Question

I have a vector A [9 3 6 9 3 6] each 2 elements represent the edge of a graph, I wan to create the adjacency matrix from this vector.

First i created the unique vector A [3 6 9] to know the size of my matrix

Second I create a matrix and fill it with 0

Third I will run a loop on A to know what edges are connected, my question is how can I tell C++ that the first element in A which is three actually represent element 0 in my matrix, same for 6 that it represent 1 and 9 is represented by 3, like this when I construct my adjacency matrix i know that 0 1 2 represent actually 3 6 9, I heard about using a map but didn t know how to construct it in my program because im new to C++.

Answer 1

You are essentially trying to track labels for your vertices. If each vertex has a sequentially assigned number, you can scan new vertex labels and insert the corresponding sequential number in a map, like so:

std::map<int, std::size_t> labels;  // "int" is the label type; could be anything!

std::size_t store_vertex_label(int v)
{
  std::map<int, std::size_t>::const_iterator const it = labels.find(v);

  if (it != labels.end()) return it->second;

  const std::size_t new_number = labels.size();
  labels.insert(std::make_pair(v, new_number);
  return new_number;
}

Now as you process your input, you read one label token v at a time, send it to store_vertex_label(), and you get the corresponding sequential vertex index back.

Your adjacency matrix will be of size labels.size() squared.

Note that your label type doesn’t have to be an integer, it could be anything (e.g. a string).