Xcode：线程1：制作邻接表时，EXC_BAD_ACCESS（代码= 1，地址= 0x0）

Question

I'm writing a program that reads a file consisting of courses and the pre-reqs for each course. I'm supposed to print an ordering in which you could take all of the listed courses so that you don't take any courses before you've taken all the required pre-req courses. To do this I made an adjacency matrix that uses an array of linked lists to store the vertices. Whenver I run it I get the Thread 1: EXC_BAD_ACCESS (code=1, address=0x0) error. It happens sometime after it outputs the number of vertices. I've been playing around with the breakpoints for a while but I havn't seen any progress. Can anyone point me in the right direction? Full Code:

Header:

#ifndef adjList3_h
#define adjList3_h
//#include <vector>
#include <string>
#include <iostream>
//#include <queue>
#include <fstream>

using namespace std;

template <class T>
class adjList
{
private:
//    class neighbor{
//    public:
//        string name;
//        neighbor * next;
//        bool mark;
//        //constructor
//        neighbor(T x)
//        {
//            name = x;
//            next = NULL;
//            mark = false;
//        }
//    };

    class vertex
    {
    public:
        T name;
        vertex * next;
        bool mark;

        vertex(T x)
        {   name = x;
            next = NULL;
            mark = false;
        }
    };

    vertex ** arr; //array of vertex objects, collection of linked lists
    int numV;//number of vertices


    vertex * findVertex(string x, int size, vertex ** arr)
    {
        for (int i = 0; i < size; i++)
        {
            if (arr[i]->name == x)
                return arr[i];
        }
        return NULL;
    }

//    neighbor * findNeighbor(string x, int size, vertex ** arr)
//    {
//        for (int i = 0; i < size; i++)
//        {
//            if(arr[i]->name == x)
//            {
//                return arr[i];
//            }
//        }
//        return NULL;
//    }


public:
    adjList(string fileName)
    {
        string adjacentVertex, firstVertex;
        ifstream inFile;

        inFile.open(fileName);
        if(!inFile)
        {
            cout << "Error opening file..." << endl;
        }

        inFile >> numV;
        arr = new vertex*[numV]; //create an array of vertices
        cout << "Number of vertices: " << numV << endl;

        for (int i = 0; i < numV; i++)
        {
            inFile >> firstVertex; //read the source vertex name
            arr[i] = new vertex(firstVertex); //add a vertex with the source name to the graph

            inFile >> adjacentVertex; //read the next adjacent's name

            //while the adjacent name isn't the -999 sentinel
            //add an edge from source->destination
            //read the next adjacent name
            while (adjacentVertex != "\n")
            {
                //add directed edge from source vertex to neihgbors (class to pre-reqs)
                addDirectedEdge(firstVertex, adjacentVertex);
                inFile >> adjacentVertex;
            }

        }
        delete [] arr;
        inFile.close();

    }

//    bool checkCopy(string name)
//    {
//        for (int i = 0; i < numV; i++)
//        {
//            if(arr[i]->name == name)
//            {
//                return true;
//            }
//        }
//        return false;
//    }
//
    //add a directed edge going from x to y (class to pre-reqs)
    void addDirectedEdge(T x, T y)
    {
        //we want to add a directed edge from the vertex to neighbors
        vertex * source = findVertex(x, numV, arr);
        vertex * destination = findVertex(y, numV, arr);

        if (source != NULL && destination != NULL)
        {
            source->next = destination;
        }
    }


};

#endif /* adjList3_h */

Main:

    #include "adjList3.h"

    int main() {

    string filename;
    cout << "What is the filename? " << endl;
    cin >> filename;

    adjList<string> G(filename);
    }

Answer 1

The problem is here:

    for (int i = 0; i < numV; i++)
    {
        inFile >> firstVertex; //read the source vertex name
        arr[i] = new vertex(firstVertex); //add a vertex with the source name to the graph

        inFile >> adjacentVertex; //read the next adjacent's name

        //while the adjacent name isn't the -999 sentinel
        //add an edge from source->destination
        //read the next adjacent name
        while (adjacentVertex != "\n")
        {
            //add directed edge from source vertex to neihgbors (class to pre-reqs)
            addDirectedEdge(firstVertex, adjacentVertex);
            inFile >> adjacentVertex;
        }

    }

You get the first vertex's name and you create a vertex object to store it. You then get the next vertex's name, but you never actually create the vertex object to store it. Then you search for it, but there's no vertex there. Furthermore, in addDirectedEdge() , you're assuming the size of the list is numV , but you haven't actually read in numV vertexes yet.

I would make sure to create and add each vertex to the list as you read them in.

So the constructor for adjList() might look like this:

adjList(string fileName)
{
    string adjacentVertex, firstVertex;
    ifstream inFile;

    inFile.open(fileName);
    if(!inFile)
    {
        cout << "Error opening file..." << endl;
    }

    inFile >> numV;
    arr = new vertex*[numV]; //create an array of vertices
    cout << "Number of vertices: " << numV << endl;

    for (int i = 0; i < numV; i++)
    {
        inFile >> firstVertex; //read the source vertex name
        arr[i] = new vertex(firstVertex); //add a vertex with the source name to the graph

        inFile >> adjacentVertex; //read the next adjacent's name

        //while the adjacent name isn't the -999 sentinel
        //add an edge from source->destination
        //read the next adjacent name
        vertex* prevVertex = arr[i];
        while (adjacentVertex != "\n")
        {
            vertex* nextVertex = new vertex(adjacentVertex);
            //add directed edge from source vertex to neihgbors (class to pre-reqs)
            prevVertex->next = nextVertex;
            prevVertex = nextVertex;

            inFile >> adjacentVertex;
        }

    }
    delete [] arr;
    inFile.close();

}