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Thread: Dijstra's Algorithm Problems

  1. #1
    Junior Member
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    Exclamation Dijstra's Algorithm Problems

    Hey guys, So I'm trying write a dijstra's implementation in java. First off, here is the algorith:
    package lab3;
     
    import java.util.HashMap;
    import java.util.Stack;
     
    /**
     * Compute shortest paths in a graph.
     *
     * Your constructor should compute the actual shortest paths and
     * maintain all the information needed to reconstruct them.  The
     * returnPath() function should use this information to return the
     * appropriate path of edge ID's from the start to the given end.
     *
     * Note that the start and end ID's should be mapped to vertices using
     * the graph's get() function.
     */
    class ShortestPaths {
     
    	Multigraph graph;
    	final int INF = Integer.MAX_VALUE;
    	PriorityQueue<Integer> Q;
    	int n;
    	int dist[];
    	Handle handles[];
    	Edge edge[];
     
    	/**
    	 * Constructor
    	 */
    	public ShortestPaths(Multigraph G, int startId) {
    		Q = new PriorityQueue<Integer>();
    		graph = G;
    		n = graph.nVertices();
    		dist = new int [n];
    		edge = new Edge [n];
    		handles = new Handle[n];
     
    		for (int i = 0; i<n; i++){
    			dist[i] = INF;
    		}
    		dist[startId] = 0;
     
    		Handle h = Q.insert(startId, dist[startId]);
    		handles[startId] = h;	
    		Q = new PriorityQueue<Integer>();
    		while (!Q.isEmpty()){
    			Vertex v = graph.get(Q.min());
    			Q.extractMin();
    			while (v.adj().hasNext()){
    				relax(v.adj().next());	
    			}		
    		}	
    	}
     
    	private void relax(Edge e) {
    		Handle h;
    		int v = e.from().id();
    		int w = e.to().id();
    		if (dist[w] > dist[v] + e.weight()) {
    			dist[w] = dist[v] + e.weight();
    			edge[w] = e;
    			if (handles[w].getIndex() != -1){
    				Q.decreaseKey(handles[w], dist[w]);
    			}
    			else{
    				h = Q.insert(w, dist[w]);
    				handles[w] = h;
    			}
    		}
    	}
     
    	/**
    	 * Calculates the list of edge ID's forming a shortest path from the start
    	 * vertex to the specified end vertex.
    	 *
    	 * @return the array
    	 */
    	public int[] returnPath(int endId) {
    		int c = 0;
    		int[] path = new int[edge.length];
    		for (Edge e = edge[endId]; e != null; e = edge[e.from().id()]) {
    			path[c] = e.id();
    			c++;
    		}
    		return path;
    	}
     
     
    }
    I followed someone else's psuedocode very closely but for whatever reason, my edge[] array is just full of null data, which means I can't actually return the shortest path. Any ideas on why that's happening/how to fix it? Maybe I'm not understanding dijstra's correctly. Anyway, thanks for your help.


  2. #2
    Junior Member
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    Exclamation Dijstra's Algorithm Problems

    Hey guys, So I'm trying write a dijstra's implementation in java. First off, here is the algorithm:
    package lab3;
     
    import java.util.HashMap;
    import java.util.Stack;
     
    /**
     * Compute shortest paths in a graph.
     *
     * Your constructor should compute the actual shortest paths and
     * maintain all the information needed to reconstruct them.  The
     * returnPath() function should use this information to return the
     * appropriate path of edge ID's from the start to the given end.
     *
     * Note that the start and end ID's should be mapped to vertices using
     * the graph's get() function.
     */
    class ShortestPaths {
     
    	Multigraph graph;
    	final int INF = Integer.MAX_VALUE;
    	PriorityQueue<Integer> Q;
    	int n;
    	int dist[];
    	Handle handles[];
    	Edge edge[];
     
    	/**
    	 * Constructor
    	 */
    	public ShortestPaths(Multigraph G, int startId) {
    		Q = new PriorityQueue<Integer>();
    		graph = G;
    		n = graph.nVertices();
    		dist = new int [n];
    		edge = new Edge [n];
    		handles = new Handle[n];
     
    		for (int i = 0; i<n; i++){
    			dist[i] = INF;
    		}
    		dist[startId] = 0;
     
    		Handle h = Q.insert(startId, dist[startId]);
    		handles[startId] = h;	
    		Q = new PriorityQueue<Integer>();
    		while (!Q.isEmpty()){
    			Vertex v = graph.get(Q.min());
    			Q.extractMin();
    			while (v.adj().hasNext()){
    				relax(v.adj().next());	
    			}		
    		}	
    	}
     
    	private void relax(Edge e) {
    		Handle h;
    		int v = e.from().id();
    		int w = e.to().id();
    		if (dist[w] > dist[v] + e.weight()) {
    			dist[w] = dist[v] + e.weight();
    			edge[w] = e;
    			if (handles[w].getIndex() != -1){
    				Q.decreaseKey(handles[w], dist[w]);
    			}
    			else{
    				h = Q.insert(w, dist[w]);
    				handles[w] = h;
    			}
    		}
    	}
     
    	/**
    	 * Calculates the list of edge ID's forming a shortest path from the start
    	 * vertex to the specified end vertex.
    	 *
    	 * @return the array
    	 */
    	public int[] returnPath(int endId) {
    		int c = 0;
    		int[] path = new int[edge.length];
    		for (Edge e = edge[endId]; e != null; e = edge[e.from().id()]) {
    			path[c] = e.id();
    			c++;
    		}
    		return path;
    	}
     
     
    }
    I followed someone else's psuedocode very closely but for whatever reason, my edge[] array is just full of null data, which means I can't actually return the shortest path. Any ideas on why that's happening/how to fix it? Maybe I'm not understanding dijstra's correctly. Anyway, thanks for your help.

  3. #3
    Super Moderator Norm's Avatar
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    Default Re: Dijstra's Algorithm Problems

    There are several class definitions missing. They are needed to compile and execute the code for testing.
    If you don't understand my answer, don't ignore it, ask a question.

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