Trying to parallelize DFS (recursive) traversal through openMP - c++

Question

It's my first time using openMP.
Below is given the code so far(which I have not yet been able to parallel,even in a very small part, since I am coming face to face with the error given at the end).
// C++ program to print DFS traversal from a given vertex in a given graph
#include <bits/stdc++.h>
#include <iostream>
#include <fstream>
#include <vector>
#include <omp.h>
using namespace std;
// Graph class represents a directed graph using adjacency list representation
class Graph
{
int V; // No. of vertices
// Pointer to an array containing adjacency lists
list<int> *adj;
// A recursive function used by DFS
void DFSUtil(int v, bool visited[]);
public:
Graph(int V); // Constructor
// function to add an edge to graph
void addEdge(int v, int w);
// DFS traversal of the vertices reachable from v
void DFS(int v);
};
Graph::Graph(int V)
{
this->V = V;
adj = new list<int>[V];
}
void Graph::addEdge(int v, int w)
{
adj[v].push_back(w); // Add w to v’s list.
}
void Graph::DFSUtil(int v, bool visited[])
{
// Mark the current node as visited and print it
visited[v] = true;
cout << v << " ";
// Recur for all the vertices adjacent to this vertex
list<int>::iterator i;
for (i = adj[v].begin(); i != adj[v].end(); ++i) {
if (!visited[*i])
DFSUtil(*i, visited);
}
}
// DFS traversal of the vertices reachable from v. It uses recursive DFSUtil().
void Graph::DFS(int v)
{
// Mark all the vertices as not visited
bool *visited = new bool[V];
for (int i = 0; i < v; i++)
visited[i] = false;
// Call the recursive helper function to print DFS traversal
DFSUtil(v, visited);
}
// ------------------Drivercode-------------------------------//
int main()
{
//Create a dynamic array to hold the values
vector<int> numbers;
//Create an input file stream
ifstream in("graph_adjacency_list.txt",ios::in);
/*
As long as we haven't reached the end of the file, keep reading entries.
*/
int number; //Variable to hold each number as it is read
//Read number using the extraction (>>) operator
while (in >> number) {
//Add the number to the end of the array
numbers.push_back(number);
}
//Close the file stream
in.close();
/*
Now, the vector<int> object "numbers" contains both the array of numbers,
and its length (the number count from the file).
*/
//Display the numbers
cout << " \n Numbers of our file (graph_adjacency_list.txt):\n";
for (int i=0; i<numbers.size(); i++) {
cout << numbers[i] << ' ';
}
cout << "\n";
int s = numbers.size();
auto start = chrono::steady_clock::now();
// Create a graph given in the above diagram
Graph g(numbers[0]); //<--Takes the number of Vertices , included in the first position of the array(numbers[])
In this part I have the problem :
#pragma omp parallel shared(g) private(i) firstprivate(s)
{
int i =0;
#pragma omp for
for ( i=0; i<s; i=i+2) {
g.addEdge(numbers[i],numbers[i+1]);
}
}
cout << "\n Following is Depth First Traversal"
" (starting from vertex 0): \n";
g.DFS(0);
cout << "\n";
auto end = chrono::steady_clock::now();
auto diff = end - start;
cout << "\n Time taken:" << chrono::duration <double, milli> (diff).count() << " ms" << endl;
cout << "\n";
return 0;
}
When i compile the file through terminal :
ex.--> g++ parallel_DFS.cpp -o parallel_DFS -fopenmp
I take this error:
parallel_DFS.cpp: In function ‘int main()’:
parallel_DFS.cpp:149:44: error: ‘i’ has not been declared
#pragma omp parallel shared(g) private(i) firstprivate(s)
^
and moreover, when i run it on VScode i take:
malloc(): memory corruption
Aborted (core dumped)
Thanks for any help in advance