/* binsearch: найти х в v[0] <= v[1] <= … <= v[n-1] */
int main(int x, int v[], int n)
{
    int low, high, mid;
    low = 0;
    high = n - 1 ;
    while (low <= high && x != v[mid]) {
        mid = (low + high) / 2;
        if (x < v[mid])
            high = mid - 1;
        else
            low = mid + 1;
        mid = (low + high) / 2;  \
    }
    if ( x == v[mid])
        return mid;
    else
        return -1; /* совпадения нет */
}