python字符串详解

字符串定义

• 字符串是由一个个字符组成的序列

• 使用成对的单引号或者双引号

  two_quote_str = 'hello python'
  one_quote_str = "hello python"
  

• 成对的三引号，保留字符串中的全部格式信息

  # 会保留换行符，制表符
  three_quote_str = '''languages:
    1.python
    2.java
    3.golang
    4.c++
  '''
  

字符串的操作

基本操作（序列通用功能）

1. 索引访问

   str = "12345678"
   print(str[0])
   print(str[2])
   print(str[8]) # IndexError: string index out of range
   print(str[-1])
   print(str[-8])
   print(str[-9]) # IndexError: string index out of range
   

2. 拼接运算符

   a = "1"
   b = "2"
   c = a + b
   print(c) # "s12"
   
   a = "1"
   b = 2
   c = a + b # TypeError: can only concatenate str (not "int") to str
   

3. 重复运算符

   c = a * b * b
   print(c) # 1111
   
   c = a * 1.2 # TypeError: can't multiply sequence by non-int of type 'float'
   print(c)
   

4. 切片运算符

   str = "12345678"
   print(str) # 12345678
   print(str[:]) # 12345678
   print(str[0:]) # 12345678
   print(str[:1]) # 1
   print(str[1:5]) # 2345
   print(str[0:-1:2]) # 1357
   

5. 成员运算符

   name = "shawn"
   print("s" in name) # True
   print("sh" in name) # True
   print("Sh" in name) # False 区分大小写
   print("a" not in name) # False
   print("  s" in name) # False
   # print([1, 3, 4] not in name) # TypeError: 'in <string>' requires string as left operand, not list
   print(1 in name) # TypeError: 'in <string>' requires string as left operand, not int
   

6. 常用函数

   print(len(name))
   print(name[len(name) - 1]) # 访问最后一个序列元素
   print(sorted(name)) # ['a', 'h', 'n', 's', 'w']
   

7. 遍历

   for c in name :
       print(c)
   

字符串特有的操作

1. 一些常用的函数(不会改变原字符串的值)

   操作	含义
   upper()	字符串中所有字母转大写
   lower()	字符串中所有字母转小写
   capitalize()	字符串中的首字母大写
   title()	字符串中的所有单词的首个字母转化为大写
   rstrip() lstrip() strip()	去掉（左或右或两边）的空格
   split()	按照指定字符串分隔字符
   isdigit()	是不是数字类型
   find()	查找。返回下标
   replace(old,new)	把所有的old子串替换成new子串

   message = "my name is shawn, i'm 20 year old"
   print(message.capitalize()) # My name is shawn, i'm 20 year old
   print(message.title()) # My Name Is Shawn, I'M 20 Year Old
   print(message) # "my name is shawn, i'm 20 year old"
   
   print(message.replace("my", "your")) # your name is shawn, i'm 20 year old
   print(message) # "my name is shawn, i'm 20 year old"
   

2. 注意点：字符串不可变，一旦生成则内容不变

   message = "my name is shawn, i'm 20 year old"
   print(message.capitalize()) # My name is shawn, i'm 20 year old
   print(message.title()) # My Name Is Shawn, I'M 20 Year Old
   print(message) # "my name is shawn, i'm 20 year old"
   
   message[0:2] = "your" #TypeError: 'str' object does not support item assignment
   
   

   如何改变？只能重新赋值

   message = "your name is shawn, i'm 20 year old"
   

字符串练习

1. 判断是否有元音字母

   AEIOU_STR = 'aeiou'
   
   
   def vowel_count(s):
       count = 0;
       for c in s:
           if c in AEIOU_STR + AEIOU_STR.upper():
               count += 1;
   
       return count
   
   
   print(vowel_count("hello world"))
   

2. 判断是否是回文字符串

   names = open("E:\\python_learning\\note\\names.txt")
   
   
   def is_palindrome_recursion(name):
       if len(name) <= 1:
           return True
       else:
           if name[0] != name[-1]:
               return False
           else:
               return is_palindrome_recursion(name[1:-1])
   
   
   def is_palindrome(name):
       left = 0
       right = len(name) - 1;
       while left < right:
           if name[left] != name[right]:
               return False;
           left += 1
           right -= 1
       return True
   
   
   for c in names:
       c = c.strip().lower();
       if is_palindrome_recursion(c):
           print(c)
   
   names.close()
   

3. 判断字符串是否是升序排列

   names = open("E:\\python_learning\\note\\names.txt")
   
   
   def is_ascending(name):
       temp = name[0]
       for n in name:
           if n < temp:
               return False
           temp = n
       return True
   
   
   for c in names:
       c = c.strip().lower();
       if is_ascending(c):
           print(c)
   
   names.close()
   

4. 模板字符串

   str_format = "my name is {}, i'm {} years old!".format("shawn", 20)
   print(str_format) # my name is shawn, i'm 20 years old!
   
   import math
   
   print("PI is {}".format(math.pi)) # PI is 3.141592653589793
   print("PI is {:.4f}".format(math.pi)) # PI is 3.1416
   print("PI is {:9.4f}".format(math.pi)) # PI is    3.1416
   print("PI is {:< 9.4f}".format(math.pi)) # PI is  3.1416
   

5. 正则表达式

   import re
   
   names = open("E:\\python_learning\\note\\names.txt")
   
   p = "(c.a)"
   for c in names:
       c = c.strip().lower()
       result = re.search(p, c)
       if result:
           print("name in {}".format(c))
   
   names.close()
   

names.txt

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