C++中rfind方法的具体使用

// 搜索整个字符串
size_type rfind(const basic_string& str, size_type pos = npos) const noexcept;
size_type rfind(const CharT* s, size_type pos = npos) const;
size_type rfind(CharT ch, size_type pos = npos) const noexcept;

// C++17 新增：带长度的版本
size_type rfind(const CharT* s, size_type pos, size_type count) const;

#include <iostream>
#include <string>
#include <cstring>

// 简化的 rfind 实现原理
size_t simplified_rfind(const std::string& str, const std::string& pattern, size_t pos = std::string::npos) {
    // 如果 pos 超过字符串长度，调整到字符串末尾
    if (pos >= str.length()) {
        pos = str.length() - 1;
    }
    
    // 如果模式串为空，返回 pos（标准行为）
    if (pattern.empty()) {
        return (pos < str.length()) ? pos : str.length();
    }
    
    // 如果模式串比原字符串长，肯定找不到
    if (pattern.length() > str.length()) {
        return std::string::npos;
    }
    
    // 从 pos 开始向前搜索
    for (size_t i = pos; i >= pattern.length() - 1; --i) {
        bool found = true;
        
        // 检查当前位置是否匹配模式串
        for (size_t j = 0; j < pattern.length(); ++j) {
            if (str[i - pattern.length() + 1 + j] != pattern[j]) {
                found = false;
                break;
            }
        }
        
        if (found) {
            return i - pattern.length() + 1;
        }
        
        // 处理无符号整数的下溢
        if (i == 0) break;
    }
    
    return std::string::npos;
}

#include <iostream>
#include <string>

int main() {
    std::string text = "Hello, World! Hello, C++!";
    
    // 1. 搜索子字符串
    size_t pos1 = text.rfind("Hello");
    std::cout << "Last 'Hello' at position: " << pos1 << std::endl; // 输出 14
    
    // 2. 搜索字符
    size_t pos2 = text.rfind('o');
    std::cout << "Last 'o' at position: " << pos2 << std::endl; // 输出 19
    
    // 3. 从指定位置向前搜索
    size_t pos3 = text.rfind("Hello", 10);
    std::cout << "Last 'Hello' before position 10: " << pos3 << std::endl; // 输出 0
    
    // 4. 搜索不存在的字符串
    size_t pos4 = text.rfind("Python");
    if (pos4 == std::string::npos) {
        std::cout << "'Python' not found" << std::endl;
    }
    
    // 5. 使用指针和长度
    const char* search_str = "Hello";
    size_t pos5 = text.rfind(search_str, text.length(), 3); // 只搜索前3个字符 "Hel"
    std::cout << "Last 'Hel' at position: " << pos5 << std::endl;
    
    return 0;
}

#include <iostream>
#include <string>

// 提取文件扩展名
std::string getFileExtension(const std::string& filename) {
    size_t dotPos = filename.rfind('.');
    if (dotPos != std::string::npos && dotPos < filename.length() - 1) {
        return filename.substr(dotPos + 1);
    }
    return "";
}

// 提取目录路径
std::string getDirectory(const std::string& path) {
    size_t slashPos = path.rfind('/');
    if (slashPos != std::string::npos) {
        return path.substr(0, slashPos + 1);
    }
    return "";
}

// 查找最后一个单词
std::string getLastWord(const std::string& sentence) {
    size_t lastSpace = sentence.rfind(' ');
    if (lastSpace != std::string::npos) {
        return sentence.substr(lastSpace + 1);
    }
    return sentence; // 没有空格，返回整个字符串
}

int main() {
    std::string filename = "document.backup.txt";
    std::string path = "/home/user/documents/file.txt";
    std::string sentence = "The quick brown fox";
    
    std::cout << "File extension: " << getFileExtension(filename) << std::endl;
    std::cout << "Directory: " << getDirectory(path) << std::endl;
    std::cout << "Last word: " << getLastWord(sentence) << std::endl;
    
    return 0;
}