  Motivated by online decision-making in time-varying combinatorial
environments, we study the problem of transforming offline algorithms to their
online counterparts. We focus on offline combinatorial problems that are
amenable to a constant factor approximation using a greedy algorithm that is
robust to local errors. For such problems, we provide a general framework that
efficiently transforms offline robust greedy algorithms to online ones using
Blackwell approachability. We show that the resulting online algorithms have
$O(\sqrt{T})$ (approximate) regret under the full information setting. We
further introduce a bandit extension of Blackwell approachability that we call
Bandit Blackwell approachability. We leverage this notion to transform greedy
robust offline algorithms into a $O(T^{2/3})$ (approximate) regret in the
bandit setting. Demonstrating the flexibility of our framework, we apply our
offline-to-online transformation to several problems at the intersection of
revenue management, market design, and online optimization, including product
ranking optimization in online platforms, reserve price optimization in
auctions, and submodular maximization. We show that our transformation, when
applied to these applications, leads to new regret bounds or improves the
current known bounds.
