Among simple LR (SLR) , canonical LR, and look-ahead LR (LALR), which of the…
2015
Among simple LR (SLR) , canonical LR, and look-ahead LR (LALR), which of the following pairs identify the method that is very easy to implement and the method that is the most powerful , in that order?
- A.
SLR, LALR
- B.
Canonical LR, LALR
- C.
SLR, canonical LR
- D.
LALR, canonical LR
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Correct answer: C
Answer: SLR, canonical LR
Why SLR is very easy to implement: builds an LR(0) automaton and uses follow sets to decide reductions, so the algorithm and tables are simpler and smaller to construct.
Why canonical LR is the most powerful: uses full LR(1) items with one-symbol lookahead attached to each item so it distinguishes more contexts, avoiding conflicts that SLR and sometimes LALR cannot; it can recognize all deterministic LR(1) grammars.
Note: LALR is a practical compromise (it merges canonical LR states to reduce table size) and is often sufficient in practice, but it is not strictly as powerful as canonical LR.
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