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Module 36 - PowerPoint PPT Presentation

Module 36. Non context-free languages Examples and Intuition Pumping lemma for CFL’s Pumping condition No proof of pumping lemma Applying pumping lemma to prove that some languages are not CFL’s. Examples and Intuition. Examples *. What are some examples of nonregular languages?

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• Non context-free languages

• Examples and Intuition

• Pumping lemma for CFL’s

• Pumping condition

• No proof of pumping lemma

• Applying pumping lemma to prove that some languages are not CFL’s

Examples and Intuition

• What are some examples of nonregular languages?

• Can we build on any of these languages to create a non context-free language?

• Try and prove that these languages are CFL’s and identify the stumbling blocks

• Why can’t we construct a CFG to generate this language?

• Why can’t we construct a PDA to accept this language?

• Compare to similar CFL languages to try and identify differences.

Comparison to regular language pumping lemma/condition

• In regular languages, a single substring “pumps”

• Consider the language of even length strings over {a,b}

• We can identify a single substring which can be pumped

• In CFL’s, multiple substrings can “pump”

• Consider the language {anbn | n > 0}

• No single substring can be pumped and allow us to stay in the language

• However, there do exist pairs of substrings which can be pumped resulting in strings which stay in the language

• This results in a modified pumping condition

A language L satisfies the regular language pumping condition if:

there exists an integer n > 0 such that

for all strings x in L of length at least n

there exist strings u, v, w such that

x = uvw and

|uv| <= n and

|v| >= 1 and

For all k >= 0, uvkw is in L

A language L satisfies the CFL pumping condition if:

there exists an integer n > 0 such that

for all strings x in L of length at least n

there exist strings u, v, w, y, z such that

x = uvwyz and

|vwy| <= n and

|vy| >= 1 and

For all k >= 0, uvkwykz is in L

Modified Pumping Condition

All languages over {a,b}

Pumping Lemma

• All CFL’s satisfy the CFL pumping condition

CFL’s

Implications

CFL

• We can use the pumping lemma to prove a language L is not a CFL

• Show L does not satisfy the CFL pumping condition

• We cannot use the pumping lemma to prove a language is context-free

• Showing L satisfies the pumping condition does not guarantee that L is context-free

Pumping Lemma

What does it mean?

• A language L satisfies the CFL pumping condition if:

• there exists an integer n > 0 such that

• for all strings x in L of length at least n

• there exist strings u, v, w, y, z such that

• x = uvwyz and

• |vwy| <= n and

• |vy| >= 1 and

• For all k >= 0, uvkwykz is in L

v and y can be pumped

1) x in L2) x = uvwyz3) For all k >= 0, uvkwykz is in L

• Let x = abcdefg be in L

• Then there exist 2 substrings v and y in x such that v and y can be repeated (pumped) in place any number of times and the resulting string is still in L

• uvkwykz is in L for all k >= 0

• For example

• v = cd and y = f

• uv0wy0z = uwz =abeg is in L

• uv1wy1z = uvwyz = abcdefgis in L

• uv2wy2z = uvvwyyz = abcdcdeffgis in L

• uv3wy3z = uvvvwyyyz = abcdcdcdefffg is in L

• A language L satisfies the CFL pumping condition if:

• there exists an integer n > 0 such that

• Since we skip this proof, we will not see what n really means

• for all strings x in L of length at least n

• x must be in L and have sufficient length

• there exist strings u, v, w, y, z such that

• x = uvwyz and

• |vwy| <= n and

• v and y are contained within n characters of x

• Note: these are NOT necessarily the first n characters of x

• |vy| >= 1 and

• v and y cannot both be l,

• One of them might be l, but not both

• For all k >= 0, uvkwykz is in L

• Let L be the set of palindromes over {a,b}

• Let x = aabaa

• Let n = 3

• What are the possibilities for v and y ignoring the pumping constraint?

• Which ones satisfy the pumping lemma?

