Problem with understanding a vector initialization - c++

Question

This might be a dumb question but there is something I can't quite understand. When using a vector, whenever I want to 'push_back' an element to a certain position I can do that only if I initialize the vector in a certain way.
For example when I use this initialization:
std::vector<int> Myvec;
int size = 0;
int x = 0;
std::cin >> size;
for(int i = 0; i < size; i++)
{
std::cin >> x;
Myvec[i].push_back(x);
}
I receive the following error:
request for member 'push_back' in 'Myvec.std::vector<_Tp, _Alloc>::operator[] >(((std::vector::size_type)i))', which is of non-class type '__gnu_cxx::__alloc_traits >::value_type {aka int}'|
But when I use the following initialization it works:
int size = 0;
int x = 0;
std::cin >> size;
std::vector<int> Myvec[size];
for(int i = 0; i < size; i++)
{
std::cin >> x;
Myvec[i].push_back(x);
}
I don't have any problem using it and can implement it in all sorts of tasks, but it's bugging me because I'm not sure why it is actually working. Thank you for your help in advance.

Answer 1

In the first block you should use:
std::vector<int> Myvec;
int size = 0;
int x = 0;
std::cin >> size;
for(int i = 0; i < size; i++)
{
std::cin >> x;
Myvec.push_back(x);
}
Or you can use:
int size = 0;
int x = 0;
std::cin >> size;
std::vector<int> Myvec(size);
for(int i = 0; i < size; i++)
{
cin>>Myvec[i];
}
And print the vector using:
for(int i = 0; i < size; i++) {
std::cout<< Myvec[i]<<" ";
}
When you initialise it using vector Myvec[size], it becomes vector of vectors with size "size", means each Myvec[i] is a vector in which you can push elements.
Read more here: https://www.geeksforgeeks.org/2d-vector-in-cpp-with-user-defined-size/

Answer 2

You have a misunderstanding on how the std::vector::push_back function works. It basically appends a new object at the end of the vector. In particular, you don't need to index into the vector with std::vector::operator[]. Instead, change your snippet to
for(int i = 0; i < size; i++)
{
std::cin >> x;
Myvec.push_back(x);
}
The solution you came up with does something that you probably don't intend, i.e., using a raw array of vectors: when transferring user input into the objects, it creates size vectors with one element each.

Answer 3

You're not putting an element at position i in a vector.
You're putting an element at the back of the ith vector in an array of vectors! Worse, the array is empty, so the access is invalid.
If it weren't, though, you'd end up with i vectors each having one element. Not good!
Instead, you can make your vector be of a certain size:
MyVec.resize(SomeSize);
…then assign the elements as if the vector were an array (which it kind of is):
MyVec[i] = thing;
The key point here is that you don't have to push_back; you only do that when you want to push a new element to the back of the vector. You can access existing values with array-like [] syntax just fine.
Read more about vectors in your C++ book.