Read integers from a text file with C++ ifstream

c++

Solution

The standard line reading idiom:

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


std::ifstream infile("thefile.txt");
std::string line;

while (std::getline(infile, line))
{
  std::istringstream iss(line);
  int n;
  std::vector<int> v;

  while (iss >> n)
  {
    v.push_back(n);
  }

  // do something useful with v
}

Here's a one-line version using a `for` loop. We need an auxiliary construction (credits to @Luc Danton!) that does the opposite of `std::move`:

namespace std
{
  template <typename T> T & stay(T && t) { return t; }
}

int main()
{
  std::vector<std::vector<int>> vv;

  for (std::string line;
       std::getline(std::cin, line);
       vv.push_back(std::vector<int>(std::istream_iterator<int>(std::stay(std::istringstream(line))),
                                     std::istream_iterator<int>())
                    )
       ) { }

  std::cout << vv << std::endl;
}

Problem

I want to read graph adjacency information from a text file and store it into a vector. the file has arbitrary number of lines each line has arbitrary number of integers ended with '\n' for example, ``` First line: 0 1 4 Second line: 1 0 4 3 2 Thrid line: 2 1 3 Fourth line: 3 1 2 4 Fifth line: 4 0 1 3 ``` If I use getline() to read one line at a time, how do I parse the line (as each line has variable number of integers)? Any suggestions?

Original source

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