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1
Bottom-up parsing
 Goal of parser : build a derivation
 Top-down parser : build a derivation by working from the
start symbol towards the input.
 Builds parse tree from root to leaves
 Builds leftmost derivation
 Bottom-up parser : build a derivation by working from the
input back toward the start symbol
 Builds parse tree from leaves to root
 Builds reverse rightmost derivation
2
Example: Shift-reduce parsing
1. S  E
2. E  E + E
3. E  E * E
4. E  num
5. E  id
Input to parse:
id1 + num * id2
Accept
$ S
Reduce (rule 1)
$ E
Reduce (rule 2)
$ E + E
Reduce (rule 3)
$ E + E * E
Reduce (rule 5)
$ E + E * id2
Shift
$ E + E *
Shift
$ E + E
Reduce (rule 4)
$ E + num
Shift
$ E +
Shift
$ E
Reduce (rule 5)
$ id1
Shift
$
ACTION
Grammar:
Handles:
underlined
STACK

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shift and reduce.ppt

  • 1. 1 Bottom-up parsing  Goal of parser : build a derivation  Top-down parser : build a derivation by working from the start symbol towards the input.  Builds parse tree from root to leaves  Builds leftmost derivation  Bottom-up parser : build a derivation by working from the input back toward the start symbol  Builds parse tree from leaves to root  Builds reverse rightmost derivation
  • 2. 2 Example: Shift-reduce parsing 1. S  E 2. E  E + E 3. E  E * E 4. E  num 5. E  id Input to parse: id1 + num * id2 Accept $ S Reduce (rule 1) $ E Reduce (rule 2) $ E + E Reduce (rule 3) $ E + E * E Reduce (rule 5) $ E + E * id2 Shift $ E + E * Shift $ E + E Reduce (rule 4) $ E + num Shift $ E + Shift $ E Reduce (rule 5) $ id1 Shift $ ACTION Grammar: Handles: underlined STACK