Is there multiple ways to cin vector elements?

Question

I have a program that adds multiple vectors in 1 vector. I saw a method of instead of .push_back to add elements and I used it, but I am wondering why this works because I thought vector[] indicated the index, so why is cin >> vector[] adding elements into the vector if square brackets indicate the index? input - output 1 2; 5 4; 9 1 - 1 2; 5 4; 9 1

'''
    #include <vector>
    #include <iostream>
    using namespace std;
    int main()
    {
    int q = 3; //total number of q2 in q1
    vector<vector<int>> q1; //vector for q2's
        for(int a = 0; a < q; a++)
            {
            vector<int> q2(2); //making each q2 vector the size of 2 elements
            for(int b = 0; b < 2; b++){
                cin >> q2[b]; //adding elements to q2**how does this work instead of push_back?
                }
            q1.push_back(q2); //adding last q2 into q1
            }
    //printing q1
        for(int a = 0; a < q1.size(); a++){
            for(int b = 0; b < q1[a].size(); b++){
                cout << q1[a][b] << " ";
                }
            cout << endl;
            }
    }
'''

Answer 1

vector<int> q2(2) makes a vector q2 that has 2 default-constructed int elements in it. You don't need to push_back() elements, because it makes 2 elements for you when it is constructed.

cin >> q2[b] merely edits the int elements that already exist in the vector.

Answer 2

#include<iostream>
#include<vector>
using namespace std;
double meannum(vector <double> &listnum);
int main()
{
    double dum {0.0};
    vector <double> listnum {};
    cout << "Enter the list of numbers to find the mean: press any non-integer key to stop"<<endl;
    while (cin >> dum) //enter any non-integer to end the loop!
    {
        listnum.push_back(dum);
    }
    cout << "List of numbers entered : "<< endl ;
    for(int i = 0;i < listnum.size();i++) {
         cout << listnum.at(i) << endl;
    }
    cout << " Finding the mean .....  " << endl;
    cout << "result: " << meannum(listnum);
}
double meannum(vector<double> &listnum)
{
    int len = listnum.size();
    double sum{ 0 };
    for (int i = 0; i < len; i++) {
        sum = sum + listnum.at(i);
    }
    double avg = sum / len;
    return avg;
}