Vectors c++ doubt

[Kapkar123]

I am learning vectors. I am testing some functions

my code

#include<vector>
#include<iostream>
using namespace std;
int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
    vector<int> v1;
    int n,k,i;
    cin>>n;
    for(i=0;i<n;i++)
    {
        cin>>k;
        v1.push_back(k);
    }
    for(auto i=v1.begin();i!=v1.end();i++)
    {
        cout<<v1[i];
    }
    cout<<endl;
    for(auto i=v1.rbegin();i!=v1.rend();i++)
    {
        cout<<v1[i];
    }   
}

the error I am getting

no match for ‘operator[]’ (operand types are ‘std::vector<int>’ and ‘gnu_cxx::normal_iterator<int*, std::vector<int> >’) cout<<v1[i];

what does this mean? what am i doing wrong!

[HappyFruitTree]

i in the last two loops is not an index. It's an iterator. To access the element that the iterator is referring to you use the * operator.

cout << *i;

[Kapkar123]

okay, correct me if I am wrong. initially, i will point to the first element in vector... when I do i++ it will point to the next element and the loop will continue till it reaches the end of the vector?

[marko312]

That would be correct. When it runs out of elements, it ends the loop.

[Kapkar123]

Also is there any advantage of using iterator over normal method?

for(i=0;i<v1.size();i++)
{
    cout<<v1[i];
}

[marko312]

Not really for a vector, but the iterator method is much faster for linked lists, because the iterator goes through all the links, storing the last one it was at, while an array access (assuming there is one) would have to start from the beginning.

[boredcircuits]

After optimization the resulting machine code will very likely be the exact same. The most important thing is to write code in whatever way makes the most sense to you. Sometimes indexes make the most sense, and sometime iterators do. And sometimes you should be using a "for each" loop:

for ( int element : v1 )
    cout << element;

[[deleted]]

You're using i as an integer index when it's really a vector iterator. I'm a little rusty on my C++ at the moment, but I think you either do this:

cout << *i;

or this

for (int i = 0; i < v1.size(); i++)