从只有一行的流中读取点

Question

I have a file which first tells me how many points will I be reading on the following line. So, for example my file look like this:

7
a,b c,d e,f g,h, i,j k,l m,n

So I know the following line after 7 is 7 pairs of integers separated by a comma and each pair separated by a blank space.

What I want: To have a vector of 7 Point elements.

I have a class called Point:

class Point {
public:
    int x;
    int y;
    bool operator==(const Point q){
        return (q.x == this->x && q.y == this->y);
    }
};

So when I read this file I'd like to have a vector V where:

V[0].x = a
V[0].y = b
V[1].x = c
V[1].y = d

and so on.

I can read the 7 fine, but how do I read each of the 7 pairs of integers individually? I need this because I'm going to store (a,b) (c,d)... in a vector.

Is not only 2 points. The first line of the file tells me how many points I'm going to store.

They're not read from standard input.

They're read from a file.

I tried using sscanf but I think that's only for when you have multiple lines with this info and I'd like to not have to modify my format.

This is what I have so far:

void process_file(string filename){
    ifstream thracklefile;
    string line;
    int set_size;
    thracklefile.open(filename);

    getline(thracklefile,line); //store set size.
    set_size = stoi(line);

    //Store points in following line
    points.clear();
    points.resize(set_size);
    getline(thracklefile,line); //store the points.
    }

I do not want to ignore commas, each comma is part of the information I want to store for each Point.

Answer 1

I think most of the discussion in the comments is about semantics. It is recommended you "ignore" the commas but you can't do that as they are in the file. Perhaps a better term is "discard". The word "ignore" is use since there is a C++ iostream function ignore .

There are many ways to handle this. One option is to override the stream insertion/extraction operators:

class Point {
public:
    int x;
    int y;

    // Don't really need this as members are public, but
    // in case you change that in the future....
    friend istream& operator>>(istream& in, Point& p);
    friend ostream& operator<<(ostream& out, const Point& p);
};

istream& operator>>(istream& in, Point& p)
{
    char separator;
    // Try to read <int><char><int>
    in >> p.x >> separator >> p.y;
    // The stream may be in an error state here. That
    // is ok. Let the caller handle that
    // Also note that we discard (ignore) "separator"
    return in;
}
ostream& operator<<(ostream& out, const Point& p)
{
    out << p.x << ',' << p.y;
    return out;
}

int main() {
    int num_points;
    std::cin >> num_points;
    Point p;
    for (int i = 0; i < num_points; i++) {
        if (!(std::cin >> p)) {
            // There was an error
            std::cout << "File format error!" << std::endl;
            break;
        }
        std::cout << p << std::endl;
    }

    return 0;
}

The example uses cin but any stream should work, including ifstream .