Question

I have a project where I’m implementing Dijkstra’s shortest path algorithm using C++ classes. It uses OpenGL but that is surely separate from my problems. I need some insight on what I’m doing wrong in my Dijkstra class method. Here is my relevant code:

class Node {
    public: 
        GLfloat x, y, z;
        int numLinks;
        Node *link1;
        Node *link2;
        GLfloat distance;
        Node *previous;

        Node(GLfloat x, GLfloat y, Node *link1, Node *link2);
        Node(GLfloat x, GLfloat y, Node *link1);
        Node();
        Node(GLfloat x, GLfloat y);
        ~Node();

        bool dijkstra(Node* graph[], Node *source, Node *target); //returns true if a path to target is found
        int dist(Node &n1, Node &n2);
};

int Node::dist(Node &n1, Node &n2) {
    GLfloat d = sqrt((pow((n2.x - n1.x), 2)) + (pow((n2.y - n1.y), 2)));
    return d;
}

bool Node::dijkstra(Node* graph[], Node *source, Node *target) {
    queue<Node> q;
    int i;
    for (i = 0; i < NUM_NODES; i++) {
        graph[i]->distance = INFIN;
    }
    source->distance = 0;   
    i = 0;
    q.push(*source);
    while (!q.empty()) {
        Node temp = q.front();

        GLfloat d1 = dist(temp, temp->link1);
        GLfloat d2 = dist(temp, temp->link2);
        temp.link1.distance = d1;
        temp.link1.distance = d2;

        GLfloat alt = temp.distance + temp->link1.distance;
        if (alt < temp->link1.distance) {
            temp->link1.distance = alt;
            temp->previous = temp;
        }

        alt = temp->distance + temp->link2->distance;
        if (alt < temp->link2->distance) {
            temp->link2->distance = alt;
            temp->previous = temp;
        }

        if(d1 > d2) {
            q.push(temp->link2);
            q.push(temp->link1);
        } else {
            q.push(temp->link1);
            q.push(temp->link2);
        }
        q.pop();
        i++;
    }

    return true;
}

My guess is that I’m using “->” and “.” operators all wrong. I get a lot of these errors when I try to compile:

error: base operand of ‘->’ has non-pointer type ‘Node’

My implementation of Dijkstra’s algorithm is somewhat retrofitted to meet my needs and is probably wrong, but I need this to compile so I can debug it.

The code listed is the only code giving me grief but if you would like to see another part of it just ask. A good explanation of what im doing wrong will be greatly appreciated.

Answer 1

You should use the ‘->’ operator when accessing data through a pointer. Otherwise you should use the ‘.’ opearator.

For example these following lines are identical

Node *node = &q.front();

(*node).link1->distance = 1;
node1->link1->distance = 1;

As for your compilation problems, the problem is with how you are accessing temp. temp is declaired as

Node temp = q.front();

Which means that temp is a copy of the node at the front of the queue and is not a pointer to it. This is why the compiler is complaining when you attempt to access it like it is a pointer. For example:

temp->link1.distance = alt;

should look like

temp.link1->distance = alt;

because temp is not a pointer but link1 is.