NLP Lab Program 4- BAI601

 

Experiment 4:

Write a program to implement top-down and bottom-up parser using appropriate context free grammar.

To implement both Top-Down and Bottom-Up parsers, we need a context-free grammar (CFG) as the input. Let's first define a simple grammar and then proceed with the parsers. We'll use LL(1) grammar for the top-down parser and LR(1) grammar for the bottom-up parser.

Example Context-Free Grammar (CFG):

S -> AB

A -> aA | ε

B -> bB | ε

Top-Down Parsing (LL(1) Parser):

In Top-Down Parsing, we start from the start symbol and try to rewrite it using production rules until we reach the input string. This is typically implemented using Recursive Descent Parsing.

Steps:

1. We define a recursive function for each non-terminal in the grammar.
2. For each non-terminal, if the first symbol of the input matches the expected terminal, we proceed to the next symbol.
3. If a non-terminal is expected and there’s no match, we backtrack (if applicable, or return an error in the case of LL(1)).

Explanation:

1. TopDownParser class initializes with the input string.
2. parse() starts the parsing from the start symbol S.
3. Functions like s(), a(), and b() correspond to non-terminal symbols and are implemented recursively to handle the rules of the grammar.
4. If parsing is successful, the function prints a success message. Otherwise, it indicates parsing failure.

Bottom-Up Parsing (LR(1) Parser):

In Bottom-Up Parsing, we start from the input string and try to reduce it to the start symbol using the production rules in reverse. This is often implemented using an LR(1) parser.

Explanation:

1. BottomUpParser uses a stack to simulate the bottom-up parsing process.
2. We start scanning the input string and push symbols onto the stack.
3. After each symbol is pushed, we attempt to reduce the stack using predefined production rules (stored in self.rules).
4. The parser continues reducing the stack until it matches the start symbol S.
5. If the stack is reduced to just S, parsing is successful.

Final Remarks:

• The Top-Down Parser is recursive and works by breaking down the start symbol using grammar rules.
• The Bottom-Up Parser simulates reductions, starting from the input and attempting to reduce it to the start symbol.

In both parsers, we've used the simple CFG S -> AB, A -> aA | ε, B -> bB | ε for illustrative purposes.

Steps:

1. We maintain a stack to simulate the parsing process.
2. We start by scanning the input and push symbols onto the stack.
3. We try to reduce the top of the stack using production rules whenever possible.
4. If the stack and input match the start symbol, we have successfully parsed the input.