LECTURE-1 (1).pptx
- 1. PARSER & ITS TYPES WEEK: 10 LECTURE: 1 Program: BS CS/SE 6th Semester Course: CS345 COMPILER CONCEPTS
- 2. Parsing • Syntax analyzers follow production rules defined by means of context-free grammar • How the production rules are implemented results in two types of parsing techniques
- 3. Top-Down Parsing • Parser starts constructing the parse tree from the start symbol • Transform the start symbol to the input Input : a + b * c S → E E → E + T E → E * T E → T T → id Input : a + b * c S → E → E * T → E * T → E + T * T → E + T * id → E + id * id → T + id * id → id + id * id
- 4. Types of Top-Down Parsing
- 5. Cont…… • Recursive Descent Parsing • Common form of top-down parsing • Called recursive as it uses recursive procedures to process the input • Suffers from backtracking. • Backtracking • If one derivation of a production fails, syntax analyzer restarts the process using different rules of same production • Input string may be processed more than once to determine the right production.
- 6. Backtracking Example S → rXd S → rZd X → oa X → ea Z → ai String = read
- 7. Predictive Parser • Capability to predict which production is to be used to replace the input string • Does not suffer from backtracking • Components of Predictive Parser • Uses a stack and a parsing table to parse the input and generate a parse tree • stack and the input contains an end symbol $ to denote that stack is empty and the input is consumed • parser refers to the parsing table to take any decision on the input and stack element combination.