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Week 9• Questions / Concerns• Hand back Test#2• What’s due:

• Final Project due next Thursday June 5. • Final Project check-off on Friday June 6 in class.

• Next class meeting: Next Friday June 6. • Please use the other class times for your final project.

• Top-Down vs. Bottom-Up Parsers• LR(1) parsers

• Parser tools• Lex&Yacc (Bottom-Up)• Microsoft’s Spirit (Top-Down)• Gold Parser (Bottom-Up)

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Top Down vs. Bottom Up parsers

• Problems in top-town parsers• Left-recursion• Common prefixes requiring left factor.

• Bottom-up parsers can handle the largest class of grammars that can be parsed deterministically.

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Top – Down Parser

• General procedure:• Put Start symbol on the stack• Grab a token and grab a rule. • The rule goes on the stack. • Try to match the tokens with rules on the stack.

S -> aSb

S -> c

a a c b b

input

Stack

S

Stack

aSb

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Top-Down Parser

S -> aSb

S -> c

a a c b b

input

Stack

S

Stack

aSb

Stack

Sb

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Top-Down Parser

S -> aSb

S -> c

a a c b b

input

Stack

S

Stack

aSb

Stack

Sb

Stack

aSbb

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Top-Down Parser

S -> aSb

S -> c

a a c b b

input

Stack

S

Stack

aSb

Stack

Sb

Stack

aSbb

Stack

Sbb

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Top-Down Parser

S -> aSb

S -> c

a a c b b

input

S

aSb

Sb

aSbb

Sbb

cbb

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Top-Down Parser

S -> aSb

S -> c

a a c b b

input

S

aSb

Sb

aSbb

Sbb

cbb

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Top-Down Parser

S -> aSb

S -> c

a a c b b

input

S

aSb

Sb

aSbb

Sbb

bb

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Top-Down Parser

S -> aSb

S -> c

a a c b b

input

S

aSb

Sb

aSbb

Sbb b

Stack emptyInput empty

DONE!!

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Bottom Up Parser

• General Procedure• Table-driven parser.• LR(1) parser table

• R – stands for rightmost derivation. Working from the bottom up to the top of the parse tree.

• There are 2 basic operations:• Shift – moves input to the stack.• Reduce – remove items on the stack and reduces to one non-terminal.

• Also called shift-reduce parsers

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Bottom Up Parser table

• Unlike LL(1), LR(1) has 3 different types of tables:• SLR(1) – simple LR(1)• LALR(1) – look ahead LR• LR(1)

• All 3 parser tables are one token look ahead. • Unlike LL(1), left recursions in the grammar are okay.

Most grammar rules do not need to be left factored. • Unit productions are also okay. • Lambdas can be very tricky. Try to reduce duplicate

lambdas. Ex: S -> aSb A -> aA

S -> A A -> bA

S -> A ->

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Bottom-Up Parser

S -> aSb

S -> c a a c b b

input

Stack

Nothing on Stack to start the parsing process

StackS

If you end up with S on the stack and nothing else in input, then you are done!!

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Bottom-Up Parser

S -> aSb

S -> c a a c b b

a

Shift onto stack

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Bottom-Up Parser

S -> aSb

S -> c a a c b b

a

aa

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Bottom-Up Parser

S -> aSb

S -> c a a c b b

aaa

caa

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Bottom-Up Parser

S -> aSb

S -> c a a c b b

aaa

caa

Wait! We have a match!

Reduce c to S

Saa

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Bottom-Up Parser

S -> aSb

S -> c a a c b b

aaa

caa

Saa

bSaa

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Bottom-Up Parser

S -> aSb

S -> c a a c b b

aaa

caa

Saa

bSaa

Wait! Another match!

Reduce aSb to S

Sa

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Bottom-Up Parser

S -> aSb

S -> c a a c b b

aaa

caa

Saa

bSaa

bSa

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Bottom-Up Parser

S -> aSb

S -> c a a c b b

aaa

caa

Saa

bSaa

Another match!

SSa

bSa

Input is empty.Stack has only S on it. DONE!!

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SLR(1) table

0: E’ -> E

1: E -> E + T

2: E -> T

3: T -> T * F

4: T -> F

5: F -> ( E )

6: F -> id

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Parser Tools

Parser Generator Tools

Grammar Specification

(Regular Expression, Context-Free Grammar, BNF grammar)

Parser Code(source files)

Most parser generator tools build table-driven parsers. They just have to produce the tables.

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Structure of Compilers

Lexical Analyzer (scanner)

Modified Source Program

Syntax Analysis(Parser)

Tokens Semantic Analysis

Syntactic Structure

Optimizer

Code Generator

Intermediate Representation

Target machine code

Symbol Table

skeletal source

programpreprocessor