Read adjacency list of digraph from a text file in C++

问题

I have a text file which has some number of integers in each line. Example:

Here 1st line means that the 1st node is connected to nodes numbered 1, 2 and 5. Blank line means 4th node isn't connected to any node.

This gives me a directed graph. This SO question might have been helpful but it assumes each line to have 2 integers, whereas in this case it can have any number of integers from 0 to N-1(N is no. of nodes).

I just want to know how to read this text file. If I had two integers per line, I could have done infile >> a >> b. How to know that "\n" or end of line has occured. I am not asking for code that makes directed graph.

回答1:

The accepted answer from the linked question already shows you how to read line by line, which you need to do for your code and the first part shows a loop how to read all numbers from your line (istringstream). Because you don't have a fixed number of entries, you might want to store them in a std::vector.

回答2:

I do not think that this is homework, since the acutal topic are digraphs.

Therefore some code. But, you have to do the error handling yourself.

#include <string>
#include <iostream>
#include <fstream>
#include <sstream>
#include <vector>

typedef std::vector<int> nl_t;
typedef std::vector<nl_t> nll_t;

int main()
{
    std::ifstream is("test.dat");
    std::string str;

    nll_t nll;

    while(std::getline(is,str)) {
        std::istringstream ss(str);
        nl_t nl;
        int i;

        while(ss >> i) {
            nl.push_back(i);
        }
        nll.push_back(nl);
    }
}
/**
     Local Variables:
     compile-command: "g++ -g test.cc -o test.exe; ./test.exe"
     End:
*/

回答3:

Follow this Link http://www.c-plusplus.de/forum/p1940874#1940874. It is enough to read the code. It is not necessary to understand the german text.

In Your case You have to change in Line 18 of the main-programm from while( datei >> ws ) to while( datei ) and delete the lines 23 and 24. Because in Your case an empty line is an information.

Or Your main can look like this:

ifstream file("input.txt");
if( !file.is_open() )
{
    cerr << ">Error by opening the file\n";
    return -2;
}
for( int node1 = 1; file.good(); ++node1 )
{
    for( int node2; !is_endl(file) && file >> node2; )
        cout << "Node " << node1 << " is connected with " << node2 << endl;
}

来源：https://stackoverflow.com/questions/22562221/read-adjacency-list-of-digraph-from-a-text-file-in-c

标签

c++

file-io

graph

directed-graph

adjacency-list