jurafsky&martin_3rdEd_17 (1).pdf

# Note that unlike the dynamic programming and search

• 499
• 100% (1) 1 out of 1 people found this document helpful

This preview shows pages 253–255. Sign up to view the full content.

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)

This preview has intentionally blurred sections. Sign up to view the full version.

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.
This is the end of the preview. Sign up to access the rest of the document.
• Fall '09

{[ snackBarMessage ]}

### What students are saying

• As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

Kiran Temple University Fox School of Business ‘17, Course Hero Intern

• I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

Dana University of Pennsylvania ‘17, Course Hero Intern

• The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

Jill Tulane University ‘16, Course Hero Intern