getting no result in terminal window - c++

Question

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.

Answer 1

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.