Displaying shortest path via bfs search in directed graph - c++

Question

I've been working on a project for college and ran into a rather large problem. I'm supposed to make a function that gets the shortest path through a directed graph from point A to point B and display the path in order.
EX. if the node holds a state name and we want to find the shortest path between California and Utah the output would show california -> nevada -> utah
Currently, my traversal shows all nodes searched with bfs instead of the list of nodes that we took to get from point A to point B.
Below is my implementation of the assignment. My only real question is how would I go about keeping track of the nodes I actually traversed instead of all nodes searched.
bool DirectedGraph::GetShortestPath(
const string& startNode, const string& endNode,
bool nodeDataInsteadOfName, vector<string>& traversalList) const
{
//Nodes are the same
if (startNode.compare(endNode) == 0)
return false;
//Stores the location of our nodes in the node list
vector<int> path;
//Queue to hold the index of the node traversed
queue<int> q;
//Create our boolean table to handle visited nodes
bool *visited = new bool[m_nodes.size()];
//initialize bool table
memset(visited, false, sizeof(bool) * m_nodes.size());
//Label the start node as visited
visited[GetNodeIndex(startNode)] = true;
//Push the node onto our queue
q.push(GetNodeIndex(startNode));
while (!q.empty())
{
//Store the nodes index
int index = q.front();
path.push_back(q.front());
q.pop();
int i = 0;
for (i = 0; i < m_nodes[index]->Out.size(); i++)
{
//If this node matches what we are looking for break/return values
if (m_nodes[index]->Out[i]->targetI == GetNodeIndex(endNode))
{
path.push_back(m_nodes[index]->Out[i]->targetI);
if (nodeDataInsteadOfName)
{
path.push_back(m_nodes[index]->Out[i]->targetI);
for (int x = 0; x < path.size(); x++)
{
traversalList.push_back(m_nodes[path[x]]->Data);
}
}
else
{
for (int x = 0; x < path.size(); x++)
{
traversalList.push_back( m_nodes[path[x]]->Name);
}
}
return true;
}
//Continue through the data
if (!visited[m_nodes[index]->Out[i]->targetI])
{
visited[m_nodes[index]->Out[i]->targetI] = true;
q.push(m_nodes[index]->Out[i]->targetI);
}
}
}
// You must implement this function
return false;
}
//definition of graph private members
struct Edge
{
int srcI; // Index of source node
int targetI; // Index of target node
Edge(int sourceNodeIndex, int targetNodeIndex)
{
srcI = sourceNodeIndex;
targetI = targetNodeIndex;
}
};
struct Node
{
string Name;
string Data;
Node(const string& nodeName, const string& nodeData)
{
Name = nodeName;
Data = nodeData;
}
// List of incoming edges to this node
vector<Edge*> In;
// List of edges going out from this node
vector<Edge*> Out;
};
// We need a list of nodes and edges
vector<Node*> m_nodes;
vector<Edge*> m_edges;
// Used for efficiency purposes so that quick node lookups can be
// done based on node names. Maps a node name string to the index
// of the node within the nodes list (m_nodes).
unordered_map<string, int> m_nodeMap;

Answer 1

The first problem is with the if inside the for loop. Your path variable can only contain two items: the starting and the ending nodes. I suggest you do no track the path with the for loop. Instead, assign each node a distance.
struct Node
{
string Name;
string Data;
int Distance;
Node(const string& nodeName, const string& nodeData)
{
Name = nodeName;
Data = nodeData;
Distance = INT_MAX;
}
// List of incoming edges to this node
vector<Edge*> In;
// List of edges going out from this node
vector<Edge*> Out;
};
and set the distance of the starting node to zero, before looping.
m_nodes[GetNodeIndex(startNode)]->Distance = 0;
At each iteration, pick a node from the queue (you called it index), loop through its adjacency list (outgoing arcs) and test if the adjacent node is visited. If the node is visited, skip it. If the node is not visited, visit it by setting its distance to
m_nodes[index]->Distance + 1
After updating the distance of each node, check if it is the final node, if so break out of the loops.
At this point you have the distance's updated properly. Work your way from the end node backwards, each time selecting the node from the adjacency list with (distance = current node's distance - 1). You can do this using m_edges vector, each time you actually know targetI, so you can check for its corresponding scrI's with the distance value mentioned above.