**Prerequisite – **Theory of Computation

**Grammar :**

It is a finite set of formal rules for generating syntactically correct sentences or meaningful correct sentences.

**Constitute Of Grammar :**

Grammar is basically composed of two basic elements –

**Terminal Symbols –**

Terminal symbols are those which are the components of the sentences generated using a grammar and are represented using small case letter like a, b, c etc.**Non-Terminal Symbols –**

Non-Terminal Symbols are those symbols which take part in the generation of the sentence but are not the component of the sentence. Non-Terminal Symbols are also called Auxiliary Symbols and Variables. These symbols are represented using a capital letter like A, B, C, etc.

**Formal Definition of Grammar :**

Any Grammar can be represented by 4 tuples – <N, T, P, S>

**N –**Finite Non-Empty Set of Non-Terminal Symbols.**T –**Finite Set of Terminal Symbols.**P –**Finite Non-Empty Set of Production Rules.**S –**Start Symbol (Symbol from where we start producing our sentences or strings).

**Production Rules :**

A production or production rule in computer science is a rewrite rule specifying a symbol substitution that can be recursively performed to generate new symbol sequences. It is of the form -> where is a Non-Terminal Symbol which can be replaced by which is a string of Terminal Symbols or Non-Terminal Symbols.

**Example-1 :**

Consider Grammar G1 = <N, T, P, S>

T = {a,b} #Set of terminal symbols P = {A->Aa,A->Ab,A->a,A->b,A-> } #Set of all production rules S = {A} #Start Symbol

As the start symbol is S then we can produce Aa, Ab, a,b,which can further produce strings where A can be replaced by the Strings mentioned in the production rules and hence this grammar can be used to produce strings of the form (a+b)*.

**Derivation Of Strings :**

A->a #using production rule 3ORA->Aa #using production rule 1 Aa->ba #using production rule 4ORA->Aa #using production rule 1 Aa->AAa #using production rule 1 AAa->bAa #using production rule 4 bAa->ba #using production rule 5

**Example-2 :**

Consider Grammar G2 = <N, T, P, S>

N = {A} #Set of non-terminals Symbols T = {a} #Set of terminal symbols P = {A->Aa, A->AAa, A->a, A->} #Set of all production rules S = {A} #Start Symbol

As the start symbol is S then we can produce Aa, AAa, a,which can further produce strings where A can be replaced by the Strings mentioned in the production rules and hence this grammar can be used to produce strings of form (a)*.

**Derivation Of Strings :**

A->a #using production rule 3ORA->Aa #using production rule 1 Aa->aa #using production rule 3ORA->Aa #using production rule 1 Aa->AAa #using production rule 1 AAa->Aa #using production rule 4 Aa->aa #using production rule 3

**Equivalent Grammars :**

Grammars are said to be equivalent id they produce the same language.

**Different Types Of Grammars :**

Grammar can be divided on basis of –

- Type of Production Rules
- Number of Derivation Trees
- Number of Strings