L3_06 字符串及其函数

一、大小写转换

1.1 C++ 中的转换

#include <iostream>
#include <string>
#include <cctype>
using namespace std;

int main() {
    string str = "Hello World!";
    
    // 转大写
    for (char &c : str) {
        c = toupper(c);
    }
    cout << str << endl;  // HELLO WORLD!
    
    // 转小写
    for (char &c : str) {
        c = tolower(c);
    }
    cout << str << endl;  // hello world!
    
    return 0;
}

1.2 Python 中的转换

str = "Hello World!"
print(str.upper())   # HELLO WORLD!
print(str.lower())   # hello world!
print(str.title())   # Hello World!
print(str.capitalize())  # Hello world!

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二、字符串搜索

2.1 C++ 中的搜索

#include <iostream>
#include <string>
using namespace std;

int main() {
    string str = "Hello World!";
    
    // 查找子串
    size_t pos = str.find("World");
    if (pos != string::npos) {
        cout << "找到 'World' 在位置: " << pos << endl;
    } else {
        cout << "未找到" << endl;
    }
    
    // 从指定位置查找
    pos = str.find("l", 3);
    cout << "从位置3开始查找 'l': " << pos << endl;
    
    return 0;
}

2.2 Python 中的搜索

str = "Hello World!"

# 查找子串
print(str.find("World"))   # 返回索引6
print(str.find("Python"))  # 未找到返回-1

# 判断是否包含
print("World" in str)  # True
print("Python" in str) # False

# 查找所有出现位置
import re
matches = [match.start() for match in re.finditer('l', str)]
print(matches)  # [2, 3, 9]

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三、字符串分割

3.1 C++ 中的分割

#include <iostream>
#include <string>
#include <sstream>
#include <vector>
using namespace std;

vector<string> split(const string &s, char delimiter) {
    vector<string> tokens;
    string token;
    istringstream tokenStream(s);
    
    while (getline(tokenStream, token, delimiter)) {
        tokens.push_back(token);
    }
    return tokens;
}

int main() {
    string str = "apple,banana,cherry";
    vector<string> fruits = split(str, ',');
    
    for (const string &fruit : fruits) {
        cout << fruit << endl;
    }
    
    return 0;
}

3.2 Python 中的分割

str = "apple,banana,cherry"

# 使用逗号分割
fruits = str.split(',')
print(fruits)  # ['apple', 'banana', 'cherry']

# 使用空格分割
sentence = "Hello World Python"
words = sentence.split()
print(words)  # ['Hello', 'World', 'Python']

# 限制分割次数
str = "a,b,c,d,e"
parts = str.split(',', 2)
print(parts)  # ['a', 'b', 'c,d,e']

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四、字符串替换

4.1 C++ 中的替换

#include <iostream>
#include <string>
using namespace std;

int main() {
    string str = "Hello World!";
    
    // 替换子串
    str.replace(6, 5, "Python");  // 从位置6开始，替换5个字符
    cout << str << endl;  // Hello Python!
    
    return 0;
}

4.2 Python 中的替换

str = "Hello World!"

# 替换所有匹配
new_str = str.replace("World", "Python")
print(new_str)  # Hello Python!

# 限制替换次数
str = "aaaabbbb"
new_str = str.replace("a", "x", 2)
print(new_str)  # xxabbbb

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五、其他常用操作

5.1 获取长度

// C++
string str = "Hello";
cout << str.size() << endl;  // 5

# Python
str = "Hello"
print(len(str))  # 5

5.2 字符串拼接

// C++
string str1 = "Hello";
string str2 = "World";
string result = str1 + " " + str2;

# Python
str1 = "Hello"
str2 = "World"
result = str1 + " " + str2

5.3 字符串切片

// C++
string str = "Hello World";
string substr = str.substr(0, 5);  // 从位置0开始，取5个字符

# Python
str = "Hello World"
substr = str[0:5]  # 从索引0到5（不包含5）

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六、示例程序

6.1 C++ 字符串处理

#include <iostream>
#include <string>
using namespace std;

int main() {
    string email;
    cout << "输入邮箱地址: ";
    cin >> email;
    
    // 查找@位置
    size_t at_pos = email.find('@');
    if (at_pos != string::npos) {
        string username = email.substr(0, at_pos);
        string domain = email.substr(at_pos + 1);
        cout << "用户名: " << username << endl;
        cout << "域名: " << domain << endl;
    } else {
        cout << "无效邮箱" << endl;
    }
    
    return 0;
}

6.2 Python 字符串处理

# 统计字符串中每个字符出现的次数
str = "Hello World!"
char_count = {}

for char in str:
    char_count[char] = char_count.get(char, 0) + 1

print(char_count)