Pumping Lemma

Applying it to prove a specific language L is not context-free

• We choose a specific language L

• For example, {ajbjcj | j > 0}

• We show that L does not satisfy the pumping condition

• We conclude that L is not context-free

A language L satisfies the CFL pumping condition if:

there exists an integer n > 0 such that

for all strings x in L of length at least n

there exist strings u, v, w, y, z such that

x = uvwyz and

|vwy| <= n and

|vy| >= 1 and

For all k >= 0, uvkwykz is in L

A language L does not satisfy the CFL pumping condition if:

for all integers n of sufficient size

there exists a string x in L of length at least n such that

for all strings u, v, w, y, z such that

x = uvwyz and

|vwy| <= n and

|vy| >= 1

There exists a k >= 0 such that uvkwykz is not in L

Showing L “does not pump”

A language L does not satisfy the CFL pumping condition if:

for all integers n of sufficient size

there exists a string x in L of length at least n such that

for all strings u, v, w, y, z such that

x = uvwyz and

|vwy| <= n and

|vy| >= 1

There exists a k >= 0 such that uvkwykz is not in L

Proof that L = {aibici | i>0} does not satisfy the CFL pumping condition

Let n be the integer from the pumping lemma

Choose x = anbncn

Consider all strings u, v, w, y, z s.t.

x = uvwyz and

|vwy| <= n and

|vy| >= 1

Argue that uvkwykz is not in L for some k >= 0

Argument must apply to all possible u,v,w,y,z

Continued on next slide

Example Proof

Proof that L = {aibici | i>0} does not satisfy the CFL pumping condition

Let n be the integer from the pumping lemma

Choose x = anbncn

Consider all strings u, v, w, y, z s.t.

x = uvwyz and

|vwy| <= n and

|vy| >= 1

Argue that uvkwykz is not in L for some k >= 0

Argument must apply to all possible u,v,w,y,z

Continued next column

Identify possible cases for vwy

What is impossible for vwy?

Case 1

vwy contains no a’s

Case 2

vwy contains no c’s

Must argue uvkwykz is not in L for bothcases described above

Can use different values of k

Continued on next slide

Example Proof Continued *

What is impossible for vwy?

Case 1

vwy contains no a’s

Case 2

vwy contains no c’s

Must argue uvkwykz is not in L for bothcases described above

Can use different values of k

Continued next column

Case 1: vwy contains no a’s

vy contains at least 1 b or c

follows from

vwy contains no a’s and

|vy| >= 1

uwz is not in L

uwz has n a’s

follows from fact vwy contains no a’s and x originally had n a’s

uwz has fewer than n b’s or fewer than n c’s

follows from vy contains at least 1 b or c and x originally only had n b’s and n c’s

Continued next slide

Example Proof Continued

vy contains at least 1 b or c

follows from

vwy contains no a’s and

|vy| >= 1

uwz is not in L

uwz has n a’s

follows from fact vwy contains no a’s and x originally had n a’s

uwz has fewer than n b’s or fewer than n c’s

follows from vy contains at least 1 b or c and x originally only had n b’s and n c’s

Continued next column

Case 2: vwy contains no c’s

vy contains at least

uv2wy2z is not in L

uv2wy2z has n c’s

follows from fact vwy contains no c’s and x originally had n c’s

uv2wy2z has more than n a’s or more than n b’s

follows from vy contains at least 1 a or b and x originally has n a’s and n b’s

Continued next slide

Example Proof Continued *

uvkwykz is not in L

Note, we used a different value of k for each case (though we didn’t have to)

Therefore L does not satisfy the CFL pumping condition

There L is not a CFL

Case 2: vwy contains no c’s

vy contains at least

uv2wy2z is not in L

uv2wy2z has n c’s

follows from fact vwy contains no c’s and x originally had n c’s

uv2wy2z has more than n a’s or more than n b’s

follows from vy contains at least 1 a or b and x originally has n a’s and n b’s

Continued next column

Example Proof Completed

Other example languages k>=0 such that

• TWOCOPIES = {ww | w is in {a,b}* }

• abbabb is in TWOCOPIES but abaabb is not

• EQUAL3 = the set of strings over {a, b, c} such that the number of a’s equals the number of b’s equals the number of c’s

• {aibjck | i < j < k}

Pumping Lemma k>=0 such that

Two rules of thumb

Two Rules of Thumb k>=0 such that

• Try to use blocks of at least n characters in x

• For TWOCOPIES, choose x = anbnanbn rather than anbanb

• Guarantees v and y cannot be in more than 2 blocks of x

• Try k=0 or k=2

• k=0

• This reduces number of occurrences of v and y

• k=2

• This increases number of occurrences of v and y

Summary k>=0 such that

• We use the Pumping Lemma to prove a language is not a CFL

• Note, does not work for all non CFL languages

• Can be strengthened to Ogden’s Lemma

• In book

• Choosing a good string x is first key step

• Choosing a good k is second key step

• Typically have several cases for v, w, y