How to accept graph from input file - c++

Question

I am accepting undirected weighted graph from input file. File contains 200 nodes.
I have written code like this.
typedef pair<int, int> ipair;
class Graph
{
int V;
list< pair <int, int> > *adj;
public:
Graph(int V);
};
Graph::Graph( int V)
{
this-> V = V;
adj = new list<ipair>[V];
}
bool Graph :: read_file()
{
const long int N = 1000000;
std::ifstream infile("Dijkstra.txt");
if(!infile.is_open()) return false;
std::string line;
int i = 0;
while ( i < N && getline(infile, line) )
{
std::istringstream str(line);
int u;
str >> u;
if ( u > N )
{
// Problem.
abort();
}
int v;
int w;
while ( str >> v >> w)
{
adj[u].push_back(make_pair(v,w));
}
++i;
}
}
int main()
{
Graph g(200);
g.read_file();
g.print_graph();
return 0;
}
I/P file :
1 80,982 163,8164 170,2620 145,648 200,8021 173,2069 92,647 26,4122 140,546 11,1913 160,6461 27,7905 40,9047 150,2183 61,9146 159,7420 198,1724 114,508 104,6647 30,4612 99,2367 138,7896 169,8700 49,2437 125,2909 117,2597 55,6399
2 42,1689 127,9365 5,8026 170,9342 131,7005 172,1438 34,315 30,2455 26,2328 6,8847 11,1873 17,5409 157,8643 159,1397 142,7731 182,7908 93,8177
Node 1 is connected to node 80 with weight 982
Node 1 is connected to node 163 with weight 8164
Node 2 is connected to node 42 with weight 1689
Node 2 is connected to node 127 with weight 9365
etc.....
Now with this I can accept node 1 is connected to node 80 with weight 982 (1 80,982 ) but what about remaining nodes which are connected with node 1 ??
i.e. How to make loop for accepting v and w ??

Answer 1

I think instead of list< pair <int, int> > *adj in Graph class you can use vector<list< pair <int, int> >> adj to store the data.
I have modified your program slightly to store all the pair of data related to a node.
#include <iostream>
#include <utility>
#include <list>
#include <fstream>
#include <sstream>
#include <cstdlib>
#include <vector>
using namespace std;
typedef pair<int, int> ipair;
class Graph
{
int V;
vector<list< pair <int, int> >> adj;
public:
bool read_file();
void print_graph();
Graph(int V);
};
Graph::Graph( int V)
{
this-> V = V;
adj.reserve(V);
}
bool Graph:: read_file()
{
const long int N = 1000000;
std::ifstream infile("Dijkstra.txt");
if(!infile.is_open()) return false;
std::string line;
int i = 0;
while ( i < N && getline(infile, line) ) {
std::istringstream str(line);
int u;
str >> u;
if ( u > N ) {
// Problem.
abort();
}
int v;
int w;
char c;
while ( str >> v >> c >> w) {
if (u <= (int)adj.size()) { // index (u-1) exists
auto list = adj[u-1]; // get the existing list
list.push_back(make_pair(v,w)); // add new data
adj[u-1] = list; // store it the same index
} else { // index (u-1) doesn't exist
list<ipair> list; // create a new list
list.push_back(make_pair(v,w)); // store the values
adj.push_back(list); // add it in the vector
}
}
++i;
}
return true;
}
void Graph::print_graph() {
int node = 1;
for (auto& x : adj) {
cout << "from node: " << node++ << endl;
for (auto it = begin(x); it != end(x); ++it) {
cout << it->first << " " << it->second << "\n";
}
}
}
int main() {
Graph g(200);
g.read_file();
g.print_graph();
return 0;
}

Answer 2

You can regex match ,.
e.g.
std::regex pairs_regex("(\d+),(\d+)");
auto pairs_it = std::sregex_iterator(line.begin(), line.end(), pairs_regex);
auto pairs_end = std::sregex_iterator();
for(;pairs_it != pairs_end; ++pairs_it)
{
std::string v = pairs_it->str(1);
std::string w = pairs_it->str(2);
adj[u].push_back(make_pair(std::stoi(v), std::stoi(w)));
}
N.B. it would be safer if adj were a std:: container, not a raw array. The loop body would populate a list<pair<int, int>> temp, then adj.emplace_back(std::move(temp))