Question

Which one of the following derivations does a top-down parser use while parsing an input string? The input is assumed to be scanned from left to right.

• A. Leftmost derivation
• B. Leftmost derivation traced out in reverse
• C. Rightmost derivation
• D. Rightmost derivation traced out in reverse

Hint:

Top-down parsing corresponds to leftmost derivation, while bottom-up parsing corresponds to rightmost derivation in reverse.

Answer 1

The Correct Option is A

Step 1: Understanding top-down parsing.
Top-down parsers begin parsing from the start symbol and try to generate the input string by expanding non-terminals.
Step 2: Direction of derivation.
In top-down parsing, the leftmost non-terminal is always expanded first, while the input is scanned from left to right.
Step 3: Distinguishing from bottom-up parsing.
Bottom-up parsers construct a rightmost derivation in reverse, which is not the case here.
Step 4: Final conclusion.
Hence, a top-down parser uses a leftmost derivation.