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Pushdown Stack Automata. Zeph Grunschlag. Agenda. Pushdown Automata Stacks and recursiveness Formal Definition. From CFG’s to Stack Machines. CFG’s naturally define recursive procedure: boolean derives(strings x, y) 1. if (x==y )return true 2. for(all u y )

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Pushdown Stack Automata

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Pushdown Stack Automata

Zeph Grunschlag

Agenda

• Pushdown Automata

• Stacks and recursiveness

• Formal Definition

From CFG’s to Stack Machines

CFG’s naturally define recursive procedure:

boolean derives(strings x, y)

1. if (x==y)return true

2. for(all uy)

if derives(x,u) return true

3. return false //no successful branch

EG:S  # | aSa | bSb

From CFG’s to Stack Machines

By general principles, can carry out any recursive computation on a stack. Can do it on a restricted version of an activation record stack, called a “Pushdown (Stack) Automaton” or PDA for short.

Q: What is the language generated by

S  # | aSa | bSb?

From CFG’s to Stack Machines

A: Palindromes in {a,b,#}* containing exactly one #-symbol.

Q: Using a stack, how can we recognize such strings?

From CFG’s to Stack Machines

A: Use a three phase process:

• Push mode: Before reading “#”, push everything on the stack.

• Reading “#” switches modes.

• Pop mode: Read remaining symbols making sure that each new read symbol is identical to symbol popped from stack.

Accept if able to empty stack completely. Otherwise reject, and reject if could not pop somewhere.

ACCEPT

read == peek?

Pop

Else: CRASH!

read a or b ?

Push it

(2)

read #?

(ignore stack)

empty

stack?

(1)

PUSH

(3)

POP

ACCEPT

read == peek?

Pop

Else: CRASH!

read a or b ?

Push it

(2)

read #?

(ignore stack)

empty

stack?

(1)

PUSH

(3)

POP

Input:

aaab#baa

ACCEPT

read == peek?

Pop

Else: CRASH!

read a or b ?

Push it

(2)

read #?

(ignore stack)

empty

stack?

(1)

PUSH

(3)

POP

Input:

aaab#baa

ACCEPT

read == peek?

Pop

Else: CRASH!

read a or b ?

Push it

(2)

read #?

(ignore stack)

empty

stack?

(1)

PUSH

(3)

POP

Input:

aaab#baa

ACCEPT

read == peek?

Pop

Else: CRASH!

read a or b ?

Push it

(2)

read #?

(ignore stack)

empty

stack?

(1)

PUSH

(3)

POP

Input:

aaab#baa

ACCEPT

read == peek?

Pop

Else: CRASH!

read a or b ?

Push it

(2)

read #?

(ignore stack)

empty

stack?

(1)

PUSH

(3)

POP

Input:

aaab#baa

ACCEPT

read == peek?

Pop

Else: CRASH!

read a or b ?

Push it

(2)

read #?

(ignore stack)

empty

stack?

(1)

PUSH

(3)

POP

Input:

aaab#baa

ACCEPT

read == peek?

Pop

Else: CRASH!

read a or b ?

Push it

(2)

read #?

(ignore stack)

empty

stack?

(1)

PUSH

(3)

POP

Input:

aaab#baa

ACCEPT

read == peek?

Pop

Else: CRASH!

read a or b ?

Push it

(2)

read #?

(ignore stack)

empty

stack?

(1)

PUSH

(3)

POP

Input:

aaab#baa

ACCEPT

From CFG’s to Stack Machines

read == peek?

Pop

Else: CRASH!

read a or b ?

Push it

(2)

read #?

(ignore stack)

empty

stack?

(1)

PUSH

(3)

POP

Input:

aaab#baa

REJECT (nonempty stack)

ACCEPT

read == peek?

Pop

Else: CRASH!

read a or b ?

Push it

(2)

read #?

(ignore stack)

empty

stack?

(1)

PUSH

(3)

POP

Input:

aaab#baaa

ACCEPT

read == peek?

Pop

Else: CRASH!

read a or b ?

Push it

(2)

read #?

(ignore stack)

empty

stack?

(1)

PUSH

(3)

POP

Input:

aaab#baaa

ACCEPT

read == peek?

Pop

Else: CRASH!

read a or b ?

Push it

(2)

read #?

(ignore stack)

empty

stack?

(1)

PUSH

(3)

POP

Input:

aaab#baaa

Pause input

ACCEPT

read == peek?

Pop

Else: CRASH!

read a or b ?

Push it

(2)

read #?

(ignore stack)

empty

stack?

(1)

PUSH

(3)

POP

Input:

aaab#baaa

ACCEPT

ACCEPT

read == peek?

Pop

Else: CRASH!

read a or b ?

