Parsing (Automata)
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This document discusses different types of parsers used in parsing strings based on production rules of a grammar. It describes: 1) Top-down parsers which start from the start symbol and derive strings using a parse tree from top to bottom, and bottom-up parsers which start from the string and build the parse tree from bottom to top. 2) Types of top-down parsers including recursive descent parsing which uses recursive procedures and may involve backtracking, and types of bottom-up parsers including SLR(1), LR(1), and LALR(1) parsers. 3) Examples of top-down and bottom-up parsing of strings based on sample grammars to illustrate the parsing process.
Parsing (Automata)
- 1. PARSING
- 2. GROUP MEMBERS • ROOP SAGAR (2016-CS-121) • GHULAM SARWAR KHAN (2016-CS-128) • SAHIL (2016-CS-106)
- 3. PARSING Parsing is used to derive a string using the production rules of a grammar. It is used to check the acceptability of a string. Compiler is used to check whether or not a string is syntactically correct. A parser takes the inputs and builds a parse tree.
- 4. TYPES OF PARSER TDP BUP PARSER TDP WITH BACKTRACKING OPERATOR PRECEDENCE PARSER LRTDP WITHOUT BACKTRACKING TDP RECURSIVE TDP NON RECURSIVE
- 5. TYPES OF PARSER • A parser can be of two types. • Top-Down Parser : Top-down parsing starts from the top with the start- symbol and derives a string using a parse tree. • Bottom-Up Parser : Bottom-up parsing starts from the bottom with the string and comes to the start symbol using a parse tree.
- 6. TYPES OF TOP DOWN PARSER • Recursive descent parsing : It is a common form of top-down parsing. It is called recursive as it uses recursive procedures to process the input. Recursive descent parsing suffers from backtracking. • Backtracking : It means, if one derivation of a production fails, the syntax analyzer restarts the process using different rules of same production. This technique may process the input string more than once to determine the right production.
- 7. TYPES OF BOTTOM UP PARSER • SLR(1) – Simple LR Parser: • Works on smallest class of grammar • Few number of states, hence very small table • Simple and fast construction • LR(1) – LR Parser: • Works on complete set of LR(1) Grammar • Generates large table and large number of states • Slow construction • LALR(1) – Look-Ahead LR Parser: • Works on intermediate size of grammar • Number of states are same as in SLR(1)
- 8. TOP DOWN PARSER For Example, consider the grammar • E ::= E+T | T (E is the start symbol) • T ::= T*F | F • F ::= id (assume some finite set of possible ids) Here is a top down parse of the sentence x + y * z. Note that it is LL: we always expand the leftmost nonterminal node. + E T E T F X F Z Y * T F
- 9. TOP DOWN PARSER For Example, consider the grammar • E ::= a A B e (E is the start symbol) • A ::= b | A b c • B ::= d Here is a top down parse of the sentence a b b c d e Note that it is LL: we always expand the leftmost nonterminal node. Aa E A b c b e B d
- 10. BOTTOM UP PARSER S A B A a b b c d e For Example, Here is a Bottom up parse of the sentence a b b c d e consider the grammar S ::= a A B e A ::= b | A b c B ::= d Note that it is LR: we always expand the Rightmost node.
- 11. BOTTOM UP PARSER E E T T + T F F * F X Y Z For Example, consider the grammar Here is a Bottom up parse of the sentence x + y * z. E ::= E+T | T T ::= T*F | F F ::= id (assume some finite set of possible ids) Note that it is LR: we always expand the Rightmost node.
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