从文件中读取 C++ Ifstream

Question

我正在尝试使用动态规划来做一个问题，以在最大化价值的同时，在一定重量下从文本文件中找到哪些项目 select。文件格式为：

item1, weight, value
item2, weight, value

项目数量可变。我在 main 方法中读取文件时遇到问题，我正在尝试对错误输入进行错误检查。我的问题来自于我检查重量是否丢失，或者是一个字符串而不是一个整数。我不知道如何单独检查一个字符串，或者那个地方是否没有任何内容。我应该根据错误输出不同的错误。谢谢

int main(int argc, char * const argv[])
{
    std::vector<Item> items;
    if (argc != 3)
    {
        std::cerr << "Usage: ./knapsack <capacity> <filename>";
        return -1;
    }

    int capacity;
    std::stringstream iss(argv[1]);

    if (!(iss >> capacity) || (capacity < 0))
    {
        std::cerr << "Error: Bad value '" << argv[1] << "' for capacity." << std::endl;
        return -1;
    }   

    std::ifstream ifs(argv[2]);
    if (ifs.is_open())
    {
        std::string description;
        unsigned int weight;
        unsigned int value;
        int line = 1;
        while (!ifs.eof())
        {
            if (!(ifs >> description))
            {
                std::cerr << "Error: Line number " << line << " does not contain 3 fields." << std::endl;
                return -1;
            }

            if (!(ifs >> weight))
            {
                if (ifs.eof())
                    std::cerr << "Error: Line number " << line << " does not contain 3 fields." << std::endl;
                else
                    std::cerr << "Error: Invalid weight '" << ifs << "' on line number " << line << "." << std::endl;
                return -1;
            }

            else if (!(ifs >> weight) || (weight < 0))
            {
                std::cerr << "Error: Invalid weight '" << ifs << "' on line number " << line << "." << std::endl;
                return -1;
            }

            if (!(ifs >> value))
            {
                if (ifs.eof())
                    std::cerr << "Error: Line number " << line << " does not contain 3 fields." << std::endl;
                else
                    std::cerr << "Error: Invalid value '" << ifs << "' on line number " << line << "." << std::endl;
                return -1;
            }

            else if (!(ifs >> value) || (value < 0))
            {
                std::cerr << "Error: Invalid value '" << ifs << "' on line number " << line << "." << std::endl;
                return -1;
            }

            Item item = Item(line, weight, value, description);
            items.push_back(item);
            line++;
        }

        ifs.close();
        knapsack(capacity, items, items.size());
    }

    else
    {
        std::cerr << "Error: Cannot open file '" << argv[2] << "'." << std::endl;
        return -1;
    }

    return 0;
}

Answer 1

您可以一次读取整行并使用正则表达式检查其格式。

Answer 2

我认为更好的方法是使用 getline 函数，通过该函数您可以获取一整行，然后您可以尝试将其拆分为三个字符串。如果找不到三个部分或三个部分中的任何一个格式不正确，您可以输出错误。

示例代码如下，

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

using namespace std;

void split(const std::string &s, std::vector<std::string> &elems, char delim) {
  elems.clear();
  std::stringstream ss(s);
  std::string item;
  while (std::getline(ss, item, delim)) {
    elems.push_back(item);
  }
}

int main()
{
  vector<string> elems;
  ifstream fi("YOUR\PATH");

  string inp;
  while(getline(fi, inp))
  {
    split(inp, elems, ' ');

    if(elems.size() != 3)
    {
      cout<<"error\n";
      continue;
    }

    int value;
    bool isint = istringstream(elems[1])>>value;
    if(!isint)
    {
      cout << "error\n";
      continue;
    }

    cout<<"value was : " << value << endl;
  }
  return 0;
}

从文件中读取 C++ Ifstream

Reading from a file C++ Ifstream

c++

file-io

ifstream