使用C ++向量的有向图的邻接表表示

Question

I'm a newcomer. I'm facing a problem with c++ vector and its iterator. I have tried to represent a adjacency list of a directed graph but failed. Here is my code: `

#include <bits/stdc++.h>

using namespace std;

int main()
{
    int Node,Edge,node1,node2,cost;
    vector<int>nodes[100],costs[100];
    vector<int>::iterator it;
    cout<<"Enter numbers of nodes: "<<endl;
    cin>>Node;
    cout<<"Enter numbers of edges: "<<endl;
    cin>>Edge;
    for(int i=0;i<Edge;i++){
        cout<<i+1<<"th edge's Node1: ";
        cin>>node1;
        cout<<i+1<<"th edge's Node2: ";
        cin>>node2;
        cout<<"Cost from "<<node1<<" to"<<node2<<": ";
        cin>>cost;
        cout<<endl;
        nodes[node1].push_back(node2);
        costs[node1].push_back(cost);
    }
    for(it=nodes.begin();it<nodes.end();it++){
        for(int i=0;i<nodes[it].size();i++){
            cout<<nodes[it][i]<<" ";
        }
        cout<<endl;
    }

    return 0;
}

`

Answer 1

You should have told us about the compilation error that you've encountered.

Trying to compile your code immediately shows an inconsistency in the loop:

for(it=nodes.begin();it<nodes.end();it++)

and on the following uses of it . In fact you use it in indexes, as if it were an int . But you've declared it as an iterator:

vector<int>::iterator it;  // you say it is an iterator that iterates through integers in ONE vector

Inexed access and iterators are two different concepts.

Solution 1: use indexes

Just use an index of type int (or better size_t ):

...
const size_t maxnodes=100; 
vector<int>nodes[maxnodes],costs[maxnodes];
// + remove the definition of it
...
for(size_t it=0;it<maxnodes;it++) {
...

Solution 2: use iterators, but correctly

Let the compiler define the right type for the iterator, and then dereference the iterator (ie handle it as if it were a pointer):

// remove the definition of the it at the begin of main
...  // rest of code unchanged except for the loop
for(auto it=begin(nodes);it!=end(nodes);it++){  // attention at the condition
    for(int i=0;i<it->size();i++){
        cout<<(*it)[i]<<" ";
    }
    cout<<endl;
}

Here a live demo

Solution 3: use the confortable range for

Forget about the iterators and let the compiler handle the job for you:

for(auto& n : nodes) {  // note the &, to get the original element and not a clone of it
    for(int i=0;i<n.size();i++){
        cout<<n[i]<<" ";
    }
    cout <<endl;
}

Here another live demo .

You will immediately realize that you could as well use the range for to handle the inner loop:

    for(int& target : n) {  // when reading the code outloud, say " in " instead of ":"
        cout<<target<<" ";
    }

Final remarks

You should always verify the user's data input, especially to ensure that it's in the valid range if you are using it as index.