为什么 getline 只读取第一笔？

Question

我的程序应该读取命令并用一个数字来执行它们。它应该是 +, -, /, * 数字，但它只读取第一笔画。

    ifstream fin("file.txt");
    string line;
    while(getline(fin,line))
    {
        if(line[0] == '+')
        {
            for(int i = 1; i < 10; i++)
            {
                n += line[i];
            }
            ss << n;
            double p = 0.0;
            ss >> p;
            res += p;
        }
        n = "";
        if(line[0] == '-')
        {
            for(int i = 1; i < 10; i++)
            {
                n += line[i];
            }
            ss << n;
            double m = 0.0;
            ss >> m;
            res -= m;
        }
        n = "";
        if(line[0] == '*')
        {
            for(int i = 1; i < 10; i++)
            {
                n += line[i];
            }
            ss << n;
            double di = 0.0;
            ss >> di;
            res *= di;
        }
        n = "";
        if(line[0] == '/')
        {
            for(int i = 1; i < 10; i++)
            {
                n += line[i];
            }
            ss << n;
            double mu = 0.0;
            ss >> mu;
            res /= mu;
        }
        n = "";
    }

文件示例：

+1
-2
*5

输出：

预期输出是-5，但它输出了1。为什么会这样，我该怎么办？ ss 是字符串流，n 是字符串，res 是双精度数，strokes[] 是字符串。

Answer 1

你已经自己发现了。您必须清除 std::istringstream 或定义一个新的，即使名称相同。所以你可以写

    ss.clear(); // Clear EOF bit
    ss.str(std::string()); // Clear buffer

在下一次插入操作之前，或者，你只需每次定义一个新变量，写成：

    std::stringstream ss{};
    ss << n;

我还将您的代码修改为最小可重现示例：

#include <iostream>
#include <sstream>
#include <fstream>
#include <string>

int main() {

    std::string line{};
    std::stringstream ss{};
    std::string n{};
    double res{};

    std::ifstream fin("file.txt");

    while (std::getline(fin, line))
    {
        if (line[0] == '+')
        {
            for (int i = 1; i < line.length(); i++)
            {
                n += line[i];
            }
            ss << n;
            double p = 0.0;
            ss >> p;
            res += p;
        }
        n = "";
        if (line[0] == '-')
        {
            for (int i = 1; i < line.length(); i++)
            {
                n += line[i];
            }
            ss.clear(); // Clear EOF bit
            ss.str(std::string()); // Clear buffer
            ss << n;
            double m = 0.0;
            ss >> m;
            res -= m;
        }
        n = "";
        if (line[0] == '*')
        {
            for (int i = 1; i < line.length(); i++)
            {
                n += line[i];
            }
            ss.clear(); // Clear EOF bit
            ss.str(std::string()); // Clear buffer
            ss << n;
            double di = 0.0;
            ss >> di;
            res *= di;
        }
        n = "";
        if (line[0] == '/')
        {
            for (int i = 1; i < line.length(); i++)
            {
                n += line[i];
            }
            ss.clear(); // Clear EOF bit
            ss.str(std::string()); // Clear buffer
            ss << n;
            double mu = 0.0;
            ss >> mu;
            res /= mu;
        }
        n = "";
    }
    std::cout << res;
    return 0;
}

我还添加了一个更现代的 C++ 代码：

#include <iostream>
#include <sstream>
#include <fstream>
#include <string>

int main() {
    
    if (std::ifstream ifs{ "file.txt" }; ifs) {

        double result{};
        for (std::string line{}; std::getline(ifs, line); ) {

            char calculationOperator{};
            double number{};

            std::istringstream iss{ line };
            if (iss >> calculationOperator >> number) {

                switch (calculationOperator) {
                case '+':
                    result += number;
                    break;
                case '-':
                    result -= number;
                    break;
                case '*':
                    result *= number;
                    break;
                case '/':
                    result /= number;
                    break;
                default:
                    std::cerr << "\n\nError: invalid mathematical operator '" << calculationOperator << "' found\n\n";
                }
            }
            else
                std::cerr << "\n\nError: Wronng input in line: " << line << "\n\n";
        }
        std::cout << "\n\nResult:  " << result << "\n\n";
    }
    else
        std::cerr << "\n\nError: Could not open 'file.txt'\n\n";
    return 0;
}

为什么 getline 只读取第一笔？

Why does getline reads only the first stroke?

c++

getline

ifstream