Question

我正在尝试实现Dijkstra算法。为此，我使用一个优先级队列存储指向该类对象的节点的指针＆＃39; Node＆＃39;它将返回到起始节点的距离最小的节点。我减少了代码，以便手动编辑起始节点和当前节点之间的距离，并从优先级队列中提取元素。通常Dijkstra会这样做。以下代码无法正常运行：

using namespace std;

#include <iostream>
#include <limits>
#include <vector>
#include <queue>

const int numberNodes = 6;
int IMAX = numeric_limits<int>::max();

class Node{
public:
    Node(float pdistance, int pid){distance = pdistance;    id = pid;}
    float distance;
    int id;     //only for debug
};

Node** nodes;       //in int main() Array of Node*

class Compare{  //Compare pointer to nodes based on distance to start node (Dijkstra)
public:
    bool operator() (Node *n1, Node *n2) const {
        return n1->distance>n2->distance;
    }
};

priority_queue<Node*, vector<Node*>, Compare> pq;

int main(){
    nodes = new Node*[numberNodes];
    for(int i=0; i<numberNodes; i++){       //create new objects and store them in pq
        nodes[i] = new Node(IMAX, i);
        pq.push(nodes[i]);
    }
    Node* sNode;        //Start node. not contained in nodes[]
    sNode = new Node(0, -1);        //distance 0, id -1
    pq.push(sNode);

    cout << "extracted: Node " << (pq.top())->id << " , distance " << (pq.top())->distance << endl;
    pq.pop();

    nodes[0]->distance = 0.5;
    nodes[1]->distance = 0.5;
    cout << "extracted: Node " << (pq.top())->id << " , distance " << (pq.top())->distance << endl;
    pq.pop();       

    cout << "extracted: Node " << (pq.top())->id << " , distance " << (pq.top())->distance << endl;
    pq.pop();   

    nodes[2]->distance = 2.5;
    nodes[3]->distance = 3.5;
    cout << "extracted: Node " << (pq.top())->id << " , distance " << (pq.top())->distance << endl;
    pq.pop();   
}

它返回：

extracted: Node -1 , distance 0
extracted: Node 0 , distance 0.5
extracted: Node 1 , distance 0.5
extracted: Node 5 , distance 2.14748e+09

pq正常工作三次，但最后它应该返回节点2，距离为2.5。

那我怎么能让它发挥作用呢？

感谢您的回答

Answer 1

由于你弹出了priority_queue的元素，它的大小正在减少，因为这行：

节点[2] - ＆gt;距离= 2.5; nodes [3] - ＆gt; distance = 3.5;

应改为：

nodes [0] - ＆gt;距离= 2.5; nodes [1] - ＆gt; distance = 3.5;

Answer 2

在顶部，变量numberNodes设置为等于6。将它减少到一个较小的数字，因为它正在初始化主函数中的6个节点。

庞大的数字是编译器初始化浮点数的默认垃圾，因为你声明它但没有为它分配任何内容。

要避免此问题，请确保在使用之前初始化所有变量。

for(int i=0; i<numberNodes; i++){       //create new objects and store them in pq
    nodes[i] = new Node(IMAX, i);
    nodes[i].distance = 0.0f; //<--set a default value
    pq.push(nodes[i]);
}

C ++：指向对象的指针的优先级队列无法正常工作

2 个答案: