Vectors array [Solved]

Question

In the following code the array having 4 elements prints the value of its 5th element and I don't know how?
May be it has something to do with Vectors, as I am new in it, I don't know much about it
Please help
https://code.sololearn.com/cBiF9vHUQm47/?ref=app

Answer 1

Oh, now I see.

First you allocate a vector with 5 elements in main.
printvec then copies the vector and you give that new vector a new range of numbers, 4 numbers this time.

That means that the vector already has a container that CAN contain 5 elements.
So why would it need to allocate a new array when it can fit those 4 new elements inside the old array?

It simply changes 88,97,95,95,86 into 97,95,95,88, [86].
That 86 is not removed from memory, doing so would waste cpu cycles for no reason.
It simply changes the internal state from 5 elements to 4.
If you then decide to access the 5th element, that's on you.

Answer 2

Just like regular arrays, a vector's index starts at 0, so the 4th element is located at n[3].
n[4] ( 5th element ) behaves the same way as it would on regular arrays with 4 elements, undefined behavior.

Most things in C++ is all about speed. Range checking and potentially throwing in every function is not the best idea.

n.at( 4 ) does do range checks and that does throw an exception into your face if you want to sacrifice speed for safety ( might be a good idea in debugging ).

Additionally, a vector is really expensive to copy, so you probably want to take it by (const) reference.

Answer 3

Dennis but what is the reason behind this
I didn't get it

Answer 4

Saurabh Tiwari
Reason about what?
C++ just doesn't hold your hand.

Answer 5

Dennis why n[4] exists and why n.at(4) throws an error
Is the compiler confused between the two vectors 😅

Answer 6

Dennis thanks
Compiler is smart 😎
What about n.at(4)
It throws an error

Answer 7

Yea, because, like I said, the internal state of the vector changed from 5 to 4 elements and the at function does a range check.
at() works something like this:

template<typename T, typename Allocator>
(const) T& vector<T, Allocator>::at( size_t index ) (const)
{
if( !(index < m_size) ) throw std::out_of_range( "vector::range_check: __index (which is " + std::to_string( index ) + ") >= this->size() (which is " + std::to_string( m_size ) + ")" );

return m_data[index];
}

https://en.cppreference.com/w/cpp/container/vector/at

Answer 8

Thanks Dennis
Gotta learn more to understand that code snippet