As we will see, they arise from simple analysis of the distribution of words in text.The goal of this chapter is to answer the following questions: Along the way, we'll cover some fundamental techniques in NLP, including sequence labeling, n-gram models, backoff, and evaluation.Back in elementary school you learnt the difference between nouns, verbs, adjectives, and adverbs.
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Consider the following analysis involving By convention in NLTK, a tagged token is represented using a tuple consisting of the token and the tag.
We can create one of these special tuples from the standard string representation of a tagged token, using the function Other corpora use a variety of formats for storing part-of-speech tags.
A word frequency table allows us to look up a word and find its frequency in a text collection.
In all these cases, we are mapping from names to numbers, rather than the other way around as with a list.
Once we start doing part-of-speech tagging, we will be creating programs that assign a tag to a word, the tag which is most likely in a given context.
We can think of this process as : Dictionary Look-up: we access the entry of a dictionary using a key such as someone's name, a web domain, or an English word; other names for dictionary are map, hashmap, hash, and associative array. When we type a domain name in a web browser, the computer looks this up to get back an IP address.
Since words and tags are paired, we can treat the word as a condition and the tag as an event, and initialize a conditional frequency distribution with a list of condition-event pairs.
This lets us see a frequency-ordered list of tags given a word: We can reverse the order of the pairs, so that the tags are the conditions, and the words are the events. We will do this for the WSJ tagset rather than the universal tagset: Finally, let's look for words that are highly ambiguous as to their part of speech tag.
Most often, we are mapping from a "word" to some structured object.
For example, a document index maps from a word (which we can represent as a string), to a list of pages (represented as a list of integers).