Pointers do not have to point to single variables. It can also point at the cells of an array. For example:

int *ip;
int a[10];
ip = &a[3];

and we would end up with ip pointing at the fourth cell of the array a (remember, arrays are 0-based, so a[0] is the first cell). We could illustrate the situation like this:

We'd use this ip just like the one in the previous section: *ip gives us what ip points to, which in this case will be the value in a[3].

Once we have a pointer pointing into an array, we can start doing pointer arithmetic. Given that ip is a pointer to a[3], we can add 1 to ip:

ip + 1

What does it mean to add one to a pointer? In C, it gives a pointer to the cell one farther on, which in this case is a[4]. To make this clear, let's assign this new pointer to another pointer variable:

ip2 = ip + 1;

Now the picture looks like this:

If we now do

*ip2 = 4;

we've set a[4] to 4. But it's not necessary to assign a new pointer value to a pointer variable in order to use it; we could also compute a new pointer value and use it immediately:

*(ip + 1) = 5;

In this last example, we've changed a[4] again, setting it to 5. The parentheses are needed because the unary ``contents of'' operator * has higher precedence (i.e., binds more tightly than) the addition operator. If we wrote *ip + 1, without the parentheses, we'd be fetching the value pointed to by ip, and adding 1 to that value. The expression *(ip + 1), on the other hand, accesses the value one past the one pointed to by ip.

Given that we can add 1 to a pointer, it's not surprising that we can add and subtract other numbers as well. If ip still points to a[3], then

*(ip + 3) = 7;

sets a[6] to 7, and

*(ip - 2) = 4;

sets a[1] to 4.

Up above, we added 1 to ip and assigned the new pointer to ip2, but there's no reason we can't add one to a pointer, and change the same pointer:

ip = ip + 1;

Now ip points one past where it used to (to a[4], if we hadn't changed it in the meantime). The shortcuts we learned in a previous chapter all work for pointers, too: we could also increment a pointer using

ip += 1;

or

ip++;

Of course, pointers are not limited to ints. It's quite common to use pointers to other types, especially char. Here is the innards of the mystrcmp function we saw in a previous chapter, rewritten to use pointers. (mystrcmp, you may recall, compares two strings, character by character.)

char *p1 = &str1[0], *p2 = &str2[0];

while(1)
{
if(*p1 != *p2)
return *p1 - *p2;
if(*p1 == '\0' || *p2 == '\0')
return 0;
p1++;
p2++;
}