Constructing a Boost graph from a file (Adjacency list)

Question

I'm trying to get started with the very good Boost graph library.

The first step is reading a graph from a file, where each line contains edges separated by spaces.

I can't seem to get it done right. My current code looks like this:

..include stuff..
using namespace boost;

typedef boost::adjacency_list<listS, vecS, undirectedS> Graph;

Graph get_graph(char* str1){

  Graph g; 

  std::ifstream infile(str1);
  std::string line;
  int e1;
  int e2;

  //std::map<int,g> VertexList;

  while (std::getline(infile, line)){

    std::istringstream iss(line);

    //std::cout<<line<<"\n";

    std::vector<std::string> strs;
    split(strs, line, is_any_of(" "));


    //std::cout<<" Adding edge: " << strs[0]<< " to " << strs[1] << "\n";

    std::istringstream ss(strs[0].substr(0,5));
    ss >> e1;

    std::istringstream s2(strs[1].substr(0,5));
    s2 >> e2;


    //ADD e1 and e2 to a graph and connect them with an edge somehow!



    //    VertexList[123]=boost::add_vertex(g);

    //Graph::vertex_descriptor v2 = boost::add_vertex(g);

    //add individual vertices to the graph..
    //e1 = add_vertex(g);
    //e2 = add_vertex(g);

    //add the corresponding edge..


  }

  std::cout<<num_vertices(g)<<std::endl;



  return g;

}

int main(int argc, char *argv[]){ 

  Graph g = get_graph(argv[1]);


  //degree_vec(g);

  return 0;

}

File example:

214328887 34428380
17116707 28465635
380580781 18996905
221036078 153460275
107830991 17868918
151338729 222261763
19705747 34428380
222261763 88323281
19933035 149538028
158419434 17434613

Things I tried and kind of worked:

• Using boost graph library: how to create a graph by reading edge lists from file, but it reads Edge lists, which do not help me as I can not call functions, bound to Adjacency lists (or can I?)

• Using add_edge(e1,e2,g) did not work, as in this case e1 and e2 can only be unsigned integers of length 1 (?), It threw some weird results if I used add_edge(123,32,g) for example. (like when I counted vertices, there were 122, instead of 1)

• Using vertex_descriptors, for e.g. Graph::vertex_descriptor e1 = boost::add_vertex(g); and then adding edges using these descriptors, but I can't seem to make it work (as I said, this is relatively new to me)

Answer 1

Using add_edge(e1,e2,g)

Sure it does! Your assumptions about how adjacency_list<..., vecS,... > should work are wrong. The vertex-descriptor IS the vertex index in that case and vertices are random-access (due to vecS).

If you want to make the numbers as read "just" an ID, make them a property of your vertex, instead of directly using them as the vertex index. Most likely you will want to use some node-based container selector for the vertex container (so instead of vecS e.g. listS). Next, you probably want an id or name property associated with each vertex. May I suggest a bundled property:

struct VertexProperty {
    int id;
};

typedef boost::adjacency_list<listS, vecS, undirectedS, VertexProperty> Graph;

std::istringstream iss(line);
if (iss >> e1 >> e2) {
    // find or insert v1 and v2, e.g. insert by:
    auto v1 = add_vertex(VertexProperty { e1 }, g);
    auto v2 = add_vertex(VertexProperty { e2 }, g);

    add_edge(v1, v2, g);
}

Notes:

• finding existing vertices by id is left as an exercise (you may want to look at http://www.boost.org/doc/libs/1_63_0/boost/graph/labeled_graph.hpp or perhaps simply keep a map from id -> vertex_descriptor.)
• consider fixing the naming of your variables (it's confusing taste to call a vertex-ID e1)

BONUS: labeled_graph demo

As a bonus, here's an example using labeled_graph adaptor:

Live On Coliru

#include <boost/graph/adjacency_list.hpp>
#include <boost/graph/labeled_graph.hpp>
#include <fstream>
#include <iostream>
#include <sstream>

using namespace boost;

typedef boost::adjacency_list<listS, vecS, undirectedS> AdjList;
typedef boost::labeled_graph<AdjList, size_t, mapS> Graph;

Graph get_graph(char* str1){

    Graph g; 

    std::ifstream infile(str1);
    std::string line;

    while (std::getline(infile, line)) {

        std::istringstream iss(line);

        size_t vid1, vid2;
        if (iss >> vid1 >> vid2) {
            auto v1 = add_vertex(vid1, g);
            auto v2 = add_vertex(vid2, g);

            add_edge(v1, v2, g);
        } else {
            std::cerr << "Skipped invalid line '" << line << "'\n";
        }

    }

    std::cout << num_vertices(g) << std::endl;
    return g;
}

int main(int argc, char *argv[]){ 
    if (argc<2) return 255;
    Graph g = get_graph(argv[1]);
}

Which prints (for the example input):

18

Which is correct since 222261763 and 34428380 occur twice in the input of 10 lines (10x2 - 2 == 18 vertices).