function ONLINE-DFS-AGENT(s') returns an action inputs: s', a percept that identifies the current state persistent: result, a table, indexed by state and action, initially empty untried, a table that lists, for each state, the actions not yet tried unbacktracked, a table that lists, for each state, the backtracks not yet tried s, a, the previous state and action, initially null if GOAL-TEST(s') then return stop if s' is a new state (not in untried) then untried[s'] <- ACTIONS(s') if s is not null then result[s, a] <- s' add s to the front of the unbacktracked[s'] if untried[s'] is empty then if unbacktracked[s'] is empty then return stop else a <- an action b such that result[s', b] = POP(unbacktracked[s']) else a <- POP(untried[s']) s <- s' return aFigure 4.21 An online search agent that uses depth-first exploration. The agent is applicable only in state spaces in which every action can be "undone" by some other action.
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