Code:

int find_array(int a[], int left, int right, int elem);
int main ()
{
int a[] = {
1, 4, 6, 8, 9, 10, 13, 16, 17, 19,
20, 21, 30, 32, 33, 35, 37, 39, 50, 70,
71, 73, 79, 81, 85, 87, 95, 96, 97, 99 };
int size = sizeof(a) / sizeof(int);
int num, index;
while (num != 0)
{
printf("Enter a positive number: ");
scanf("%d", &num);
if (num == 0)
{
;
}
else
{
index = find_array(a, 0, size-1, num);
if (index == -1)
{
printf("%d is not included\n", num);
}
else
{
printf("%d is at index %d\n", num, index);
}
}
}
return 0;
}
int find_array(int a[], int left, int right, int elem)
{
int middle;
if (left > right)
{
return -1;
}
else
{
middle = left + (right-left) / 2;
if (elem == a[middle])
{
return middle;
}
else
{
if (elem < a[middle])
{
find_array(a, 0, middle-1, elem);
}
else
{
find_array(a, middle+1, right, elem);
}
}
}
}

Please help, thanks.