Question

I am trying to input data from a text file in C++.
The text file is in that format:

4 15
3 516
25 52 etc.

Each line contains two integers. I don’t know the number of lines in the file so I can bind enough memory and this is what I have come into as a way to solve that:

ifstream filein;
filein.open("text.txt",ios::in);
int count=0;

while (!filein.eof())
    {
        count++;
        filein>>temporary;
    }
count=count/2; // This is the number of lines in the text file.

My problem is that I can’t figure out a way to reset

filein

into the initial state (to the beggining of the file so I can actually input the data) other than closing the input stream and opening it again. Is there any other way to do that?

Answer 1

Rather than answer the question you asked, I’m going to answer the question you didn’t ask, namely:

Q: How can I read in all the lines of the file if I don’t know how many lines there are?

A: Use a std::vector<>.

If you want to read in all of the numbers, regardless of pairing:

// all code fragments untested. typos are possible
int i;
std::vector<int> all_of_the_values;
while(filein >> i)
    all_of_the_values.push_back(i);

If you want to read in all of the numbers, putting alternating numbers into different data structures:

int i, j;
std::vector<int> first_values;
std::vector<int> second_values;
while(filein >> i >> j) {
    first_values.push_back(i);
    second_values.push_back(j);

If you want to read in all of the numbers, storing them in some sort of data structure:

int i, j;
struct S {int i; int j;};
std::vector<S> values;
while(filein >> i >> j) {
    S s = {i, j};
    values.push_back(s);
}

Finally, if you want to read the file a line at a time, keeping the first two numbers from each line, discarding the remainder of each line, and storing them a user-defined data structure:

std::vector<MyClass> v;
std::string sline;
while(std::getline(filein, sline)) {
  std::istringstream isline(sline);
  int i, j;
  if(isline >> i >> j) {
    values.push_back(MyClass(i, j));
  }
}

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Aside: never use eof() or good() in a loop conditional. Doing so almost always produces buggy code, as it would have in your case. Instead prefer invoking the input function in the condition, as I have done above.