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Natural Language Processing with Python Phần 8

Có một quy ước được chấp nhận rộng rãi abbreviates đại diện của các tính năng e Boolean, thay vì aux = + hoặc aux =-, chúng tôi sử dụng + aux-aux. | may will and do with the Boolean feature AUX. Then the production V TENSE pres aux - can means that can receives the value pres for TENSE and or true for AUX. There is a widely adopted convention that abbreviates the representation of Boolean features f instead of aux or aux - we use aux and -aux respectively. These are just abbreviations however and the parser interprets them as though and - are like any other atomic value. 17 shows some representative productions 17 V TENSE pres aux - can V TENSE pres aux - may V TENSE pres -aux - walks V TENSE pres -aux - likes We have spoken of attaching feature annotations to syntactic categories. A more radical approach represents the whole category that is the non-terminal symbol plus the annotation as a bundle of features. For example N NUM sg contains part-of-speech information which can be represented as POS N. An alternative notation for this category therefore is POS N NUM sg . In addition to atomic-valued features features may take values that are themselves feature structures. For example we can group together agreement features e.g. person number and gender as a distinguished part of a category serving as the value of AGR. In this case we say that AGR has a complex value. 18 depicts the structure in a format known as an attribute value matrix AVM . 18 POS N AGR PER 3 NUM pl GND fem In passing we should point out that there are alternative approaches for displaying AVMs Figure 9-1 shows an example. Although feature structures rendered in the style of 18 are less visually pleasing we will stick with this format since it corresponds to the output we will be getting from NLTK. Figure 9-1. Rendering a feature structure as an attribute value matrix. 336 Chapter 9 Building Feature-Based Grammars On the topic of representation we also note that feature structures like dictionaries assign no particular significance to the order of features. So 18 is equivalent to 19 AGR NUM pl per 3 GND fem POS N Once we have the possibility