b.c

#include <stdio.h>
#include <assert.h>

#define LEN 8
int a[LEN] = { 1, 2, 2, 2, 5, 6, 8, 9 };

int is_sorted(void)
{
	int i;
	for (i = 1; i < LEN; i++)
		if (a[i-1] > a[i])
			return 0;
	return 1;
}

int mustbe(int start, int end, int number)
{
	int i;
	for (i = 0; i < start; i++)
		if (a[i] == number)
			return 0;
	for (i = end+1; i < LEN; i++)
		if (a[i] == number)
			return 0;
	return 1;
}

int contains(int n)
{
	int i;
	for (i = 0; i < LEN; i++)
		if (a[i] == n)
			return 1;
	return 0;
}

int binarysearch(int number)
{
	int mid, start = 0, end = LEN - 1;

	assert(is_sorted()); /* Precondition */
	while (start <= end) {
		assert(mustbe(start, end, number)); /* Maintenance */
		mid = (start + end) / 2;
		if (a[mid] < number)
			start = mid + 1;
		else if (a[mid] > number)
			end = mid - 1;
		else {
			assert(mid >= start && mid <= end
			       && a[mid] == number); /* Postcondition 1 */
			return mid;
		}
	}
	assert(!contains(number)); /* Postcondition 2 */
	return -1;
}

int main(void)
{
	printf("%d\n", binarysearch(5));
	return 0;
}