jurafsky&martin_3rdEd_17 (1).pdf

Note that unlike the dynamic programming and search

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Note that unlike the dynamic programming and search-based approaches dis- cussed in Chapters 12 and 13, this approach is a straightforward greedy algorithm — the oracle provides a single choice at each step and the parser proceeds with that choice, no other options are explored, no backtracking is employed, and a single parse is returned in the end. Figure 14.7 illustrates the operation of the parser with the sequence of transitions leading to a parse for the following example. (14.5) Book me the morning flight dobj iobj det nmod root Let’s consider the state of the configuration at Step 2, after the word me has been pushed onto the stack. Stack Word List Relations [root, book, me] [the, morning, flight] The correct operator to apply here is RIGHT A RC which assigns book as the head of me and pops me from the stack resulting in the following configuration. Stack Word List Relations [root, book] [the, morning, flight] (book ! me)
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254 C HAPTER 14 D EPENDENCY P ARSING Step Stack Word List Action Relation Added 0 [root] [book, me, the, morning, flight] SHIFT 1 [root, book] [me, the, morning, flight] SHIFT 2 [root, book, me] [the, morning, flight] RIGHT A RC (book ! me) 3 [root, book] [the, morning, flight] SHIFT 4 [root, book, the] [morning, flight] SHIFT 5 [root, book, the, morning] [flight] SHIFT 6 [root, book, the, morning, flight] [] LEFT A RC (morning flight) 7 [root, book, the, flight] [] LEFT A RC (the flight) 8 [root, book, flight] [] RIGHT A RC (book ! flight) 9 [root, book] [] RIGHT A RC (root ! book) 10 [root] [] Done Figure 14.7 Trace of a transition-based parse. After several subsequent applications of the SHIFT and LEFT A RC operators, the con- figuration in Step 6 looks like the following: Stack Word List Relations [root, book, the, morning, flight] [] (book ! me) Here, all the remaining words have been passed onto the stack and all that is left to do is to apply the appropriate reduce operators. In the current configuration, we employ the LEFT A RC operator resulting in the following state. Stack Word List Relations [root, book, the, flight] [] (book ! me) (morning flight) At this point, the parse for this sentence consists of the following structure. (14.6) Book me the morning flight dobj nmod There are several important things to note when examining sequences such as the one in Figure 14.7 . First, the sequence given is not the only one that might lead to a reasonable parse. In general, there may be more than one path that leads to the same result, and due to ambiguity, there may be other transition sequences that lead to different equally valid parses. Second, we are assuming that the oracle always provides the correct operator at each point in the parse — an assumption that is unlikely to be true in practice. As a result, given the greedy nature of this algorithm, incorrect choices will lead to incorrect parses since the parser has no opportunity to go back and pursue alternative choices. Section 14.4.2 will introduce several techniques that allow transition-based approaches to explore the search space more fully.
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