Pointer

Question

Can anyone tell me what does p[-1] mean?

int arr[]={-20,-10,0,10,20};

int *p, i;
p=arr;

for(p=arr+2,i=0;i<3;++i)
{
printf("%d %d\n",p[-i],p[i]);
}

Answer 1

Remember that an array is actually a pointer to a continuous block of memory. A call like p[ i ] is actually translated into pointer arithmetic *( p + i ). The same goes for a negative index. A call p[ -i ] equals *( p - i ), meaning you access the memory location i steps before where p points to.
In your example, p is set to point to the middle of the array, so p[ -1 ] retrieves the location before the third element, which is of course the second element, p[ -2 ] is the first element, and so on.