getting no result in terminal window

I have just started coding in C++ and I am using codeblocks. My build log is giving me 0 errors and 0 warning but I do not know why when I run it, it is giving me no result in the terminal. Terminal Window Result:

Process returned -1073741571 (0xC00000FD) execution time : 1.252 s Press any key to continue.

my code:

    #include <iostream>
    #include<math.h>
    using namespace std;
    int main() {
    int n;
    cin>>n;

    int a[n];
    for(int i = 0; i <n ; i++){
        cin>>a[i];
    }
    const int N = pow(10, 6);
    int idx[N];
    for(int i = 0; i< N; i++){
        idx[i] = -1;
    }

    int minidx = INT_MAX;

    for(int i = 0; i<n; i++){
        if(idx[a[i]] != -1){
            minidx = min(minidx, idx[a[i]]);
        }
        else{
            idx[a[i]] = i;
        }
    }
    if (minidx == INT_MAX){
        cout<<"-1"<<endl;
    }
    else{
        cout<<minidx+1<<endl;
    }
    return 0;
}

Please help me in finding my mistake in the code.

This:

int n;
std::cin >> n;

int a [n];

for (int i = 0; i < n ; i++) {
    std::cin >> a [i];
}

is bad practice. Don't use VLAs whose size you don't know at compile time. Instead, if I guess correctly that this is some Competitive Programming problem, you'll probably know what the max size will be as stated in the problem. So, do it this way instead:

int n;
std::cin >> n;

constexpr int max_size = 1000000;
int a [max_size];

for (int i = 0; i < n; i++) {
    std::cin >> a [i];
}

However, even doing it this way will crash your program anyway. This is simply because of stack overflow when you declare an array that size inside a function. For slightly smaller sizes however, that would be okay. Just don't use VLAs the way you're using them.

One solution is to use a standard container like std::vector as the allocation takes place on the heap. Note that using std::array will crash too as the allocation is not on the heap.

Another solution is to make your array a global. This way you can increase to sizes well over 1e6. Not really recommended though.

In your code above, irrespective of what the size n for array a is (even if it's a fairly small size to fit on the stack), your code will definitely crash when you declare the array idx [1000000]. Reason is the same, stack overflow.

Also, please post indented code and use good indentation practices.