How to get a vector of strings from input?

Question

I'm trying to get a vector of string from input to create a graph , but i don't know why in middle my code it crashes. please help me fix it. I use Visual Studio.

#include <iostream>
#include <vector>
#include <iterator>

void main(void)
{
    {
        using namespace std;
        int8_t n{ 0 };
        cout << "enter the size of graph : ";
        cin >> n;
        vector<string> graph(n);
        string connectionsWith;
        vector<string>::iterator i;
        string::iterator e;
        int p{ 0 };
        for (i = graph.begin(); i != graph.end(); ++i)
        {
            cout << '\n' << "enter the vertices that are connected to " << p << " : ";
            cin >> connectionsWith;
            graph.push_back(connectionsWith);
            p++;
        }
        p = 0;
        for (i = graph.begin(); i != graph.end(); ++i)
        {
            cout << p << " is connected to " << *i;
            p++;
        }
    }
}

Your iterators are invalidated when you do a `push_back`. I'm not sure what you are trying to do exactly, but you probably just want a simple `for ( size_t i = 0; i < graph.size(); ++i )` — ChrisMM, Dec 16 '19 at 13:01
`void main(void)` no. Use [`int main()`](https://stackoverflow.com/q/204476/) — L. F., Dec 16 '19 at 13:03
Also don't do [`using namespace std`](https://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice) — ChrisMM, Dec 16 '19 at 13:04
@ShahroozLeon didn't know Visual Studio was a debugger. Everyday I learn something new — RoQuOTriX, Dec 16 '19 at 13:08
I thought they must be used like this , I'm newbie to these iterators and I'm not sure about using them correctly. please if you can tell me where in this code I should use iterator and where not. @user253751 — Shahrooz Leon, Dec 16 '19 at 13:29

score 2 · Answer 1 · answered Dec 16 '19 at 13:10

2

In your constructor of graph, you allocate n string. Inside the loop you add on top of the constructed n strings, n more strings via push back. That potentially invalidates your iterators, as ChrisMM said, also not the most efficient way to implement such a scenario.

So as a solution, instead of

vector<string> graph(n);

do

vector<string> graph;
graph.reserve(n);

and iterate over the index, e.g. from 0 to n, and push back.

Especially in the first loop you are not dereferencing the iterator at all, which suggests that using index based loop would show your intent better.

answered Dec 16 '19 at 13:10

life_steal

55
7

could you please give me an example of iterate over the index , I'm newbie to iterators. thank you – Shahrooz Leon Dec 16 '19 at 13:24
1

@ShahroozLeon no iterator is involved. It's just a counter loop. `for (int i=0; i – WhozCraig Dec 16 '19 at 13:46

score 0 · Accepted Answer · answered Dec 16 '19 at 14:08

life_steal pointed out the problem. I would like to add few information and other way to solve the problem.

int8_t n{ 0 }; 

vector<string> graph(n); // Here initialization happening with ASCII. If input is "1" then it would be "49". Consider changing int8_t to int if you unaware of that. 

graph.push_back(connectionsWith); //Instead of this line Use: *i = connectionsWith; it directly assign the value.

How to get a vector of strings from input?

2 Answers2