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CSC 302: COMPILER CONSTRUCTION 1

LECTURE 5

DEPARTMENT OF COMPUTER SCIENCE

ON MONDAY @ 9AM; TUESDAY @ 10AM

INSTRUCTOR: OBIENU, A. C. (obienuac@gmail.com)

CSC 302

SECOND SEMESTER

© 2018 Obienu, A. C. All rights reserved.Students enrolled in Csc 302 at MCIU and other educational institutions have explicit permission to make copies of these materials for theirpersonal use, provided this copyright notice is preserved.

Parser Generator

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PARSER

PARSERS

Top Down Parser (TDP) Bottom Up Parser (BUP)

TDP with full TDP without

Backtracking Backtracking

Bruteforce Recursive Non-Recursive

Method descent descent (LL(1))

Operator precedence LL Parsers

Parser

LR(0) SLR(0) LALR(1) CLR(1)

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TDP VERSUS BUP

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Top-down parsers

❑ start at the root of derivation tree and fill in

❑ picks a production and tries to match the input

❑ may require backtracking

❑ some grammars are backtrack-free (predictive)

Bottom-up parsers

❑ start at the leaves and fill in

❑ start in a state valid for legal first tokens

❑ as input is consumed, change state to encode possibilities (recognize

valid prefixes)

❑ use a stack to store both state and sentential forms

Top Down Parser

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S → aABe w = abbcde

A → Abc | b

B → D

S

Top Down a A B e

A b c d

b

NB: TDP uses leftmost derivation

Issue with TDP

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S → aABe w = abbcde

A → Abc | b

B → D

S

Top Down a A B e

A b c d

b

(Issue : What to use)

Bottom Up Parser

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S → aABe w = abbcde

A → Abc | b

B → D

S

Bottom Up B

A

A

a b b c d e

NB: BUP follow rightmost derivation

Issues with BUP

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S → aABe w = abbcde

A → Abc | b

B → D

S

Bottom Up B

A

A

a b b c d e

(Issue : When to reduced)

TDP REVISITED

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Top Down Parsing (TDP)

TDP

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Once grammar is on its optimal form:

I. Goal is to construct the parse tree for some input (token) string

a) starting with the root,

b) recursively descending through the tree,

c) while building the parse tree on preorder form (depth-first).

2. Find the left-most derivation to guide the preorder construction.

3. At each step of the parse tree construction:

a) determine which (unique) production to be applied,

b) match terminal symbols in the production body with the input (token)

string symbols.

Example

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First, bring grammar on optimal form

E → E + T | T

T → T * F | F

F → (E) |id

Can be rewritten (left-recursion elimination):

E → T E’

E’ → + T E’ / 𝜀T → F T ’

T → *F T ‘ / 𝜀F → ( E ) / id

EXAMPLE

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Then, Top down parsing

Example: Top Down Parser

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Example: Top Down Parser

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Classes of Top Down Parser

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Recursive-Descent Parser: general form of top-down parsers.

❑ May require backtracking.

Predictive Parser special case of recursive-descent parsers.

❑ No backtracking necessary.

❑ Constructed from LL(k) grammars, k >= 1.

An LL(k) grammar implies scanning input from Left, retrieving

Leftmost derivation, using k lookahead symbols to reach a

resolution.

LL(1) PARSER

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A predictive parser looks like:

TABLE-DRIVEN PARSERS

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A parser generator system often looks like:

This is true for both top-down (LL) and bottom-up (LR) parsers

Definition of Terms

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LL(k) implies scanning input from Left, retrieving

Leftmost derivation, using k lookahead symbols to reach a resolution.

Stack: A data structure used for the procedure of parsing

Input Buffer holds the input string

NB: Both Stack and Input buffer has a “$”, which is used as

a stopping criterium.

Choosing the right production

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Key parsing problem: determining which production to apply

whilst constructing the parse tree.

FIRST and FOLLOW sets of a grammar helps choose a production

based on the next input symbol.

Definition

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NEXT CLASS

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For the next week, we will look at:

THE FIRST AND FOLLOW SETS

THE END

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QUESTIONS ?

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