Parsing with
Context Free Grammars
Outline
• Why should you care?
• Parsing
• Top-Down Parsing
• Bottom-Up Parsing
• Bottom-Up Space (an example)
• Top - Down Space (an example)
• Top - Down Space (an exercise)
• Conclusion (Top - Down Parse versus
Bottom - up Parse)
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Why should you care?
• Grammar checkers
• Question answering
• Information extraction
• Machine translation
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Outline
• Why should you care?
• Parsing
• Top-Down Parsing
• Bottom-Up Parsing
• Bottom-Up Space (an example)
• Top Down Space (an example)
• Top Down Space (an exercise)
• Conclusion (Top Down Parse versus
Bottom up Parse)
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Parsing
• Parsing with CFGs refers to the task of assigning
correct trees to input strings
• Correct here means a tree that covers all and only
the elements of the input and has an S at the top
• It doesn’t actually mean that the system can
select the correct tree from among the possible
trees
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Parsing
• As with everything of interest, parsing involves a
search that involves the making of choices.
Parsing a string with a CFG:
Finding a derivation of the string consistent with
the grammar.The derivation gives us a PARSE
TREE
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Outline
• Why should you care?
• Parsing
• Top-Down Parsing
• Bottom-Up Parsing
• Bottom-Up Space (an example)
• Top Down Space (an example)
• Top Down Space (an exercise)
• Conclusion (Top Down Parse versus
Bottom up Parse)
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Top-Down Parsing
• Since we are trying to find trees rooted with an S
(Sentences) start with the rules that give us an S.
• Then work your way down from there to the
words.
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Top Down Space
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Outline
• Why should you care?
• Parsing
• Top-Down Parsing
• Bottom-Up Parsing
• Top Down Space (an example)
• Top Down Space (an exercise)
• Conclusion (Top Down Parse versus
Bottom up Parse)
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Bottom-Up Parsing
• Of course, we also want trees that cover the input
words. So start with trees that link up with the
words in the right way.
• Then work your way up from there.
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Bottom-Up Space (an example)
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Outline
• Why should you care?
• Parsing
• Top-Down Parsing
• Bottom-Up Parsing
• Bottom-Up Space (an example)
• Top Down Space (an example)
• Top Down Space (an exercise)
• Conclusion (Top Down Parse versus
Bottom up Parse)
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Top Down Space
Example (Does this flight …..)
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Example … (cont’d)
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Example (cont’d)
flight
flight
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Example (cont’d)
flight
flight
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TOP-DOWN, DEPTH-FIRST,
LEFT-TO-RIGHT
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Outline
• Why should you care?
• Parsing
• Top-Down Parsing
• Bottom-Up Parsing
• Bottom-Up Space (an example)
• Top Down Space (an example)
• Top Down Space (an exercise)
• Conclusion (Top Down Parse versus
Bottom up Parse)
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Top Down Space (an exercise)
Using Top Down Space draw the parse tree for the
following sentence:
1- Book that flight.
2- John hit the ball.
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Top Down Space (result parse tree no.1)
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Top Down Space (result parse tree no.2)
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Avoiding Repeated Work
Parsing is hard, and slow. It’s wasteful to redo stuff over and over and
over.
Consider an attempt to top-down parse the following as an NP
A flight from Indianapolis to Houston on TWA
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Step by step parsing…1
flight
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Parsing step …2
flight
flight
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Step by step parsing…3
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The parse tree.
s
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Top Down Space (an exercise)
Using Top Down Space draw the parse tree for the
following sentence:
I shot an elephant in my pajamas.
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Top Down Space (result parse tree )
NP
NP
Pro
I
V Det
shot
PP
NP
Nom P
an elephant
in
poss
Nom
my
pajamas
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Top Down Space (result parse tree)
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Outline
• Why should you care?
• Parsing
• Top-Down Parsing
• Bottom-Up Parsing
• Bottom-Up Space (an example)
• Top Down Space (an example)
• Top Down Space (an exercise)
• Conclusion (Top Down Parse versus
Bottom up Parse)
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TOP-DOWN AND BOTTOM-UP
SEARCH STRATEGIES
The search has to be guided by the INPUT and the
GRAMMAR
TOP-DOWN search: the parse tree has to be
rooted in the start symbol S
EXPECTATION-DRIVEN parsing
BOTTOM-UP search: the parse tree must be an
analysis of the input
DATA-DRIVEN parsing
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TOP-DOWN vs
BOTTOM-UP
TOP-DOWN:
Only search among grammatical answers
BUT: suggests hypotheses that may not be
consistent with data
Problem: left-recursion
BOTTOM-UP:
Only forms hypotheses consistent with data
BUT: may suggest hypotheses that make no
sense globally
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