？如果有空格，是不是要去掉呢？等等。或许还有别的，看看能不能在这个练习中，将以前学习过的东西综合应用一下？
>>> a[:] #表示截取全部
'Hello,wor ld'
>>> a[3:] #表示从a[3]开始，一直到字符串的最后
'lo,wor ld'
>>> a[:4] #表示从字符串开头一直到a[4]前结束
'Hell'
S.lstrip() 去掉字符串的左边空格
S.rstrip() 去掉字符串的右边空格
看官在看下面示例之前，请先自己用上面的内置函数，是否可以？
>>> b=" hello "
>>> b
' hello '
>>> b.strip()
'hello'
>>> b
' hello '
>>> b.lstrip()
'hello '
>>> b.rstrip()
' hello'
#coding:utf-8
print "please write your name:"
name=raw_input()
print "Hello,%s"%name
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