[英]Segmentation fault (core dumped) implementing union-find C++
我正在做Kruskal最小生成樹,並編寫了一些union-find函數來進行設置操作。 以前它可以正常工作,直到我將KruskalMST()函數從按值傳遞更改為按引用傳遞。 現在看起來像這樣:
vector<Edge*> KruskalMST(vector<Edge>& EV, vector<Node>& mynodelist){
int setcount = NVAR;
vector<Edge*> mst; //mst
vector<int> umark; //setmark
for(int i=0;i<NVAR;i++)
{
umark.push_back(i);
}
/*Kruskal*/
while(setcount>1){
cout<<"# USER DEFINED EDGES = "<<USER_DEFINED_EDGES<<endl;
cout<<"Threshold = "<<THRESHOLD<<endl;
cout<<"# edges = "<<EV.size()<<endl;
for(int j = 0; j < EV.size(); j++){
cout<<"Find(v1) = "<<Find(&mynodelist[EV[j].readv1()->readi()])->readi()<<endl;
cout<<"Find(v2) = "<<Find(&mynodelist[EV[j].readv2()->readi()])->readi()<<endl;
if(Find(&mynodelist[EV[j].readv1()->readi()])->readi()!= Find(&mynodelist[EV[j].readv2()->readi()])->readi()){
mst.push_back(&EV[j]); //add this edge to mst
cout<<"mst.size() = "<<mst.size()<<endl;
Union(&mynodelist[EV[j].readv1()->readi()], &mynodelist[EV[j].readv2()->readi()]); //
cout<<"after Find(v1) = "<<Find(&mynodelist[EV[j].readv1()->readi()])->readi()<<endl;
cout<<"after Find(v2) = "<<Find(&mynodelist[EV[j].readv2()->readi()])->readi()<<endl;
}
}
cout<<"it never reaches here"<<endl;
std::list<int> listmark;
for(int i=0;i<NVAR;i++){
mynodelist[i].setmark(Find(&mynodelist[i]));
listmark.push_back(mynodelist[i].readmark());
}
listmark.sort();
listmark.unique();
setcount = listmark.size();
umark.clear();
umark.insert(umark.begin(), listmark.begin(),listmark.end()); //assign list to vector (clear first)
cout << "# disjoint components: " << setcount << endl;
cout << endl;
}
return mst;
}
奇怪的是它運行了一段時間並墜毀,屈服
All Vertices: #100
All Edges: #4950
# USER DEFINED EDGES = 0
Threshold = 15
# edges = 4950
Find(v1) = 96
Find(v2) = 36
mst.size() = 1
after Find(v1) = 96
after Find(v2) = 96
Find(v1) = 81
Find(v2) = 48
mst.size() = 2
after Find(v1) = 81
after Find(v2) = 81
...
...
Find(v1) = 57
Find(v2) = 84
mst.size() = 26
after Find(v1) = 84
after Find(v2) = 84
Find(v1) = 29
Segmentation fault (core dumped)
似乎它在隨機迭代次數時崩潰,但從未退出for循環。 可能的原因是什么? 請幫幫我!
編輯
查找和聯合功能:
Node* Find(Node* node) { //FINDING THE REPRESENTATIVE FOR EACH SET
Node* temp;
Node* root = node;
while (root->readp() != NULL)
root = root->readp(); //readp() reads its parent
/* Updates the parent pointers */
while (node->readp() != NULL) {
temp = node->readp();
node->setp(root); //setparent
node = temp;
}
return root;
}
/* Merges two nodes based on their rank */
void Union(Node* node1, Node* node2) {
Node* root1 = Find(node1);
Node* root2 = Find(node2);
if (root1->readrank() > root2->readrank()) { //WHOSE RANK IS LARGER, WHO'S THE PARENT. RANK IS THE NUMBER OF CHILDREN FOR THE VERTEX
root2->setp(root1);
} else if (root2->readrank() > root1->readrank()) {
root1->setp(root2);
} else { //EQUAL, ROOT1'S RANK INCREMENTS AND IS THE PARENT
root2->setp(root1);
root1->setrank((root1->readrank())+1);
}
}
我沒有更改聯合查找部分,只是將KruskalMST(vector<Edge> EV, vector<Node> mynodelist)
更改為KruskalMST(vector<Edge>& EV, vector<Node>& mynodelist)
您的返回向量是指針,但是傳遞的向量按值取值。 生成結果時,您將指針指向傳遞的EV向量中的值。 這不是安全的。 如果您在任何時候更改了傳遞的EV向量(在函數執行期間或什至在KrustalMST()
返回之后KrustalMST()
,該向量中值的地址都會更改。 因此,訪問返回的指針將是無效的。
如果KruskalMST()
方法按值獲取參數,那會使情況變得更糟,因為您將返回指向銷毀向量中元素的指針。
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