像列表一样处理字符串

知识点

Python中字符串可以看作字符的列表，支持下标索引取值，但字符串是不可变对象，不能修改其中某个字符。

代码

booklist = ["java","csharp","cplusplus"]

# booklist[0] 获取列表第0个元素，得到字符串 "java"
print(booklist[0])

# booklist[0][0] 双重下标：先取列表第0项字符串，再取这个字符串的第0个字符
print(booklist[0][0])Code language: PHP (php)

运行输出：

java
j

索引规则

1. 正向索引：从0开始
   "java"：j[0]、a[1]、v[2]、a[3]
2. 负向索引：-1代表最后一个字符

booklist[0][-1]   # 输出 a，取java最后一位Code language: CSS (css)

3. 索引越界报错
   字符串长度是4，最大下标为3，访问[4]会抛出：
   IndexError: string index out of range 字符串索引超出范围

strd = "java"
print(strd[1])  # 输出 aCode language: PHP (php)

字符串首字母作为字典key

需求：把列表里的字符串存入字典，用每个字符串首字母作为字典的key

books = ["java","csharp","cplusplus","python"]
booksdict = {}

booksdict[books[0][0]] = books[0]   # books[0][0]拿到"j"
booksdict[books[1][0]] = books[1]   # books[1][0]拿到"c"
booksdict[books[2][0]] = books[2]
booksdict[books[3][0]] = books[3]

print(booksdict)
# {'j': 'java', 'c': 'csharp', 'p': 'python'}Code language: PHP (php)

注意

字符串可以读取下标字符，但不能赋值修改

s = "hello"
s[0] = "H"  # 报错！字符串不可修改Code language: PHP (php)

循环简化写法

books = ["java","csharp","cplusplus","python"]
booksdict = {}
for item in books:
    booksdict[item[0]] = item
print(booksdict)Code language: PHP (php)

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练习

1. s = "apple"，取出第一个字符、最后一个字符怎么写？
2. 列表lst=["cat","dog","bird"]，取出dog的第一个字符。