14.13 Pointer Increment and Decrement

The ‘++’ operator adds 1 to a variable. We have seen it for integers (see Increment/Decrement), but it works for pointers too. For instance, suppose we have a series of positive integers, terminated by a zero, and we want to add them up. Here is a simple way to step forward through the array by advancing a pointer.

int
sum_array_till_0 (int *p)
{
  int sum = 0;

  for (;;)
    {
      /* Fetch the next integer.  */
      int next = *p++;
      /* Exit the loop if it’s 0.  */
      if (next == 0)
        break;
      /* Add it into running total.  */
      sum += next;
    }

  return sum;
}

The statement ‘break;’ will be explained further on (see break Statement). Used in this way, it immediately exits the surrounding for statement.

*p++ uses postincrement (++; see Postincrement/Postdecrement) on the pointer p. that expression parses as *(p++), because a postfix operator always takes precedence over a prefix operator. Therefore, it dereferences the entering value of p, then increments p afterwards.

Incrementing a variable means adding 1 to it, as in p = p + 1. Since p is a pointer, adding 1 to it advances it by the width of the datum it points to—in this case, sizeof (int). Therefore, each iteration of the loop picks up the next integer from the series and puts it into next.

This for-loop has no initialization expression since p and sum are already initialized, has no end-test since the ‘break;’ statement will exit it, and needs no expression to advance it since that’s done within the loop by incrementing p and sum. Thus, those three expressions after for are left empty.

Another way to write this function is by keeping the parameter value unchanged and using indexing to access the integers in the table.

int
sum_array_till_0_indexing (int *p)
{
  int i;
  int sum = 0;

  for (i = 0; ; i++)
    {
      /* Fetch the next integer.  */
      int next = p[i];
      /* Exit the loop if it’s 0.  */
      if (next == 0)
        break;
      /* Add it into running total.  */
      sum += next;
    }

  return sum;
}

In this program, instead of advancing p, we advance i and add it to p. (Recall that p[i] means *(p + i).) Either way, it uses the same address to get the next integer.

It makes no difference in this program whether we write i++ or ++i, because the value of that expression is not used. We use it for its effect, to increment i.

The ‘--’ operator also works on pointers; it can be used to step backwards through an array, like this:

int
after_last_nonzero (int *p, int len)
{
  /* Set up q to point just after the last array element.  */
  int *q = p + len;

  while (q != p)
    /* Step q back until it reaches a nonzero element.  */
    if (*--q != 0)
      /* Return the index of the element after that nonzero.  */
      return q - p + 1;

  return 0;
}

That function returns the length of the nonzero part of the array specified by its arguments; that is, the index of the first zero of the run of zeros at the end.