如何在 C++ 中使用 map 存储双向图？

Question

I am trying to store a bi-directional graph as an adjacency list using std::map<int,vector<int>> . The idea here is to store n nodes, from 1 to n in this map. The input is given as uv , which denotes an edge between node u and node v. We get n such inputs on n lines. My code for storing the graph:

    int u,v;
    map<int,vector<int>> graph();
    for(int i=0;i<n;i++) {
        cin >> u >> v;
        graph[u].push_back(v);
        graph[v].push_back(u);
    }

This should work, but it gave me errors when complied with C++ 14. I then compiled this with C++ 17 and the errors still persists. Errors(as displayed on my terminal):

/home/chirag/chiragC/forces/try.cpp: In function ‘void solve()’:
/home/chirag/chiragC/forces/try.cpp:68:10: warning: pointer to a function used in arithmetic [-Wpointer-arith]
   graph[u].push_back(v);
          ^
/home/chirag/chiragC/forces/try.cpp:68:12: error: request for member ‘push_back’ in ‘*(graph + ((sizetype)u))’, which is of non-class type ‘std::map<int, std::vector<int> >()’
   graph[u].push_back(v);
            ^~~~~~~~~
/home/chirag/chiragC/forces/try.cpp:69:10: warning: pointer to a function used in arithmetic [-Wpointer-arith]
   graph[v].push_back(u);
          ^
/home/chirag/chiragC/forces/try.cpp:69:12: error: request for member ‘push_back’ in ‘*(graph + ((sizetype)v))’, which is of non-class type ‘std::map<int, std::vector<int> >()’
   graph[v].push_back(u);
            ^~~~~~~~~
[Finished in 1.5s with exit code 1]

I feel the script I am using to store the graph is correct, as my friend was able to run this code completely fine on his system. I don't know why it is giving errors.

My System: debian 10

Headers used: #include<bits/stdc++.h>

Some info on my Compiler:

chirag@debian10:~/chiragC/forces$ ls /usr/bin | grep g++
arm-none-eabi-g++
avr-g++
g++
g++-8
x86_64-linux-gnu-g++
x86_64-linux-gnu-g++-8
chirag@debian10:~/chiragC/forces$ g++ --version
g++ (Debian 8.3.0-6) 8.3.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

chirag@debian10:~/chiragC/forces$ 

Answer 1

Your bug is that you are trying to make a function named graph() remove the parenthesis then all will be fine.