需要帮助来了解指针练习的输出

Question

Hi i'm studying a book and there is a question that displays this code:

What is the output of the following code?

int main()
{
int x;
int *p;
int *q;
p = new int[10];
q = p;
*p = 4;
for(int j = 0; j<10; j++)
{
    x = *p;
    p++;
    *p = x+j;
}
for(int k= 0; k<10; k++)
{
    cout << *q << " ";
    q++;
}
cout << endl;
return 0;
}

I know the output is:

4 4 5 7 10 14 19 25 32 40

but i cannot understand why, i know p = q and since the first p in the array of 10 equals 4 so does q but after that shouldnt it just increment by one each time since j is?

Answer 1

p++ increment the pointer so it points to the next element. This is known as pointer arithmetic. This is not affecting the value. For that, you need to dereference the pointer using * as in *p = x+j

*p = x+j; sets the value pointed to by p to x+j . x = value pointed to previous p (before increment) and j goes from 0 to 9. So it gives:

4+0(4), 4+1(5), 5+2(7), 7+3(10), ...

Since it initialize first element to 4 and start writing at second element, that is why you have two 4s at the beginning.

However, I think the loop should be

for(int j = 0; j<9; j++)
{
    x = *p;
    p++;
    *p = x+j;
}

else there will be out of bound access to p .