[PDF] Solutions for CSE303 Homework 5 Solution - Stony Brook University





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Give pushdown automata that recognize the following languages. Give both a drawing and 6-tuple specification for each PDA. (a) A = { w ? {0 1} 



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Solutions for CSE303 Homework 5 1 Construct nondeterministic pushdown automata (npda) that accept the following regular languages Note: Observe that all the languages are regular languages so the solutions are essentially NFA’s (ornpda’s with inactive stack) For all the languagesfis the ?nal state (a)L1=L(aaa?b)



Searches related to pushdown automata examples solutions filetype:pdf

• A DPDA is simply a pushdown automata without non-determinism – i e no epsilon transitions or transitions to multiple states on same input – Only one state at a time • DPDA not as powerful a non-deterministic PDA – This machine accepts a class of languages somewhere between regular languages and context-free languages

Solutions for CSE303 Homework 5

1. Construct nondeterministic pushdown automata (npda) that accept the following regular languages.

Note: Observe that all the languages are regular languages, so the solutions are essentially NFA"s (or

npda"s with inactive stack). For all the languages,fis the final state. (a)L1=L(aaa?b) Solution: The npdaM= (K,Σ,Γ,Δ,s,F), whereK={q0,q1,q2,f}, Σ ={a,b},F={f}, and

Δ as the set of following rules acceptsL1.

((q0,a,e),(q1,e)) ((q1,a,e),(q2,e)) ((q2,a,e),(q2,e)) ((q2,b,e),(f,e)) (b)L2=L(abb?aba?) Solution: The npdaM= (K,Σ,Γ,Δ,s,F), whereK={r0,r1,r2,r3,f}, Σ ={a,b},F={f}, and Δ as the set of following rules acceptsL2. ((r0,a,e),(r1,e)) ((r1,b,e),(r2,e)) ((r2,b,e),(r2,e)) ((r2,a,e),(r3,e)) ((r3,b,e),(f,e)) ((f,a,e),(f,e)) (c) the union ofL1andL2 Solution: The npdaM= (K,Σ,Γ,Δ,s,F), whereK={s,q0,q1,q2,r0,r1,r2,r3,f}, Σ ={a,b}, F={f}, and Δ as the set of following rules acceptsL1?L2 ((s,e,e),(q0,e)) ((s,e,e),(r0,e)) whereq0andr0are as defined above.

2. Construct npda"s that accept the following languages on Σ ={a,b,c}.

(a)L={anb2n:n≥0} Solution: The npdaM= (K,Σ,Γ,Δ,s,F), whereK={s,f}, Σ ={a,b},F={f}, and Δ as the set of following rules acceptsL. ((s,e,e),(f,e)) ((s,a,e),(s,aa)) ((s,b,a),(f,e)) ((f,b,a),(f,e)) (b)L={anbmcn+m:n≥0,m≥0} Solution: The npdaM= (K,Σ,Γ,Δ,s,F), whereK={s0,s1,f}, Σ ={a,b},F={s1}, and Δ as the set of following rules acceptsL. ((s0,e,e),(f,e)) ((s0,a,e),(s0,a)) ((s0,c,a),(f,e)) ((s0,b,e),(s1,b)) ((s1,b,e),(s1,b)) ((s1,c,b),(f,e)) ((f,c,b),(f,e)) ((f,c,a),(f,e)) 1

3. Find an npda on Σ ={a,b,c}that accepts the languageL={w1cw2:w1,w2? {a,b}?,w1?=wR2}

Solution: Idea: Observe that ifw1=wR2, then the npda should not accept the input. Butw1=wR2is equivalent towR1=w2. Hence, we first, pushw1onto stack, then while matchingw2, if a mismatch is

found (thenwR1?=w2), consume all the input and empty the stack then go to final state; otherwise (then

w R1=w2which meansw1=wR2), the input is not accepted. Thus the npdaM= (K,Σ,Γ,Δ,s,F), whereK={s0,s1,d,f}, Σ ={a,b},F={f}, and Δ as the set of following rules acceptsL. ((s0,a,e),(s0,a)) ((s0,b,e),(s0,b)) ((s0,c,e),(s1,e)) ((s1,a,a),(s1,e)) ((s1,b,b),(s1,e)) ((s1,a,b),(d,e)) ((s1,b,a),(d,e)) ((d,e,a),(d,e)) ((d,e,b),(d,e)) ((d,a,e),(d,e)) ((d,b,e),(d,e)) ((d,e,e),(f,e))

4. Construct an npda corresponding to the grammar

S→aABB|aAA,

A→aBB|a,

B→bBB|A

Solution: The npdaM= (K,Σ,Γ,Δ,s,F), whereK={s0,s1}, Σ ={a,b},F={s1}, and Δ as the set of following rules implements the grammar above. ((s0,e,e),(s1,S) ((s1,a,S),(s1,ABB) ((s1,a,S),(s1,AA) ((s1,a,A),(s1,BB) ((s1,a,A),(s1,e) ((s1,b,B),(s1,BB) ((s1,a,B),(s1,BB) ((s1,a,B),(s1,e)

Another solution:((s0,e,e),(s1,S)

((s1,e,S),(s1,aABB)) ((s1,e,S),(s1,aAA)) ((s1,e,A),(s1,aBB)) ((s1,e,B),(s1,bBB)) ((s1,e,B),(s1,A)) ((s1,a,a),(s1,e)) ((s1,b,b),(s1,e))

5. Show that the languageL={ww:w? {a,b}?}is not context-free.

Solution: Consider the stringambmambm. Now, letvandycontain only the firsta"s and letv=ap andy=aq. Then consideruxz=akbmambm,k < m, which is not inL. For other choices ofvandy, we can make similar arguments (since each symbol occurs exactlymtimes). Thus,Lis not context-free. 2

6. Show that the languageL={an!:n≥0}is not context-free.

Solution: In this case, this is same as showing thatLis not regular (since the language consists entirely of alphabet over single symbol). Letw=uvxyzsuch thatuv=?. Then we need to show that we havem!-k >(m-1)!. ThereforeLis not context-free (or regular, either).

7. Construct Turing machines that will accept the following languages on{a,b}.

(a)L={w:|w|is even} Solution: Here we keep checking off two input symbols and if ultimately we encounter end of input in the start state, we accept the input. The Turing machineM= (K,Σ,δ,s,H), whereK={q0,q1,h},Σ ={a,b,?,?},s=q0,H={h}

andδas follows acceptsL.q σδ(q,σ)q0a(q1,→)q0b(q1,→)q1a(q0,→)q1b(q0,→)q0?(h,?)(b)L={w:na(w) =nb(w)}

wherena(w) is number ofa"s inwandnb(w) is number ofb"s inw. Solution: First find leftmostaorb. Ifa(orb) is found, then replace it withxand find a matchingb (ora), replace it withxand then go back to the left end of the tape (so as to find next leftmostaor b). Repeat. The Turing machineM= (K,Σ,δ,s,H), whereK={q0,q1,q2,q3,h},Σ ={a,b,?,?},s=q0,H={h}

andδas given below acceptsL.q σδ(q,σ)q0x(q0,→)q0a(q1,x)q0b(q2,x)q1x(q1,→)q1a(q1,→)q1b(q3,x)q2x(q2,→)q2a(q3,x)q2b(q2,→)q3x(q3,←)q3a(q3,←)q3b(q3,←)q3?(q0,→)q0?(h,?)3

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