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What does the 'reduce' action do in an LR parser?

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Question: What does the \'reduce\' action do in an LR parser?

Options:

  1. It shifts the next input symbol onto the stack.
  2. It pops symbols from the stack and replaces them with a non-terminal.
  3. It accepts the input string.
  4. It generates an error.

Correct Answer: It pops symbols from the stack and replaces them with a non-terminal.

Solution: 

The \'reduce\' action in an LR parser pops a number of symbols from the stack and replaces them with a non-terminal according to a production rule.

What does the 'reduce' action do in an LR parser?

Practice Questions

Q1
What does the 'reduce' action do in an LR parser?
  1. It shifts the next input symbol onto the stack.
  2. It pops symbols from the stack and replaces them with a non-terminal.
  3. It accepts the input string.
  4. It generates an error.

Questions & Step-by-Step Solutions

What does the 'reduce' action do in an LR parser?
  • Step 1: Understand that an LR parser uses a stack to keep track of symbols while parsing.
  • Step 2: Know that a production rule defines how a non-terminal can be replaced by a sequence of symbols.
  • Step 3: When the parser encounters a situation where a sequence of symbols on the stack matches the right side of a production rule, it performs a 'reduce' action.
  • Step 4: The 'reduce' action means that the parser will remove (pop) those symbols from the stack.
  • Step 5: After popping the symbols, the parser will push (replace) a non-terminal onto the stack that corresponds to the left side of the production rule.
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