jurafsky&martin_3rdEd_17 (1).pdf

# Or postpone doing anything with the current word

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Or postpone doing anything with the current word, adding it to a store for later processing. To make these actions more precise, we’ll create three transition operators that will operate on the top two elements of the stack: LEFT A RC : Assert a head-dependent relation between the word at the top of stack and the word directly beneath it; remove the lower word from the stack. RIGHT A RC : Assert a head-dependent relation between the second word on the stack and the word at the top; remove the word at the top of the stack; SHIFT : Remove the word from the front of the input buffer and push it onto the stack. This particular set of operators implements the what is known as the arc stan- dard approach to transition-based parsing ( Covington 2001 , Nivre 2003 ). There are arc standard two notable characteristics to this approach: the transition operators only assert re- lations between elements at the top of the stack, and once an element has been assigned its head it is removed from the stack and is not available for further pro- cessing. As we’ll see, there are alternative transition systems which demonstrate different parsing behaviors, but the arc standard approach is quite effective and is simple to implement.

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14.4 T RANSITION -B ASED D EPENDENCY P ARSING 253 To assure that these operators are used properly we’ll need to add some pre- conditions to their use. First, since, by definition, the ROOT node cannot have any incoming arcs, we’ll add the restriction that the LEFT A RC operator cannot be ap- plied when ROOT is the second element of the stack. Second, both reduce operators require two elements to be on the stack to be applied. Given these transition opera- tors and preconditions, the specification of a transition-based parser is quite simple. Fig. 14.6 gives the basic algorithm. function DEPENDENCY P ARSE ( words ) returns dependency tree state { [root], [ words ], [] } ; initial configuration while state not final t O RACLE ( state ) ; choose a transition operator to apply state A PPLY ( t , state ) ; apply it, creating a new state return state Figure 14.6 A generic transition-based dependency parser At each step, the parser consults an oracle (we’ll come back to this shortly) that provides the correct transition operator to use given the current configuration. It then applies that operator to the current configuration, producing a new configuration. The process ends when all the words in the sentence have been consumed and the ROOT node is the only element remaining on the stack. The efficiency of transition-based parsers should be apparent from the algorithm. The complexity is linear in the length of the sentence since it is based on a single left to right pass through the words in the sentence. More specifically, each word must first be shifted onto the stack and then later reduced.
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