1. COP 4620 / 5625 Programming Language Translation / Compiler Writing Fall 2003 Lecture 7, 10/09/2003
Prof. Roy Levow

2. Bottom-up Parsing Almost always automatic generation
Because algorithm is complex
Uses Push-down Automata
Basic principles are can be seen with LR(0) though it is of little practical value
“handle” is complete sequence of children that can be “reduced”

3. LR(0) Using dotted notation
A shift item has dot anywhere but end
A reduce item has dot at end
Item sets are sets of items that can occur in a state

4. Sample Grammar for LR(0)

5. Transition Diagram

6. LR(0) PDA

7. Example LR(0) Parse

8. LR Parsing with Lookahead
There are a number of variants for LR parsing with lookahead
Problem is that tables for direct approaches can be very large
1M for LR(1)
10K for SLR(1)
1K for LALR(1)
Smaller size reduces class of languages that can be parsed

9. Example for LR(1)

10. LALR(1) Automaton, fig

11. Conflicts & Error Recovery
Grammars that are not LR(1) exhibit
Shift-Reduce Conflicts
Reduce-Reduce Conflicts
Adding actions often generates conflicts
Error recovery
Add symbol in input
Pop items from stack and input to recovery point