Dijkstra algorithm implementation - Code not working for bigger inputs

Question

#include <iostream>
#include <list>

using namespace std;

class edge
{
    public:
        int dest;
        int dist;
        edge(int a,int b)
        {
            dest=a;
            dist=b;
        }
};

class Graph
{
    public:
        int v;      
        list<edge> *adj;
        list<int> remEdges;
        int *dist;
        int *parent;
        int src;
        Graph(int);
        void addEdge(int,int,int);
        void printEdges(int);
        bool isPresent(int);
        int findMin();
        void dijkstra(int);
};

Graph::Graph(int v)
{
    adj = new list<edge>[v];
    this->v=v;
    dist=new int(v);
    parent=new int(v);
    for(int i=0;i<v;i++)
    {
        remEdges.push_back(i);
        dist[i]=INT_MAX;
    }   
}

bool Graph::isPresent(int num)
{
    list<int> :: iterator i;

    for(i=remEdges.begin();i!=remEdges.end();i++)
        if(num==*i)
            return true;

    return false;
}

int Graph::findMin()
{
    int min=INT_MAX;
    int index=-1;
    for(int i=0;i<v;i++)
        if(dist[i]<min && isPresent(i))
        {
            min=dist[i];
            index=i;
        }
    return index;
}

void Graph :: addEdge(int i,int j,int k)
{
    adj[i].push_back(edge(j,k));
    adj[j].push_back(edge(i,k));
}

void Graph :: printEdges(int src)
{
for(int i=0;i<v;i++)
if(i!=src)
cout<<dist[i]<<" ";


}

void Graph::dijkstra(int src)
{
    dist[src]=0;
    while(!remEdges.empty())
    {
        int min=findMin();
        list<edge> :: iterator i;

        for(i=adj[min].begin();i!=adj[min].end();i++)
        {
            if(isPresent((*i).dest))
            {
            if(dist[min]+(*i).dist<dist[(*i).dest])
                dist[(*i).dest] = dist[min]+(*i).dist;
            }
            parent[(*i).dest]=min;                  
        }
        remEdges.remove(min);
    }
}


int main()
{

    Graph g(9);
    g.addEdge(0, 1, 4);
    g.addEdge(0, 7, 8);
    g.addEdge(1, 2, 8);
    g.addEdge(1, 7, 11);
    g.addEdge(2, 3, 7);
    g.addEdge(2, 8, 2);
    g.addEdge(2, 5, 4);
    g.addEdge(3, 4, 9);
    g.addEdge(3, 5, 14);
    g.addEdge(4, 5, 10);
    g.addEdge(5, 6, 2);
    g.addEdge(6, 7, 1);
    g.addEdge(6, 8, 6);
    g.addEdge(7, 8, 7);
    g.dijkstra(0);
    g.printEdges(0);
    cout<<endl;

    return 0;
}

Code not working for bigger inputs.

I am newbie to algorithms and I wanted to implement dijkstra algorithm through CPP. Spent lot of hours to fix the code.

Its showing correct output during debug mode but it's not working when executed directly from Run button.

I am using DEV C++ to run the code and it is executing fine when the driver function is

int main()
{
    Graph g(4);
    g.addEdge(0, 1, 24);
    g.addEdge(0, 3, 20);
    g.addEdge(2, 0, 3);
    g.addEdge(3, 2, 12);
    g.dijkstra(0);
    g.printEdges(0);
    cout<<endl; 
    return 0;
}

But it is not working when I am adding too many edges.

Please help me in this regard.

*How* does it not work? *How* do you "execute directly"? Does the program crash? Does it give the wrong output (what output do you get, and what did you expect)? Maybe you miss initializing a pointer somewhere (stop using pointers instead of `std::vector`)? — Some programmer dude, Sep 20 '16 at 13:15
However, a good start might be checking your allocations. You don't always allocate arrays, but only *single values*. Again, ***use `std::vector`*** instead of dynamically allocated arrays. — Some programmer dude, Sep 20 '16 at 13:18

flogyu · Accepted Answer · 2016-09-21T07:52:41.467

1

//You should use: 
dist=new int[v]; 
parent=new int[v];

//instead of 
//dist=new int(v);  means: dist = new int[1]; dist[0] = v;
//parent=new int(v);

Use of unallocated memory results in undefined behaviour, so it was only luck, your code didn't crash with few vertices.

You should also set the child’s parent only if the child was not visited before, although it couldn't cause the crash. =)

if(isPresent((*i).dest))
{
   if(dist[min]+(*i).dist<dist[(*i).dest])
      dist[(*i).dest] = dist[min]+(*i).dist;

   parent[(*i).dest]=min;
}

edited Sep 21 '16 at 07:52

answered Sep 21 '16 at 06:18

flogyu

34
4

This parent and child relationship between nodes is not used anywhere. I forgot to remove those lines which created some ambiguity in the code. The problem is that if I am adding too many vertices and edges, the program is getting crashed. – Sai Krishna Palagummi Sep 21 '16 at 06:26
Oh, I see! You should use: "dist=new int[v]; parent=new int[v];" instead of "dist=new int(v); parent=new int(v);" "new int(v)" allocates only one int, and initializes it to v. – flogyu Sep 21 '16 at 06:45
Thanks bro!!! Its working now. Thank you very much. But can you explain me why it is working for few vertices with the previous syntax dist=new int(v); Also please edit your answer with the above comment. – Sai Krishna Palagummi Sep 21 '16 at 06:50

Dijkstra algorithm implementation - Code not working for bigger inputs

1 Answers1