Push it

(2)

read #?

(ignore stack)

empty

stack?

(1)

PUSH

(3)

POP

Input:

aaab#baaaa

ACCEPT

read == peek?

Pop

Else: CRASH!

read a or b ?

Push it

(2)

read #?

(ignore stack)

empty

stack?

(1)

PUSH

(3)

POP

Input:

aaab#baaaa

ACCEPT

read == peek?

Pop

Else: CRASH!

read a or b ?

Push it

(2)

read #?

(ignore stack)

empty

stack?

(1)

PUSH

(3)

POP

Input:

aaab#baaaa

Pause input

ACCEPT

read == peek?

Pop

Else: CRASH!

read a or b ?

Push it

(2)

read #?

(ignore stack)

empty

stack?

(1)

PUSH

(3)

POP

Input:

aaab#baaaa

CRASH

PDA’s à la Sipser

To aid analysis, theoretical stack machines restrict the allowable operations. Each text-book author has his own version. Sipser’s machines are especially simple:

• Push/Pop rolled into a single operation: replace top stack symbol

• No intrinsic way to test for empty stack

• Epsilon’s used to increase functionality, result in default nondeterministic machines.

ACCEPT

read == peek?

Pop

Else: CRASH!

read a or b ?

Push it

(2)

read #?

(ignore stack)

empty

stack?

Becomes:

(1)

PUSH

(3)

POP

a , ea

b , eb

a , ae

b , be

e , e\$

#, ee

e , \$e

Sipser’s Version

x, y z

p

q

Meaning of labeling convention:

If at p and next input x and top stack y,

thengo to qand replace y by z on stack.

• x = e: ignore input, don’t read

• y = e: ignore top of stack and push z

• z = e: pop y

Sipser’s Version

a , ea

b , eb

a , ae

b , be

e , e\$

#, ee

e , \$e

push \$ to detect empty stack

a , ea

b , eb

a , ae

b , be

e , e\$

#, ee

e , \$e

Input:

aaab#baaa

a , ea

b , eb

a , ae

b , be

e , e\$

#, ee

e , \$e

Input:

aaab#baaa

a , ea

b , eb

a , ae

b , be

e , e\$

#, ee

e , \$e

Input:

aaab#baaa

a , ea

b , eb

a , ae

b , be

e , e\$

#, ee

e , \$e

Input:

aaab#baaa

a , ea

b , eb

a , ae

b , be

e , e\$

#, ee

e , \$e

Input:

aaab#baaa

a , ea

b , eb

a , ae

b , be

e , e\$

#, ee

e , \$e

Input:

aaab#baaa

a , ea

b , eb

a , ae

b , be

e , e\$

#, ee

e , \$e

Input:

aaab#baaa

a , ea

b , eb

a , ae

b , be

e , e\$

#, ee

e , \$e

Input:

aaab#baaa

a , ea

b , eb

a , ae

b , be

e , e\$

#, ee

e , \$e

Input:

aaab#baaa

a , ea

b , eb

a , ae

b , be

e , e\$

#, ee

e , \$e

Input:

aaab#baaa

a , ea

b , eb

a , ae

b , be

e , e\$

#, ee

e , \$e

Input:

aaab#baaa

a , ea

b , eb

a , ae

b , be

e , e\$

#, ee

e , \$e

Input:

aaab#baaa

ACCEPT!

PDAFormal Definition

DEF: A pushdown automaton (PDA) is a 6-tuple M = (Q, S, G, d, q0, F ). Q, S, and q0, are the same as for an FA. G is the tape alphabet. d is as follows:

So given a state p, an input letter x and a tape letter y, d(p,x,y) gives all (q,z) where q is a target state and z astack replacement for y.

PDAFormal Definition

Q: What is d(p,x,y) in each case?

• d(0,a,b)

• d(0,e,e)

• d(1,a,e)

• d(3,e,e)

a , ea

b , eb

a , ae

b , be

a , e e

0

1

2

3

e , e\$

b, e\$

e , \$e

PDAFormal Definition

A:

• d(0,a,b) = 

• d(0,e,e) = {(1,\$)}

• d(1,a,e) = {(0,e),(1,a)}

• d(3,e,e) = 

a , ea

b , eb

a , ae

b , be

a , e e

0

1

2

3

e , e\$

b, e\$

e , \$e

PDA Exercise

(Sipser 2.6.a) Draw the PDA acceptor for

L = { x  {a,b}* | na(x) = 2nb(x) }

NOTE: The empty string is in L.

a, \$ \$

a, AA

e, eA

a , ae

e, Ae

e, \$\$

b, Ae

e , e\$

e , \$e

e, ea

b, \$ \$

b, a a

e, ea