alleryStart learning →The FIRST(α) set of a string α in LL parsing is
Athe set of terminals that can begin some derivation of α
Bthe first non-terminal in α
Cthe set of α's children in the parse tree
Dalways equal to the empty set
Answer & Solution
Correct answer: A. the set of terminals that can begin some derivation of α
1. FIRST(α) = the set of TERMINAL symbols that can begin a string DERIVABLE FROM α.
2. Formally: $a \in \text{FIRST}(\alpha)$ iff $\alpha \Rightarrow^* a\beta$ for some $\beta$.
3. If $\alpha \Rightarrow^* \varepsilon$, then $\varepsilon \in \text{FIRST}(\alpha)$.
4. USE: LL(1) PARSING TABLE construction. For rule $A \to \alpha$, this production is selected when the lookahead is in $\text{FIRST}(\alpha)$.
5. FIRST and FOLLOW sets together drive the entire LL(1) parser table.
6. Other options misstate FIRST.
_Source: Bob Nystrom, "Crafting Interpreters", Ch 6.2 + classical compiler theory._
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