Binary Search

Binary Search is an algorithm that counts the number of true values in an array of $n$ booleans $f_0,f_1,\dots ,f_{n-1}$ satisfying $\forall i\in \{1,2,\dots ,n-1\},f_i\Rightarrow f_{i-1}$ in $\mathcal{O}(\log n)$ time and $\mathcal{O}(1)$ space.

Algorithm

Consider solving the problem after the number of true values $k$ has been determined to satisfy $l<k\le r$.

Let $m=\lfloor \frac{l+r}{2}\rfloor$.

• If $f_m$ is true, $f_0,f_1,\dots ,f_m$ must all be true, which implies $m<k\le r$. Solve for the problem with $l^{\prime }=m$ and $r^{\prime }=r$ recursively.
• Otherwise, $f_m,f_{m+1},\dots ,f_{n-1}$ must all be false, which implies $l<k\le m$. Solve for the problem with $l^{\prime }=l$ and $r^{\prime }=m$ recursively.

This algorithm solves the problem in $\mathcal{O}(\log n)$ time and $\mathcal{O}(1)$ space.

int binary_search(int n, const std::vector<bool> &f) {
	if (!f[0]) {
		return 0;
	}
 
	int l = 0, r = n;
	while (l + 1 < r) {
		int mid = std::midpoint(l, r);
		if (f[mid]) {
			l = mid;
		} else {
			r = mid;
		}
	}
	return r;
}