How to implement Breadth First Search? - c++

Question

I was trying to implement BFS in C++, the function should work like this: takes a graph & a vertex as parameters, then create two vectors, one of these is used to store the vertices adjacent with the parameter vertex, then check those vertices, if one of them is unvisited, then get it's adjacencies and add them to vector, after that the vertex should be marked as VISITED and printed; I wrote this code but it did output anything. NOTE: I ensured that the other functions & data structures like the graph are working, the problem is in the BFS function, I hope some of you can help me to fix it.
#include <iostream>
#include <vector>
#include <queue>
using namespace std;
enum Mark {VISITED, UNVISITED};
struct Vertex {
char name;
Mark mark;
};
struct Edge{
Vertex v1;
Vertex v2;
Edge(Vertex vertex1, Vertex vertex2): v1(vertex1), v2(vertex2){};
};
struct Graph{
vector<Vertex>vertices;
vector<Edge>edges;
vector<pair<Vertex, Edge>> adjacent(char u){
vector<pair<Vertex, Edge>>res;
for(Edge e : edges){
if(e.v1.name == u){
res.push_back(make_pair(e.v2, e));
}else if(e.v2.name == u){
res.push_back(make_pair(e.v1, e));
}
}
return res;
}
vector<Vertex> getAdj(char u){
vector<Vertex>result;
for(Edge e: edges){
if(e.v1.name == u){
result.push_back(e.v2);
}else if(e.v2.name == u){
result.push_back(e.v1);
}
}
return result;
}
};
void BFS(Graph g, Vertex u){
vector<Vertex>vec = g.getAdj(u.name);
for(Vertex v : vec){
if(v.mark == UNVISITED){
vector<Vertex>q = g.getAdj(v.name);
for(int i=0; i<q.size(); i++){
vec.push_back(q[i]);
}
v.mark = VISITED;
vec.pop_back();
cout << v.name << " ";
}
}
}
int main(int argc, const char * argv[]) {
Graph g;
Vertex v1, v2, v3, v4, v5, v6;
v1.name = 'A';
v2.name = 'B';
v3.name = 'C';
v4.name = 'D';
v5.name = 'E';
v6.name = 'Z';
g.edges.push_back(Edge(v1, v2));
g.edges.push_back(Edge(v1, v3));
g.edges.push_back(Edge(v2, v3));
g.edges.push_back(Edge(v2, v4));
g.edges.push_back(Edge(v3, v4));
g.edges.push_back(Edge(v3, v5));
g.edges.push_back(Edge(v4, v5));
g.edges.push_back(Edge(v4, v6));
g.edges.push_back(Edge(v5, v6));
BFS(g, v1);
cout << endl;
}

Answer 1

You're not fully initialising your Vertexs. You give them names, but not the initial mark tag.
You can add this in your main:
v1.mark = UNVISITED;
v2.mark = UNVISITED;
v3.mark = UNVISITED;
v4.mark = UNVISITED;
v5.mark = UNVISITED;
v6.mark = UNVISITED;
You can also do something like this (might be better to replace with enum class):
enum Mark {VISITED, UNVISITED};
struct Vertex {
char name;
Mark mark = UNVISITED;
};
(I would also create a constructor to initialise the names to make sure you're not forgetting to do that).
UPDATE:
There are several problems with your BFS:
- Since you're adding elements to vec inside your inner loop, the outer loop isn't working properly (I think it has to do with iterators and happens because you change the size of the vector all the time). I managed to solve it using a regular for loop.
even if you managed to use a ranged for-loop, you're going from "left to right" in the vector. And you're using pop_back() which removes from the end of a vector (so at the rightmost of the vector). I don't think this is what you want.
You have the right idea by marking eacher Vertex as visited and not visited. But you're only updating the Vertexs in vec which are in the BFS function. But they are only copies from the getADj method, which means each time you add the new neighbors to your vec, anytime there are vertices you already visited, that value is not known. Effectively you're adding a new Vertex which is never visited.
There might be other issues, but already with those ones you should have plenty on your hands!
I'd recommend you use a debugger and/or a tool like 'valgrind', to help find problems in your code.