Lecture Notes for CPSC 326 (Spring 2013) - Final Exam Preparation, Study notes of Programming Languages

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Today ...

• Final exam overview
• Semantic analysis (cont.)

Final Exam

Basics

• Wed May 8, 10:30–12:30 (HK 301)
• Closed book, notes, computer, etc.
• 20% of final grade
• likely around 8 questions
• Cummulative ... split between:
  • Haskell, Prolog, language implementation, big ideas (reading)

Dealing with left-associative operators

• One approach is to rewrite the AST after parsing
  • similar to applying rotations in Red-Black or AVL trees
• Another is to modify the grammar and recursive-descent parser
• ... to construct the correct AST

Example:

e → val ( ÷ val )∗

• for left-associative ops use iteration (Kleene star)
• for right-associative ops use (tail) recursion

Expression* expr() { Expression* v1 = val(); advance(); while(tok.type() == DIV) { advance(); Expression* v2 = val(); v1 = new Div(v1, v2); } return v1; }

• For instance: v1 = Num(40) v2 = Num(10), v1 = Div(Num(40), Num(10)) v2 = Num(2), v1 = Div(Div(Num(40), Num(10)), Num(2))

Operator precedence

• Division (/) has higher precedence than addition (+)
• For example:

2 + 3 / 4 ≡ 2 + (3 / 4) 2 / 3 + 4 ≡ (2 / 3) + 4

One solution: Encode precedence in the grammar

e → t ( ‘+’ t )∗

t → num ( ’/’ num )∗

• This is equivalent to ... e → t e′ e′^ → ‘+’ t e′^ |  t → num t′ t′^ → ‘/’ num t′^ | 

Q: What is the parse tree for: 2 + 3 / 4?