How to make formatted file read for Adjacency list?

Question

I have adjacency list for graph typedef map<int, vector<int> > AdjacencyList;

Now I want to fill it from file which looks like this:

1 234 432 654 876 ...
2 32 521 323 122  ...
3 654 4 75 652    ...
....

So the first element in a row is vertex and the rest elements are adjacent vertices. How do I read it?

Answer 1

Use getline() to read each line into a string, and then construct istringstream from the string and read numbers from there. Something like this, but with better error checking.

std::ifstream file;
// open file etc.
std::string line;

AdjacencyList al;
while (!file.eof())
{
    getline(file, line);

    std::istringstream buffer(line);
    int num;
    buffer >> num;
    auto it = al.insert(std::make_pair(num, AdjacencyList::mapped_type()).first;
    while (!buffer.eof())
    {
        buffer >> num;
        it->push_back(num);
    }
}