Is there anyway to reset the filein to the initial state? - c++

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));
}
}
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.

Answer 2

I think #Robᵩ has pretty much the right idea -- instead of reading through all the data just to count the number of lines, then reading through the whole file again to actually read the data, using something like std::vector (or std::deque) that will expand as needed as you read the data.
In a typical case, however, the two numbers on a line are going to be related to each other, and you typically want to store them in a way that shows that association directly. For example, they might be the X and Y coordinates of points, in which case you want to read points:
class point {
int x, y;
};
std::istream &operator>>(std::istream &is, point &p) {
return is >> p.x >> p.y;
}
std::ifstream in("myfile.txt");
// create the vector from the data in the file:
std::vector<point> points((std::istream_iterator<point>(in)),
std::istream_iterator<point>());
On a slightly different note: even if you decide you want to use an explicit loop, please don't use while (!whatever.eof()) to do it -- that's pretty much guaranteed to fail. You want to check that reading data succeeded, so (for example) using the point class above, you could use something like:
point p;
while (infile >> p)
points.push_back(p);

Answer 3

The function is: filein.seekg (0, ios::beg);
Here is a Reference
You should also use filein.clear() to reset the eof bit in the file if you do it this way.
And, of course, if you want the best method for what you are ultimately trying to do, Robᵩ's answer is much better, albeit more involved.