UP Lecture 13

Compilers
Parser I
(202503)

Transcript

1. jgs Compilers Lecture 13: Implementing a Parser I Dr. Javier Gonzalez-Sanchez jgonzalezs@up.edu.mx
2. jgs Previously …
3. Dr. Javier Gonzalez-Sanchez | Compilers | 3 jgs Rules PROGRAM BODY C EXPRESSION X Y R E A B { 0; 1 + 2; 3 * (4 + hello); }
4. Dr. Javier Gonzalez-Sanchez | Compilers | 4 jgs Rules PROGRAM BODY C A B { 0; 1 + 2; 3 * (4 + hello); } EXPRESSION X Y R E
5. Dr. Javier Gonzalez-Sanchez | Compilers | 5 jgs Let’s Work
6. jgs Parser
7. Dr. Javier Gonzalez-Sanchez | Compilers | 7 jgs Parser BNF EBNF A | AA & B ! Parser Grammar
8. Dr. Javier Gonzalez-Sanchez | Compilers | 8 jgs PREDICTIVE DESCENDENT RECURSIVE PARSER
9. Dr. Javier Gonzalez-Sanchez | Compilers | 9 jgs Parser | Step by Step For each rule in the grammar { ▪ Step 1. The left-hand side (method declaration) ▪ Step 2. The right-hand side (body of the method) as follows: ▪ Step 2.1 Non-terminals (boxes) will be a call to a method ▪ Step 2.2. Terminals (ovals) will be if-conditions ▪ Step 2.3. Lines in a loop (EBNF items) will be loops }
10. Dr. Javier Gonzalez-Sanchez | Compilers | 10 jgs Parser public class Parser { private static Vector<Token> tokens; private static int currentToken; public static void RULE_PROGRAM () {} public static void RULE_BODY () {} public static void RULE_EXPRESSION () {} public static void RULE_X () {} public static void RULE_Y () {} public static void RULE_R () {} public static void RULE_E () {} public static void RULE_A () {} public static void RULE_B () {} public static void RULE_C () {}
11. Dr. Javier Gonzalez-Sanchez | Compilers | 11 jgs Parser public static void RULE_PROGRAM() { if (tokens.get(currentToken).getWord().equals(“{”)) { currentToken++; else error(1); RULE_BODY(); if (tokens.get(currentToken).getWord().equals(“}”)) currentToken++; else error(2); } PROGRAM
12. Dr. Javier Gonzalez-Sanchez | Compilers | 12 jgs Parser public static void RULE_BODY() { while (!tokens.get(currentToken).getWord().equals(“}”)) { RULE_EXPRESSION(); if (tokens.get(currentToken).getWord().equals(“;”)) currentToken++; else error(3); } } BODY
13. Dr. Javier Gonzalez-Sanchez | Compilers | 13 jgs Parser public static void RULE_EXPRESSION() { RULE_X(); while (tokens.get(currentToken).getWord().equals(“|”)) { currentToken++; RULE_X(); } } EXPRESSION
14. Dr. Javier Gonzalez-Sanchez | Compilers | 14 jgs Parser public static void RULE_X() { RULE_Y(); while (tokens.get(currentToken).getWord().equals(“&”)) { currentToken++; RULE_Y(); } } X
15. Dr. Javier Gonzalez-Sanchez | Compilers | 15 jgs Parser public static void RULE_Y() { if (tokens.get(currentToken).getWord().equals(“!”)) { currentToken++; } RULE_R(); } Y
16. Dr. Javier Gonzalez-Sanchez | Compilers | 16 jgs Parser public static void RULE_R() { RULE_E(); while ( tokens.get(currentToken).getWord().equals(“<”) |tokens.get(currentToken).getWord().equals(“>”) |tokens.get(currentToken).getWord().equals(“==”) |tokens.get(currentToken).getWord().equals(“!=”) ) { currentToken++; RULE_E(); } } R
17. Dr. Javier Gonzalez-Sanchez | Compilers | 17 jgs Parser public static void RULE_E() { RULE_A(); while (tokens.get(currentToken).getWord().equals(“-”) | tokens.get(currentToken).getWord().equals(“+”) ) { currentToken++; RULE_A(); } } E
18. Dr. Javier Gonzalez-Sanchez | Compilers | 18 jgs Parser public static void RULE_A() { RULE_B(); while (tokens.get(currentToken).getWord().equals(“/”) | tokens.get(currentToken).getWord().equals(“*”) ) { currentToken++; RULE_B(); } } A
19. Dr. Javier Gonzalez-Sanchez | Compilers | 19 jgs Parser public static void RULE_B() { if (tokens.get(currentToken).getWord().equals(“-”)) { currentToken++; } RULE_C(); } B
20. Dr. Javier Gonzalez-Sanchez | Compilers | 20 jgs Parser public static void RULE_C() { if (tokens.get(currentToken).getToken().equals(“integer”)) { currentToken++; } else if (tokens.get(currentToken).getToken().equals(“identifier”)) { currentToken++; } else if (tokens.get(currentToken).getWord().equals(“(”)) { currentToken++; RULE_EXPRESSION(); if (tokens.get(currentToken).getWord().equals(“)”)) { currentToken++; } else error(4); } } else { error (5); } } C
21. Dr. Javier Gonzalez-Sanchez | Compilers | 21 jgs Questions
22. jgs Compilers Javier Gonzalez-Sanchez, Ph.D. jgonzalezs@up.edu.mx Spring 2025 Copyright. These slides can only be used as study material for the Compilers course at Universidad Panamericana. They cannot be distributed or used for another purpose.