Reading from a file C++ Ifstream

Question

I am trying to do a problem using dynamic programming to find which items to select from a text file under a certain weight capacity while maximizing value. The file is in the format:

item1, weight, value
item2, weight, value

With a variable number of items. I am having a problem reading the file in the main method, where I am trying to error check for bad input. My problem comes from when I check if the weight is either missing, or is a string instead of an int. I can't figure out how to check separately for a string or if there is nothing in that spot. I am supposed to output a different error depending on which it is. Thanks.

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

I think better approach will be using getline function, by which you will take a whole line and then you can try to split it into three strings. If three parts are not found or any of the three parts is not in a right format you can output an error.

An example code is give below,

#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;
}