CPP array memory allocation

Question

Here I am declaring array length before taking input for n. At the time of array declaration n=0.

#include <iostream>
using namespace std;

int main() {
    int n;
    int sum=0;
    int arr[n]={};
    cin >> n;

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

for below input 6 1 2 3 4 5 6

I am getting this output 1234.

Can someone please explain the reason?

You invoked undefined behavior. Determining the reason for this specific result will require detailed information of your environment. — MikeCAT
– MikeCAT, Commented Aug 23, 2020 at 15:03
@MikeCAT trying to run code on hackerrank platform. hackerrank.com/challenges/simple-array-sum/problem — Katha patel
– Katha patel, Commented Aug 23, 2020 at 15:06
Even though some compilers allow int arr[n]={};, it's not valid in the standard C++ as the length must be known at compile-time. You should be using std::vector instead. — HolyBlackCat
– HolyBlackCat, Commented Aug 23, 2020 at 15:06
@Kathapatel You got lucky. n is uninitialized and could have any other value, and reading from it causes undefined behavior. — HolyBlackCat
– HolyBlackCat, Commented Aug 23, 2020 at 15:07

Lukas-T · Accepted Answer · 2020-08-23 15:11:38Z

2

Here I am declaring array length before taking input for n.

Yes, but that's no valid C++. It's a compiler specific extension. See Why aren't variable-length arrays part of the C++ standard?

At the time of array declaration n=0.

That's wrong. n has not been initialized, so it contains garbage data (which could be 0). Reading n invokes undefined behaviour.

Even if n would be 0, the loop would access the array out of bounds and undefined behaviour is invoked again.

Discussing the output of a program that invokes undefined behaviour is pointless - anything might happen. The only reasonbale thing is to avoid UB. Some good practices to do this:

Always initialize variables to a reasonable value
use std::vector instead of plain arrays/VLAs.

answered Aug 23, 2020 at 15:11

Lukas-T

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