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- Apr 13, 2013

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Could you explain me how I could find the context-free grammar,that produces a language?Because I am not familiar yet with context-free grammars

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- Thread starter
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- Apr 13, 2013

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Could you explain me how I could find the context-free grammar,that produces a language?Because I am not familiar yet with context-free grammars

- Jan 30, 2012

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I have seen some examples from an other book I have,but I haven't understood how to find the context-free grammars. Could you explain me how I can find it,for example,for this language: [tex]\{w \epsilon [/tex]{a,b}[tex]^{*}[/tex]: (number of a in w)=2 * (number of b in w)} ?

- Jan 30, 2012

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By trial and error, using common sense.Could you explain me how I can find it,for example,for this language: [tex]\{w \epsilon [/tex]{a,b}[tex]^{*}[/tex]: (number of a in w)=2 * (number of b in w)} ?

First we need to cover the empty string, so there should be the rule

\[

S\to\epsilon

\]

Next, an obvious approach is to have all other rules introduce one $b$ and two $a$s. By induction on derivation, this condition guarantees that each generated word belongs to the language, but there are grammars that don't have this restriction. The challenge is to generate

\[

\begin{aligned}

S&\to SbSaSaS\\

S&\to SaSbSaS\\

S&\to SaSaSbS

\end{aligned}

\]

By inserting $S$ between symbols we don't lose the ability to make the intermediate string (with nonterminal symbols) into an arbitrary string in the language. We don't want to commit, for example, to $baa$ with no way to insert something between $b$ and $a$.

It seems to me that these rules should cover the language, though this grammar is probably highly ambiguous, i.e., each terminal word has many derivations. In other words, inserting $S$ everywhere is probably an overkill. Anyway, the next step is proving that this grammar generates precisely the required language.

It is possible to try to provide a more economical grammar, which is less ambiguous and more suitable for parsing.

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- Apr 13, 2013

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I understand.But how can I prove that this grammar generates precisely the required language?By trial and error, using common sense.

First we need to cover the empty string, so there should be the rule

\[

S\to\epsilon

\]

Next, an obvious approach is to have all other rules introduce one $b$ and two $a$s. By induction on derivation, this condition guarantees that each generated word belongs to the language, but there are grammars that don't have this restriction. The challenge is to generateallstrings in the language. The newly introduced $b$ may be located to the left of the newly introduced $a$'s, between them, or to the right. So we may define the following rules.

\[

\begin{aligned}

S&\to SbSaSaS\\

S&\to SaSbSaS\\

S&\to SaSaSbS

\end{aligned}

\]

By inserting $S$ between symbols we don't lose the ability to make the intermediate string (with nonterminal symbols) into an arbitrary string in the language. We don't want to commit, for example, to $baa$ with no way to insert something between $b$ and $a$.

It seems to me that these rules should cover the language, though this grammar is probably highly ambiguous, i.e., each terminal word has many derivations. In other words, inserting $S$ everywhere is probably an overkill. Anyway, the next step is proving that this grammar generates precisely the required language.

It is possible to try to provide a more economical grammar, which is less ambiguous and more suitable for parsing.

- Jan 30, 2012

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I'll wait for your ideas and respond to them.But how can I prove that this grammar generates precisely the required language?

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Do I have maybe to do the following:I'll wait for your ideas and respond to them.

-to prove that every string produced by the grammar has the property (number of a in w)=2 * (number of b in w)

-to prove that every string with this property is produced by the grammar.

If yes,could you give me a hint how to start?

- Jan 30, 2012

- 2,589

Certainly.Do I have maybe to do the following:

-to prove that every string produced by the grammar has the property (number of a in w)=2 * (number of b in w)

-to prove that every string with this property is produced by the grammar.

Concerning your first point:If yes,could you give me a hint how to start?

Thenan obvious approach is to have all other rules introduce one $b$ and two $a$s. By induction on derivation, this condition guarantees that each generated word belongs to the language

Take an arbitrary string in the language. Suppose it starts with a $b$. Then somewhere in the string there are two $a$'s. (That's an understatement!) These three symbols break the string into four (possibly empty) parts. What can be said about them? That is, this direction uses induction on the string length.The challenge is to generateallstrings in the language.

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- #9

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For the first step,I tried this:Certainly.

Concerning your first point:

Then

Take an arbitrary string in the language. Suppose it starts with a $b$. Then somewhere in the string there are two $a$'s. (That's an understatement!) These three symbols break the string into four (possibly empty) parts. What can be said about them? That is, this direction uses induction on the string length.

-to prove that every string produced by the grammar has the property (number of a in w)=2 * (number of b in w), by induction on derivation:

Claim:For every natural number n, if w is a sentential form whose derivation is of length n, then w has the property: (number of a in w)=2*(number of b in w)

Base case: The claim holds for n=0,as the only sentential form with derivation length 0 is S,which satisfies the property. The claim also holds for n=1, as the only possible sentential forms are SbSaSaS, SaSbSaS, SaSaSbS, or the empty string.

