C++ Bitset array, accessing values

I've got the task to create a graph's adjacency matrix from a list of adjacent nodes, stored in a file (don't need the weights) into a bitset array in C++. I successfully read the adjacent nodes from the file, but when I try to store it in the bitset array the outcome is not right. My function is the following:

bitset<N>* read_graph(string filepath)
{
    FILE *fp;
    char line[100];
    bitset<N> bs[N];

    fp = fopen(filepath.c_str(), "r");

    if(fp != NULL)
    {
       while(!feof开发者_运维百科(fp))
       {   
          fgets(line, 100, fp);
          //cout << line << endl;
          if(line[0] == 'a')
          {
               string str = ""; 
               int i(0);    
               for(i = 2; line[i] != ' '; i++)
               {
                  if(line[i] != ' ')
                  {
                    str += line[i];
                  }     
               }
               int fi = atoi(str.c_str());
               i++;
               str = "";
               for(int j = i; line[j] != ' '; j++)
               {
                  if(line[j] != ' ')
                  {
                    str += line[j];
                  }     
               }    
               int si = atoi(str.c_str());
               si--;
               fi--;
               //cout << "string: " << str << endl;
               cout << "fi: " << fi;
               //cout << "string: " << str << endl;
               cout << "   si: " << si << endl;
               bs[fi][si]= 1;
          }
       }          
    }
    fclose(fp);

    return bs;
}

The outcome is the following (fi stands for first index and si stands for second index):

sample.gr

fi: 0 si: 1

fi: 0 si: 2

fi: 1 si: 3

fi: 2 si: 4

fi: 3 si: 2

fi: 3 si: 5

fi: 4 si: 1

fi: 4 si: 5

fi: 4 si: 5

000000

000001

011000

001000

000000

000000

The indexes are right, I've checked them, but the matrix should be the following (it is mirrored because of the bitset's right side positioning):

000000

010001

001001

000010

000100

011000

I guess the error is somewhere around the bitset element accessing but I cannot find out what exactly is wrong.

I appreciate any help. Thanks.

With the pointer to a local array problem fixed, your code runs for me and prints what's expected (except mirrored).

But wouldn't it be easier to use C++ I/O in this case?

#include <vector>
#include <bitset>
#include <iterator>
#include <fstream>
#include <sstream>
#include <iostream>
const int N=6;
std::vector<std::bitset<N> > read_graph(const std::string& filepath)
{
    std::vector<std::bitset<N> > bs(N);
    std::ifstream fp(filepath.c_str());
    for(std::string line; getline(fp, line); )
    {
        if(line.size() > 1 && line[0] == 'a')
        {
            std::istringstream str(line.substr(1));
            int fi, si;
            str >> fi >> si;
            std::cout << "fi: " << fi << "   si: " << si << '\n';
            bs[--fi][--si]= 1;
        }
    }
    return bs;
}
int main()
{
    std::vector<std::bitset<N> > v = read_graph("sample.gr");
    copy(v.rbegin(), v.rend(),
         std::ostream_iterator<std::bitset<N> >(std::cout, "\n"));
}

test run: https://ideone.com/z7Had

You are returning a pointer to a local array. Undefined Behavior.

Use a vector<bitset<N> > or similar instead.