向量迭代器提供错误的值 [英] Vector iterator providing wrong value

问题描述

我的Graph实现面临问题，特别是函数printGraph().该函数包含一个循环，用于打印图形的邻接表表示形式.如果我使用成员对象变量adj循环，那么它将向我显示正确的输出为:

I'm facing issue with my Graph implementation, specifically the function printGraph(). This function contains a loop to print the adjacency list representation of the graph. If I loop though using the member object variable adj then it shows me the correct output which is:

0 : 1 2 2 
1 : 0 2 
2 : 0 1 0 3 
3 : 2 3 3 

但是，如果我使用吸气剂方法adjL()，那么它将为我提供错误的输出，即:

However, If I use the getter method adjL() then it provides me a wrong output which is:

0 : 0 0 2 
1 : 0 0 
2 : 0 0 0 3 
3 : 0 0 3 

我很可能犯了一个愚蠢的错误，但我似乎无法抓住它.任何帮助表示赞赏.我想我无法理解如何使用getter方法adjL()返回的值.

Most likely I am making a stupid mistake but I can't seem to catch it. Any help is appreciated. I think I am not able to understand how to use the value returned by the getter method adjL().

class UndirectedGraph {
    //vector<vector <int> > adj;   
public:
    vector<vector <int> > adj;
    UndirectedGraph(int vCount);     /* Constructor */
    void addEdge(int v, int w);      /* Add an edge in the graph */
    vector<int> adjL(const int v) const ;    /* Return a vector of vertices adjacent to vertex @v */
    void printGraph();
};

UndirectedGraph::UndirectedGraph(int vCount): adj(vCount) {
}

void UndirectedGraph::addEdge(int v, int w) {
    adj[v].push_back(w);
    adj[w].push_back(v);
    edgeCount++;
}

vector<int> UndirectedGraph::adjL(const int v) const {
    return adj[v];
    //return *(adj.begin() + v);
}

void UndirectedGraph::printGraph() {
    int count = 0;
    for(vector<vector <int> >::iterator iter = adj.begin(); iter != adj.end(); ++iter) {
        cout << count << " : ";

        /*
        for(vector<int>::iterator it = adj[count].begin(); it != adj[count].end(); ++it) {
            cout << *it << " ";         
        }
        */  
        for(vector<int>::iterator it = adjL(count).begin(); it != adjL(count).end(); ++it) {
            cout << *it << " ";         
        }           

        ++count;
        cout << endl;
    }
}

int main() {
    UndirectedGraph g(4);
    g.addEdge(0, 1);
    g.addEdge(0, 2);
    g.addEdge(1, 2);
    g.addEdge(2, 0);
    g.addEdge(2, 3);
    g.addEdge(3, 3);

    g.printGraph();
}

推荐答案

由于adjL奇怪地返回值，因此以下行被破坏了:

Since adjL curiously returns by value, the following line is broken:

for(vector<int>::iterator it = adjL(count).begin(); it != adjL(count).end(); ++it) {

您要比较两个不同容器(和)中的迭代器，并将迭代器存储到立即超出范围的临时变量中，而在没有引起海森堡的情况下，无法立即读取其值可能会转过他的坟墓.

You're comparing iterators from two different containers, and you're storing an iterator to a temporary that immediately goes out of scope, its value(s) immediately becoming impossible to read without causing Heisenberg to possibly turn in his grave.

adjL应该返回const vector<int>&.

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