1. Shift-Reduce Parsing Example 1
Mr. Lupoli
F2012

2. Mechanics Goal Shift Reduce
trace that the given input can be reduced BACK to the original start symbol
Shift
marker that moves along the input
from left to right
what is on left needs to be reduced
match with production table
Reduce
apply the inverse production

3. What’s Given E → int E  E + (E) Original Production
Syntax to be checked
int + (int) + (int)

4. E → int
E  E + (E)
| int + (int) + (int)

5. E → int E  E + (E) int | + ( int ) + ( int ) E | + ( int ) + ( int )
we have a match!! Reduce
E | + ( int ) + ( int )

6. E → int
E  E + (E)
E + | ( int ) + ( int )
no match shift 

7. E → int
E  E + (E)
E + ( | int ) + ( int )
no match shift 

8. E → int E  E + (E) E + ( int | ) + ( int ) E + ( E | ) + ( int )
match !!! Reduce
E + ( E | ) + ( int )

9. E → int
E  E + (E)
E + ( E ) | + ( int )
match!! Reduce
E | + ( int )

10. E → int
E  E + (E)
E + | ( int )
no match, shift 

11. E → int
E  E + (E)
E + ( | int )
no match, shift 

12. E → int
E  E + (E)
E + ( int | )
found a match!! Reduce!!
E + ( E | )

13. E → int
E  E + (E)
E + ( E ) |
found a match!! Reduce!! E

14. E → int
E  E + (E) E
starting production!!! ACCEPT!!!

15. In total  int + (int) + (int)$ shift
int  + (int) + (int)$ red. E  int
E  + (int) + (int)$ shift 3 times
E + (int  ) + (int)$ red. E  int
E + (E  ) + (int)$ shift
E + (E)  + (int)$ red. E  E + (E)
E  + (int)$ shift 3 times
E + (int  )$ red. E  int
E + (E  )$ shift
E + (E)  $ red. E  E + (E)
E  $ accept

16. Sources
CS 671 – University of Virginia