c++ - Not trying to get random numbers for the values in my array

Question

I am new to programming in C++. I just would like to know why this code works, giving me actual numbers:

#include <iostream>
using namespace std;

int main(){ 

    const int ARRAYSIZE = 10;
    int inc = 0;
    int arrayy[ARRAYSIZE];

    while (inc < ARRAYSIZE) {

        arrayy[inc]=inc;
        std::cout << arrayy[inc]<< endl;
        inc++;  
    }
}

But this code gives me random numbers for the values in my array:

#include <iostream>
using namespace std;

int main(){ 

    const int ARRAYSIZE = 10;
    int inc = 0;
    int arrayy[ARRAYSIZE];

    while (inc < ARRAYSIZE) {

        arrayy[inc]=inc;
        inc++;
        std::cout << arrayy[inc]<< endl;
        
    }
}

question from:https://stackoverflow.com/questions/65914354/not-trying-to-get-random-numbers-for-the-values-in-my-array

Answer 1

In the second one, you're setting a value to 0, then incrementing inc, so the value you print out is the one just after what you just set to 0. Since it hasn't been initialized (yet), it produces arbitrary ("random") values.

It also prints out the value one past the end of the array, giving undefined behavior when it does so.