A C++ Program which works fine in online compilers and linux operating systems, but not on Windows

Question

While compiling a problem:
Geeks For Geeks: First Repeating Element on a Windows operating system I noticed that I was not getting any output for my solution. But when I compiled the same code on a Linux operating system and on online compilers, it worked absolutely fine without producing any errors.

Code:

#include <bits/stdc++.h>
using namespace std;

int main() {

    int n;
    cin>>n;
    int arr[n];

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

    int size= 1e6+1; // WA on windows
    int A[size];
    int min_index=INT_MAX;

    for(int i=0; i<size; i++) {
        A[i]=-1;
    }

    for(int i=0; i<n; i++) {
        if(A[arr[i]]!=-1)
            min_index=min(min_index, A[arr[i]]);
        else
            A[arr[i]]=i;
    }

    if(min_index==INT_MAX) 
        cout<<"-1";
    else
        cout<< min_index+1;

    return 0;
}

Sample Test Case:

7
1 5 3 4 3 5 6

Expected output:

Output on Windows: Screenshot

Output on Linux: Screenshot

Explanation for the program from line 14 of code:

I created an array A of size 1e6+1 to store value i on its arr[i]th index.
Array A was earlier initialized with value -1. It runs for n number of times and variable min_index stores the index of the least repeating number from the array arr.
After initializing smaller values of array int size = 10 and using very small test cases(also max value of arr[i] is lesser than size of A; I realize that the program runs perfectly in Windows.

As far as I understand, Windows might be having some trouble intializing arrays of such large length (Please correct me if I'm wrong). But why isn't it the same in the case of Linux?

To begin with: [Why should I not #include ?](https://stackoverflow.com/questions/31816095/why-should-i-not-include-bits-stdc-h) To continue: [Why aren't variable-length arrays part of the C++ standard?](https://stackoverflow.com/questions/1887097/why-arent-variable-length-arrays-part-of-the-c-standard) And while `using namespace std;` is [a bad habit](https://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice) it's often considered okay for small and simple examples. — Some programmer dude, Jan 20 '22 at 12:40
And to finish: Don't use so-called "competition" or "online judge" sites as any kind of teaching or learning resources. They are *not* that, and could be actively harmful for your proper learning. Get [some good books](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list/388282#388282) and take classes to learn C++ (and programming in general) properly. — Some programmer dude, Jan 20 '22 at 12:42
And I'm actually surprised that the program even run on Windows, considering that your VLA (Variable-Length Array) `A` is four million bytes large. The default stack-size (and local variables are usually stored on the stack) on Windows is only a single MiB. Also `1e+1` is a *floating point* value (of type `double`) and not more readable than `1000001`. — Some programmer dude, Jan 20 '22 at 12:47
Or perhaps the problem is that the VLA `A` really is too large and your program *crashes*? That means you need to learn something else not taught by such sites: ***Debugging***. — Some programmer dude, Jan 20 '22 at 12:50
`int A[size];` No. The following is proper C++ code: `std::vector A(size);`. Second, as already mentioned, use a C++ book to learn C++. If you had done that, you would not have made the mistake, since no good C++ book shows declaring arrays using a runtime variable. — PaulMcKenzie, Jan 20 '22 at 13:44
The problem could be: `int A[1e6+1]` that is a large array. Checkout this: [Is there a max array length limit in C++?](https://stackoverflow.com/a/216731/14065). A simple solution may be to change that to a `std::vector`. — Martin York, Jan 24 '22 at 21:45

score 3 · Accepted Answer · answered Jan 20 '22 at 12:47

#include <bits/stdc++.h> is not standard C++, hence you should not expect it to be portable.

int arr[n]; is not standard C++. Some compilers offer variable length arrays as extension, but it isnt portable. Same goes for int A[size];.

Sadly most of the C++ code presented on that site is not proper C++ code but some dialect.

For more details see: Why should I not #include <bits/stdc++.h>? and Why aren't variable-length arrays part of the C++ standard?. That Q&As should also explain the standard portable alternatives.

A C++ Program which works fine in online compilers and linux operating systems, but not on Windows

1 Answers1