Local Variable being changed without manipulation - c++

Question

I'm creating a simple quick sort program that initializes an empty array and asks the users for inputs to determine how many elements are going to be sorted and what elements are going to be sorted.
The problem I am encountering is a local variable is being changed despite being only being referenced to once assigned. Attached below is the code.
int main()
{
int amount;
int numbersarray[] = {};
std::cout << "How many numbers do you want to sort? " << std::endl;
std::cin >> amount;
for(int i = 0; i <= amount; i++){
std::cout << "Enter number to be sorted: " << std::endl;
std::cin >> numbersarray[i];
}
std::cout <<"Amount to be sorted: " << amount << std::endl;
for(int i = 0; i <= amount; i++){
std::cout << numbersarray[i] << std::endl;
}
}
What I expect to be occurring, when I input the amount as 5, I should be able to input 5 elements into the array, instead however the Amount printed is 2 and the maximum elements I can put into the array is 3.
Below is the execution output.
How many numbers do you want to sort?
5
Enter number to be sorted:
5
Enter number to be sorted:
2
Enter number to be sorted:
5
Amount to be sorted: 2
5
2
5
I've tried messing around with the for statement but I don't think I'm doing it right as it hasn't fixed the problem, The manipulation of the for statement I'm doing is changing the condition (i.e !=, <, <=)

Answer 1

You have undefined behavior. Anything can happen. Local variable can change without reason, the program can crash and your computer can format itself with linux 6.9
There are many problem. The first is that your program is invalid according to the standard:
int numbersarray[] = {};
This is not valid. Array need a size:
constexpr int max_amount = 32;
int numbersarray[max_amount] = {};
If you need it to be dynamic, use vector:
std::vector<int> numbersarray;
numbersarray.resize(amount);
Second, you have another source of undefined behavior:
// Iterates too much, numbersarray[amount] is past the end
// ~~~v~~~~~~~
for(int i = 0; i <= amount; i++){
std::cout << "Enter number to be sorted: " << std::endl;
std::cin >> numbersarray[i];
}
It should be:
for(int i = 0; i < amount; i++){
std::cout << "Enter number to be sorted: " << std::endl;
std::cin >> numbersarray[i];
}
To avoid invalid code and undefined behavior, you should enable warnings.

Answer 2

numbersarray is a C-style array with a size of zero and doesn't adjust its size dynamically (most compilers might even fail to compile int numbersarray[] = {}; since an empty initializer isn't allowed for an array with unspecified size).
Writing or reading its elements causes undefined behaviour (which can be access violations, changing unrelated variables, and much more). This is the reason why you might see the values of local variables changing. Someone else that uses the same code could get completely different behaviour, since it is undefined.