vector out of range/ range check

Question

new user here. I have been writing this code which creates a network with nodes and uses a random number to create edges between them. I keep track of the whole graph as a vector, each entry being a vector representing a node whose elements are its neighbors. It then uses a depth-first search to find the number of components, which are seperated parts of the graph (my count variable). Then I output the node and the number of neighbors it is connected to into a txt file. The code compiles, but the command prompt gives me an error:

terminate called after throwing an instance of 'std::out_of_range' what(): vector::_M_range_check

This application has requested the Runtime to terminate it in an unusual way. Please contact support...

So... what does this mean and how can I fix it?

Also, I need to keep track of how many nodes are in each component, any ideas?

Thanks in advance, here is my code:

#include <iostream>
#include <fstream>
#include <stdlib.h>
#include <vector>
using namespace std;

void gengraph(int v, float p, vector <vector <int> >& G);
void DFS(vector <vector <int> > G, int v, vector<int>& M);

int main()
{
    int a = 1000;
    float b = 0.004;
    vector <vector <int> > G;
    gengraph(a,b,G);
    vector <int> M (1000);
    int count = 0;
    int i;
    for (i = 0; i < a; i++)
    {
        if (M[i]==0)
        {
            DFS(G, i, M);
            count += 1;
        }
    }
    ofstream myfile;
    myfile.open ("data.txt");
    for (int l=0; l<1000; l++)
    {
        myfile << "v   len(G[v])\n";
    }
    myfile.close();
}
void gengraph(int v, float p, vector <vector <int> >& G)
{
    for (int i = 0; i<1000; i++)
    {  
        for (int j = 0; j<1000; j++)
        {
            int y = rand();
            bool Prob = (y <= p);
            if (i == j)
                continue;
            else
            {
                if(Prob == true)
                {
                    G.at(i).push_back (j);
                    G.at(j).push_back (i);
                }
            }
        }
    }
}
void DFS(vector <vector <int> >& G, int v, vector<int>& M)
{
    M[v]=1;
    for(unsigned int j = 0; j < G[v].size(); j++)
    {
        if (M[j]==0)
        {
            DFS(G, j, M);
        }
    }
}

Answer 1

You created the vector > but it has initial size 0.

Now when you access it using M.at() it checks whether this index is out of bound and throws an exception if this is case.

defining the vector as:

vector<vector<int> > M(1000);

should solve your problem.

You should also use gdb or other debugger. it will make life much easier on you

Answer 2

vector <vector <int> > G;

This creates a vector of vectors-of-ints, but initially there are no vectors-of-int elements. Despite that, you call G.at(i)... which - for any value of i, immediately accesses a non-existant element.

Separately, rand() returns a random 32-bit integer, so will almost always be more than your 0.004 float. You may want to use something like (rand() % 1000000 / 1000000.0). The random number subsystem should be initialised with a call ala srand(time(NULL));

More generally, you're best off using a few std::cerr << "x is now " << x << '\n'; - printing variables, vector sizes etc. - scattered through your code so you can see what it's doing and where it goes wrong. Alternatively, see if you can get an interactive debugger and step through your code line by line.