Question

I have a text file that contains the following data.

The first line is this:

5 4 3 2 1

The second line is this:

1 2 3 4 5

I am trying to read data from one line at a time because my first linked-list object is going to use the data from the first line and my second linked-list object is going to use data from the second line. The best I have been able to come up with is the following function:

void polynomial::allocate_poly(std::ifstream& in, const char* file, const char* number)
{

    in.open(file);

    std::string str;
    char b;
    int m = 0;

    for(int i = 0; !in.eof(); ++i)
    {
        in >> b;
        m = b - '0';
        a.insert(m);
    }

There is a few problems with this approach. I have tried different binary operators in my for loop such as b == '\n' and none of them seem to trigger when b is a newline character.

Also allocating the numbers from the file this way it looks like
5 5 4 3 2 1 1 2 3 4 5 , so it seems to be copying an extra 5 somewhere, I am not sure if this is the eof bit or not.

I have also attempted to use the getline function but for some reason it seems to only copy the first integer and then dumps the rest of the file. I know certainly I am not using it correctly but all the examples I can find are for typing the file name such as cin.getline and I want to be able to pass my file name as a command line argument when running the program instead.

My question is how can I allocate the numbers on the first row up to the newline char and then pass the ifstream in variable to another object to allocate the second line? Thanks for your help.

Answer 1

You don’t say what a is, but never mind: If you want line based parsing, you need to have getline in the outer loop. Also, never use eof, as it doesn’t do what you want. Rather use the implicit conversion to bool to check if an operation succeeded.

Here’s the typical gig:

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

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

while (std::getline(infile, line))  // this does the checking!
{
  std::istringstream iss(line);
  char c;

  while (iss >> c)
  {
    int value = c - '0';
    // process value
  }
}

However, this conversion from char to int is cumbersome and fragile. Why not read an integer directly?

  int value;
  while (iss >> value) { /* ... */ }

---

Edit: Based on your comment, here’s the code to read exactly two lines:

std::string line;
int val;

if (std::getline(infile, line))
{
  std::istringstream iss(line);
  while (iss >> value) { /* process first line */ }
}
if (std::getline(infile, line))
{
  std::istringstream iss(line);
  while (iss >> value) { /* process second line */ }
}