Parsing With Context Free Grammars

7/23/2019 Parsing With Context Free Grammars

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Parsing

Parsing with CFGs refers to the task of assigning correct trees toinput strings.

Correct here means a tree that covers all and only the elements of theinputand has an S at the top.

It doesnt actually mean that the system can select the correct treefrom among the possible trees

s with everything of interest! parsing involves a search that involvesthe making of choices


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Some assumptions..

ssume"#ou have all the words already in some buffer $he input is%isnt pos taggedll the words are known


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Parsing

Top-Down Parsing

Since we are trying to find trees rooted with an S &Sentences' startwith the rules that give us an S.

$hen work your way down from there to the words.

( Bottom-Up Parsing

)f course! we also want trees that cover the input words. So start with trees

that link up with the words in the right way.

$hen work your way up from there.


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Top Down Space


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Bottom-Up Space


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Top-Down VS. Bottom-Up

$op(down )nly searches for trees that can be answers *ut suggests trees that are not consistent with the words Guarantees that tree starts with S as root +oes not guarantee that tree will match input words

*ottom(up

)nly forms trees consistent with the words

Suggest trees that make no sense globally Guarantees that tree matches input words +oes not guarantee that parse tree will lead to S as a root

Combine the advantages of the two by doing a search constrained

from both sides &top and bottom'


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Possible Problem: Left-Recursion

,hat happens in the following situationS (- P /P

S (- u0 P /PP (- P PPP (- +et ominal"

,ith the sentence starting with+id the flight"


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Solution: Rule Orering

S (- u0 P /PS (- P /PP (- +et ominalP (- P PP

$he key for the P is that you want the recursive option after any base

case.


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!"oiing Repeate #or$

Parsing is hard! and slow. Its wasteful to redo stuff over and over andover.

Consider an attempt to top(down parse the following as an P

flight from Indianapolis to 1ouston on $,


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flight


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flight

flight


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Parsing With Context Free Grammars

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