Question

Below in c++ program,

include<iostream>
#include<vector>
using namespace std;

int main()
{
     vector<int> numbers;

    numbers.push_back(2);
    numbers.push_back(10);
    numbers.push_back(5);
    numbers.push_back(3);
    numbers.push_back(7);

    numbers[3] = 8;
    numbers[5] = 11;

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

see it on ideone.

here, numbers[3] is working but numbers[5].
It looks like, vector::operator[] doesn’t increase the size of vector like vector::push_back.
so, is this the only difference between these two or something else is there?

Answer 1

push_back creates a new element on the back with the value specified. operator[] requires the element to be there; it just accesses it. The reason [5] doesn’t work is because you have 5 elements, so your indices range from 0 to 4.

Generally, when adding new elements, push_back is preferred over resize, followed by operator[]. Only one can be used for reading, though, and operator[] is also needed to maintain normal array syntax.