Question

I’m trying to append two arrays together. The first array {0, 1, 2} when appended to the second array {3, 4, 5} should yield {0, 1, 2, 3, 4, 5}. Let me show you what I have before I move on:

#include <iostream>

int main() {

    int i = 0;

    int arr1[] = {3, 4, 5}, arr2[] = {0, 1, 2};

    while (i < 3) {
        arr2[3 + i] = arr1[i];
        i++;
    }

    std::cout << std::endl;

    for (int i = 0; i < 6; i++) std::cout << arr2[i] << std::endl; // print

}

I think the way I implemented it is correct. But I find that when I print out the contents of the new array (arr2) this is what I get:

0
1
2
-1219315227
-1218166796
134514640

0, 1, 2 is the original array, but then 3, 4, 5 has some how been turned into these weird numbers. However, this is somehow fixed with I add an arbitrary std::cout statement here in the while loop:

...
while (i < 3) {
    std::cout << 5 << '\n'; // just a random #

    arr2[3 + i] = arr1[i];
    i++;
}
...

And I print the array once again and it works!:

5

0
1
2
3
4
5

My question is why is this working when I use a std::cout statement inside the while loop as opposed to not doing that, in which case it gives me those numbers?

EDIT:

So it turns out what I have here is undefined behavior. With that, my question still stands: why does my code work with an std::cout call inside the while loop?

Answer 1

You are going beyond the bounds of arr2 here:

while (i < 3) {
    arr2[3 + i] = arr1[i];
    i++;
}

arr2 has size 3. What you are doing is undefined behaviour. All observed behaviours are consistent with undefined behaviour.