Abstract

We consider the problem of selecting, from among the arms of a stochastic n-armed bandit, a subset of size m of those arms with the highest expected rewards, based on efficiently sampling the arms. This “subset selection” problem finds application in a variety of areas. In the authors’ previous work (Kalyanakrishnan & Stone, 2010), this problem is framed under a PAC setting (denoted “Explore-m”), and corresponding sampling algorithms are analyzed. Whereas the formal analysis therein is restricted to the worst case sample complexity of algorithms, in this paper, we design and analyze an algorithm (“LUCB”) with improved expected sample complexity. Interestingly LUCB bears a close resemblance to the wellknown UCB algorithm for regret minimization. The expected sample complexity bound we show for LUCB is novel even for singlearm selection (Explore-1). We also give a lower bound on the worst case sample complexity of PAC algorithms for Explore-m.