[英]C++: priority queue with pointer to object does not work correctly
我正在尝试实现Dijkstra-Algorithm。 为此,我正在使用优先级队列来存储指向“节点”类的对象的指针,该指针将返回距离起始节点距离最短的节点。 我减少了代码,以便它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。 以下代码无法正常工作:
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可以正常工作三倍,但最后应该返回距离2.5的节点2。
那么我该如何运作呢?
谢谢答案
由于您正在弹出您的priority_queue的元素,因此其大小正在减小,因为该行:
节点[2]->距离= 2.5; 节点[3]->距离= 3.5;
应更改为:
节点[0]->距离= 2.5; 节点[1]->距离= 3.5;
在顶部,变量numberNodes设置为等于6。将其减小为较小的数字,因为它将在您的main函数中初始化6个节点。
巨大的数字是自从您声明float以来,编译器将float初始化为的默认垃圾,但没有为其分配任何内容。
为避免此问题,只需确保在使用它们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]);
}
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