How to Read Integers from a Text File with Variable-Length Lines in C ?

Read Integers from a Text File with C ifstream

When dealing with text files containing variable-length lines of integers, the standard line reading idiom proves useful:

<code class="cpp">#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
}</code>

This approach uses getline to read each line and istringstream to parse the integers within each line.

Alternatively, a more concise one-line solution utilizes a for loop and the stay auxiliary template:

<code class="cpp">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>())
                )
     ) { }</code>

Both approaches effectively parse the variable-length integer sequences from the input text file.

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