Which one of the following is a top-down parser?*An LALR(k) parserAn LR(k) parserRecursive descent parserOperator precedence parser

Step 1: Break Down the Problem

We need to identify which of the given types of parsers is a top-down parser from the options provided.

Step 2: Relevant Concepts

1. Top-Down Parsers: These parsers start from the root of the parse tree and work down to the leaves. One common method of top-down parsing is Recursive Descent parsing.
2. Bottom-Up Parsers: These parsers start from the leaves and work up to the root. LALR(k), LR(k), and Operator Precedence parsers are all examples of bottom-up parsers.

Step 3: Analysis and Detail

• LALR(k) Parser: A bottom-up parser that uses Look-Ahead LR parsing, typically does not work top-down.
• LR(k) Parser: Another type of bottom-up parser that uses a more complex state machine.
• Recursive Descent Parser: This is a classic example of a top-down parser, where each non-terminal in the grammar corresponds to a function in the code.
• Operator Precedence Parser: This parser is generally used for specific grammars to handle operator precedence and is also considered a bottom-up parser.

Step 4: Verify and Summarize

The only parser from the list that operates in a top-down manner is the Recursive Descent Parser. The other options provided are bottom-up parsers.

Final Answer

The top-down parser from the given options is Recursive Descent Parser.