(Conceptual) Iterating through an array of ints using a pointer

Question

I am a newbie trying to wrap his head around the concept of pointers. It has been going well so far except for when I am trying to play around with arrays using a for loop. Instead of indexing the array in a normal manner, I am using a pointer instead. This is where I have noticed some peculiarities that I cannot understand.

So, for example. say I wanted to print the first four values of numArray:

int numArray = [1, 5, 10, 15, 20]

Now with a pointer, my approach was this:

int *ptr = numArray;
for (ptr; *ptr < *(ptr + 4); ptr++)
{
    std::cout << *ptr << std::endl;
}

Which doesn't work, despite my thinking that *(ptr + 4) would be the same as numArray[4].

However, if I deal with the stopping condition slightly differently:

int *ptr = numArray;
int end = *(ptr + 4);
for (ptr; *ptr < end; ptr++)
{
    std::cout << *ptr << std::endl;
}

Then it suddenly gives me what I want, despite the code not being drastically different.

So why does this happen? I feel like the answer might be very obvious but I'm not grasping it. Why does putting *(ptr + 4) inside the for loop cause such a big difference? Why exactly do pointers behave this way?

Answer 1

for (ptr; *ptr < *(ptr + 4); ptr++)   
/* after ptr is incremented (ptr+4) is again calculated in iterations  */

In this loop after first iteration *(ptr+4) will go out of bounds(as it is calculated in every iteration) as you increment ptr and dereferencing it causes undefined behaviour. And same thing happens in next iterations .

Above doesn't happen in latter case as end remains constant and does not change with ptr as ptr is incremented and , therefore it works.