Question

我试图实现使用邻接列表的Graph数据结构。要填充，我必须从文件中读取数据。该文件是一个文本文件，其中第一行包含两个数字。第一个是顶点数n，第二个是边数m。在此行之后将有m行，其中包含三个数字。前两个数字表示无向边的源和目标顶点。第三个数字（正整数）是该边缘的权重。

文件的内容如下所示：

但由于某种原因，我到目前为止编写的代码使程序崩溃。并且编译器没有给出任何关于原因的提示。我真的很感激一些建议。我已经阅读了很多关于指针，C ++中的引用，但仍然发现它们令人困惑。因此，更好地理解它们的良好资源确实会有所帮助。

#include <string>
#include <iostream>
#include <fstream>
#include <sstream>
#include <vector>

using namespace std;

struct Vertex
{
    unsigned value;
    vector<Vertex*> adjList;
    vector<unsigned> weights;
};

class Graph
{
private:
    unsigned vertex_count, edge_count;
    vector<Vertex*> vertices;


public:
    Graph(string fileName)
    {
        ifstream myFile(fileName);

        if (myFile.is_open())
        {
            // Processing the first line of the file
            string aLine;
            getline(myFile, aLine);
            stringstream aString(aLine);

            aString >> vertex_count;
            aString >> edge_count;

            // Processing the rest of the file
            unsigned vert1, vert2, weight;
            while (getline(myFile, aLine))
            {
                aString= stringstream(aLine);
                aString >> vert1;
                aString >> vert2;
                aString >> weight; 
                addRelation(vert1, vert2, weight);
            }
        }
        else
            cout << "Unable to open file.";
    }

    ~Graph()
    {
        for (unsigned i = 0; i < vertices.size(); i++)
            delete vertices[i];
    }

    void addVertex(unsigned val)
    {
        Vertex* newVertex = new Vertex;
        newVertex->value = val;
        vertices.push_back(newVertex);
    }

    Vertex* findVertex(unsigned val)
    {
        for (unsigned i = 0; i < vertices.size(); i++)
            if (vertices[i]->value = val)
                return vertices[i];
        return nullptr;
    }

    void addRelation(unsigned vert1, unsigned vert2, unsigned weight)
    {
        Vertex* vertex1 = findVertex(vert1);
        if (vertex1 == nullptr) {
            addVertex(vert1);
            vertex1 = findVertex(vert1);
        }

        Vertex* vertex2 = findVertex(vert2);
        if (vertex2 == nullptr) {
            addVertex(vert2);
            vertex2 = findVertex(vert2);
        }

        vertex1->adjList.push_back(vertex2);
        vertex1->weights.push_back(weight);

        vertex2->adjList.push_back(vertex1);
        vertex2->weights.push_back(weight);
    }
};

int main()
{
    Graph myG("graph.txt");
    return 0;
}

Answer 1

你的几个if语句使用=而不是==。如果在编译器中启用警告，则会发现类似：

test.cpp:69:36: warning: using the result of an assignment as a condition without parentheses [-Wparentheses]
            if (vertices[i]->value = val)
                ~~~~~~~~~~~~~~~~~~~^~~~~
test.cpp:69:36: note: place parentheses around the assignment to silence this warning
            if (vertices[i]->value = val)
                                   ^
                (                       )
test.cpp:69:36: note: use '==' to turn this assignment into an equality comparison
            if (vertices[i]->value = val)

Answer 2

你的if表达式指的不是比较。

Vertex* findVertex(unsigned val)
{
    for (unsigned i = 0; i < vertices.size(); i++)
        if (vertices[i]->value = val)  // WHOOPS!
            return vertices[i];
    return nullptr;
}

更改为：

Vertex* findVertex(unsigned val)
{
    for (unsigned i = 0; i < vertices.size(); i++)
        if (vertices[i]->value == val)  // FIXED
            return vertices[i];
    return nullptr;
}

C ++程序崩溃了。图形实现

2 个答案: