how can i split an uninitialized array in half?

Question

so i have this code in C++ where i have an array of an undefined initial value, and the user is supposed to keep entering values for the array until exit code -1. once that is entered the array size is determined by the number of inputs. then it has to be split in half into two arrays the first having the first half of the original array and the second having the second half of the original array.

#include <iostream>
using namespace std;

int main()
{
    int n;
    n++;
    int a[n];
    for (int i = 0; i < n; i++)
    {
        cin >> a[i];
        n++;
        if (a[i] == -1)
        {
            n--;
            break;
        }
    }
    cout << n << endl;
    int e[n / 2];
    for (int j = 0; j < (n / 2); j++)
    {
        e[j] = a[j];
    }
    int q = 0;
    int o[n / 2];
    for (int l = (n / 2); l < n; l++)
    {
        o[q] = a[l];
        q++;
    }
    for (int h = 0; h < n / 2; h++)
    {
        cout << e[h] << "  ";
    }
    cout << '\n';

    for (int h = 0; h < n / 2; h++)
    {
        cout << o[h] << "  ";
    }
    return 0;
}

the problem i'm facing is that ,when the array if sized 4, it works fine, but when it's sized 6 , the 5th and 6th value become 3 0 . instead of the value i entered . and making a larger array has a similar problem certain values in the array are damaged.

Answer 1

int n;
n++;

You're incrementing a variable that has an indeterminate value. The behaviour of the program is undefined. How big is the array supposed to be?

int a[n];

The size of your array variable isn't compile time constant. The program is ill-formed. Either use a compile time constant size, or allocate the array dynamically. Simplest way to achieve the latter is to use std::vector.

then it has to be split in half into two arrays

I suspect that there may be a XY-problem here. Would it be sufficient to have iterators to the different halves of the original array? This way there is no need to create additional arrays, nor to copy values into those arrays.

Here is one such solution:

auto first  = std::begin(a);
auto middle = std::next(first, n/2);
auto last   = std::end(a);
std::span first_half  {first,  middle};
std::span second_half {middle, last  };

Answer 2

Your code splits the array just fine. It only has errors when taking the array elements from input and in printing the arrays.

As largest-prime-is-463035818 said, std::vector is a better data structure to use. However, if you are only allowed to use arrays, assume a maximum size for your array size since arrays need a size when declared.

#include <iostream>
#define MAX 1000
using namespace std;
int main()
{
    int n;
    n++;
    int a[MAX];
    for (int i = 0; i < MAX; i++)

Initialize n to 0. This is to avoid getting n with garbage values. Remove n++; as you shouldn't be incrementing it at the start.

int main()
{
    int n = 0;
    int a[MAX];
    for (int i = 0; i < MAX; i++)

To print the other array correctly, use n - n / 2 instead to compute for the correct size of the other array.

int o[n - n / 2];
for (int l = (n / 2); l < n; l++)
{
    o[q] = a[l];
    q++;
...
for (int h = 0; h < n - n / 2; h++)
{
    cout << o[h] << "  ";
}

Note: You could omit the braces in the for loops when the block has only one statement. Also, use more meaningful identifiers to make it more readable (What is q? What does the array o represent?)