/*
 * return the index of number x in array a or -1 if not found.
 * Array a must be sorted by student number.
 */
int search(int x, int nstudents, struct studentinfo *a)
{
    int low = 0;
    int high = nstudents;
    while (low < high) {
	/*
	 * invariant: if search item is in array, it is at an index i such
	 * that low <= i < high.
	 */
        int mid = (low + high) / 2;
        if (a[mid].number == x)
            return mid;
        else if (a[mid].number > x)
            high = mid;
        else
            low = mid + 1;
    }
    return -1;
}