**Pushdown Automata (PDA)**is a computational model equivalent to

**Context Free Grammar (CFG)**.

A

**Pushdown Automata (PDA)**is an

**Non-Deterministic Finite Automata (NFA)**augmented with an infinitely large stack.

The additional memory enables recognition of some non-regular languages.

A

**Pushdown Automata (PDA)**is essentially a Finite Automata with a Stack data structure as shown above.

A

**Pushdown Automata (PDA)**can write an unbounded number of stack symbols on the stack and read these symbols later.

Writing a symbol onto the stack is called

**Pushing**and it pushes all symbols on the stack one stack cell down.

Removing a symbol off the stack is called

**Poping**and every symbol on the stack moves one stack cell up.

A

**Pushdown Automata (PDA)**can access only the stack's topmost symbol in

**LIFO**manner.

Now, We Consider how to describe precisely the abstract machine whose operations.we have sketched each move of the machine will be determined by three things.

**(1) The Current State**

**(2) The Next Input**

**(3) The Symbol on top of the Stack**

and will consist of two parts.

**(1) Changing state ( Or staying in the same state)**

**(2) Replacing the top stack symbol by a string of zero or more symbols.**

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