Induction Hypothesis: Let the claim be true for n<=k. So, assume that for all sentential forms of length less than k satisfy the property.

Induction Step: Consider a sentential form w whose derivation is of length k+1.

Let the sentential form z have a derivation of length k, and by induction hypothesis it satisfies the property. We can write z=xSy, where x and y are sentential forms and we replace S in the (k+1)-step to obtain w. This S is one of the strings SbSaSaS, SaSbSaS, SaSaSbS, or ε. In any case the property is satisfied.

Is this right?

- Jan 30, 2012

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Up to this point everything is fine. A couple of remarks about the following.Base case: The claim holds for n=0,as the only sentential form with derivation length 0 is S,which satisfies the property. The claim also holds for n=1, as the only possible sentential forms are SbSaSaS, SaSbSaS, SaSaSbS, or the empty string.

First you say, for $\le k$ and then for $<k$.Induction Hypothesis: Let the claim be true for n<=k. So, assume that for all sentential forms of length less than k satisfy the property.

Stating induction hypothesis is a part of the induction step. That is, you should signal the beginning of the induction step before stating the hypothesis.Induction Step:

From the following, $z$ is not an arbitrary sentential form with a derivation of length $k$, but a direct ancestor of $w$. This should be stated from the start.Consider a sentential form w whose derivation is of length k+1.

Let the sentential form z have a derivation of length k

The last sentence has to be written more carefully. $S$ is not $SbSaSaS$.and by induction hypothesis it satisfies the property. We can write z=xSy, where x and y are sentential forms and we replace S in the (k+1)-step to obtain w. This S is one of the strings SbSaSaS, SaSbSaS, SaSaSbS, or ε.

Up to now, your reasoning is very detailed, which is fine, but this last statement is comparatively unsupported. How do you use $x$, $y$ and $z$ to prove it?In any case the property is satisfied.

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- #11

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x,y,z are strings that satisfy the property.Or can't I do it like that?Up to this point everything is fine. A couple of remarks about the following.

First you say, for $\le k$ and then for $<k$.

Stating induction hypothesis is a part of the induction step. That is, you should signal the beginning of the induction step before stating the hypothesis.

From the following, $z$ is not an arbitrary sentential form with a derivation of length $k$, but a direct ancestor of $w$. This should be stated from the start.

The last sentence has to be written more carefully. $S$ is not $SbSaSaS$.

Up to now, your reasoning is very detailed, which is fine, but this last statement is comparatively unsupported. How do you use $x$, $y$ and $z$ to prove it?

- Jan 30, 2012

- 2,589

Why do $x$ and $y$ satisfy the property?x,y,z are strings that satisfy the property.Or can't I do it like that?

I just wanted to say that it would be nice to write in more detail why the fact that $z$ satisfies the property implies that $w$ does, too.

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- #13

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Can I explain it like that:Why do $x$ and $y$ satisfy the property?

I just wanted to say that it would be nice to write in more detail why the fact that $z$ satisfies the property implies that $w$ does, too.

Since S can be replaced by one of the strings SbSaSaS, SaSbSaS, SaSaSbS, ε, then we obtain w cause z has now a derivation of length k+1.

Or am I wrong?

- Jan 30, 2012

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Unfortunately, this is not even wrong. You are saying, "Since ..., we obtain $w$". But what kind of conclusion is this? It's as if "we obtain $w$" can be true or false; it can't at this point. In the induction step we started by considering a derivation of length $k+1$ and called its conclusion $w$. We already obtained $w$ and this fact needs no justification.Can I explain it like that:

Since S can be replaced by one of the strings SbSaSaS, SaSbSaS, SaSaSbS, ε, then we obtain w cause z has now a derivation of length k+1.

Or am I wrong?

Second, $z$ is a fixed string, a direct predecessor of $w$ in the derivation. You can't say, "$z$ has now a derivation of length $k+1$" because $z$ has not changed, and the derivation we fixed in the induction step has not changed. There is $z$, and by one more rule application we obtain $w$.

Finally, you have not said anything about the number of $a$'s and $b$'s.

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- Apr 13, 2013

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I will think about it and I will tell you what I think that I could change to improve my answer..Unfortunately, this is not even wrong. You are saying, "Since ..., we obtain $w$". But what kind of conclusion is this? It's as if "we obtain $w$" can be true or false; it can't at this point. In the induction step we started by considering a derivation of length $k+1$ and called its conclusion $w$. We already obtained $w$ and this fact needs no justification.

Second, $z$ is a fixed string, a direct predecessor of $w$ in the derivation. You can't say, "$z$ has now a derivation of length $k+1$" because $z$ has not changed, and the derivation we fixed in the induction step has not changed. There is $z$, and by one more rule application we obtain $w$.

Finally, you have not said anything about the number of $a$'s and $b$'s.

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