Parsing and it’s classification in Compiler Design- Top down parsing/bottom up parsing/Crecursive

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Parsing and Its Classification in Compiler Design

 What is Parsing?

Parsing is the process of analyzing a sequence of tokens (from lexical analysis) to check if they follow the syntax rules of a grammar. It helps in syntax analysis and is a crucial part of a compiler.

 Classification of Parsing

Parsing is broadly classified into two types:
Top-Down Parsing
Bottom-Up Parsing

 Top-Down Parsing

In top-down parsing, the parser starts from the start symbol (S) and tries to derive the input string step by step by applying production rules.

Types of Top-Down Parsing:

• Recursive Descent Parsing (Uses recursive functions)
• Predictive Parsing (Uses a parsing table, LL(1) Parsing)

 Recursive Descent Parsing

• Uses recursive functions for each non-terminal.
• Backtracking may be required (which is inefficient).

Example:

For input id + id, the parser starts from E and applies rules step by step.

 Predictive Parsing (LL(1) Parsing)

• No backtracking required.
• Uses a parsing table to decide the next step.
• Works for LL(1) grammars (Left-to-right scan, Leftmost derivation, 1 token lookahead).

Example:
Grammar:

LL(1) table helps decide which production to apply next.

 Bottom-Up Parsing

In bottom-up parsing, the parser starts with the input string and reduces it step by step to the start symbol using reverse derivations.

Types of Bottom-Up Parsing:

• Shift-Reduce Parsing (LR Parsing)
• Operator Precedence Parsing

 Shift-Reduce Parsing (LR Parsing)

• Uses a stack to hold symbols.
• Shift: Push tokens onto the stack.
• Reduce: Replace a sequence of symbols with a non-terminal based on production rules.
• Common types: LR(0), SLR(1), LALR(1), CLR(1).

Example:
Grammar:

For input id + id, the parser shifts tokens onto the stack and reduces them using rules.

 Key Differences: Top-Down vs. Bottom-Up Parsing

Feature	Top-Down Parsing	Bottom-Up Parsing
Start Point	From the start symbol	From the input string
Direction of Parsing	Left-to-right derivation	Right-to-left reduction
Backtracking	May require (in Recursive Descent)	No backtracking
Common Techniques	Recursive Descent, Predictive Parsing (LL(1))	Shift-Reduce Parsing (LR, SLR, CLR, LALR)
Complexity	Easier to implement	More powerful but complex

 Conclusion

Top-Down Parsing is simpler but may not handle all types of grammars.
Bottom-Up Parsing is more powerful and can handle complex grammars.

 Would you like an example of LL(1) or LR(1) parsing? Let me know!

Here’s a clear and concise explanation of:

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Parsing and Its Classification in Compiler Design

Includes Top-Down Parsing, Bottom-Up Parsing, and Recursive Descent Parsing

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What is Parsing?

Parsing is the process of analyzing a string of symbols (source code) according to the rules of a formal grammar.

It helps in:

• Checking syntax errors
• Converting code into a parse tree
• Guiding the compiler toward semantic analysis and code generation

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Goal of Parsing:

To determine if the input string can be generated by the grammar and to produce a derivation or parse tree.

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Classification of Parsing:

Parsing techniques are broadly classified into two categories:

Type	Direction	Works With
Top-Down Parsing	Left-to-right (L→R), builds tree from root	LL Parsers
Bottom-Up Parsing	Left-to-right (L→R), builds tree from leaves	LR Parsers

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1. Top-Down Parsing

Definition:

Starts from the start symbol and tries to derive the input string using production rules.

Techniques:

• Recursive Descent Parsing
• Predictive Parsing (LL(1))

Process:

• Begin with the start symbol
• Expand non-terminals using productions
• Try to match input tokens

Issues:

• May suffer from left recursion
• Requires backtracking (unless LL(1))

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Recursive Descent Parsing

• Uses one function per non-terminal
• Functions call each other recursively
• Easy to implement manually
• May involve backtracking

Suitable for:

• Small grammars
• LL(1) grammars

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2. Bottom-Up Parsing

Definition:

Starts from the input string and tries to reduce it to the start symbol using reverse productions.

Techniques:

• Shift-Reduce Parsing
• LR(0), SLR(1), LALR(1), CLR(1)

Process:

• Shift tokens to a stack
• Reduce using grammar rules
• Continue until start symbol is reached

Advantages:

• More powerful
• Handles a larger class of grammars (LR)

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Comparison Table

Feature	Top-Down Parsing	Bottom-Up Parsing
Direction	Left-to-right	Left-to-right
Tree Construction	Root to leaves	Leaves to root
Backtracking	May need it	Not required
Handles Left Recursion	No	Yes
Examples	Recursive Descent, LL	LR, SLR, LALR, CLR

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Summary:

• Top-Down → Start from root, expand down (predictive)
• Bottom-Up → Start from input, reduce up (more powerful)
• Recursive Descent → A hand-written top-down parser using functions

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Bonus:

Would you like:

• A diagram of parsing tree construction?
• Practice problems with parse trees?
• Video script or explainer?

Let me know what you’d like next!

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