UP Lecture 16

Compilers
First Set
(202603)

Transcript

1. jgs Compilers Lecture 16: Error Handling – First Set Dr. Javier Gonzalez-Sanchez jgonzalezs@up.edu.mx
2. Dr. Javier Gonzalez-Sanchez | Compilers | 2 jgs First Attempt public void error () { System.out.println(”error”); System.exit(); }
3. Dr. Javier Gonzalez-Sanchez | Compilers | 3 jgs Second Attempt private void error(int error) throws Exception { throw new Exception( "Error " + error + " at word " + tokens.get(currentToken).getValue() ); }
4. Dr. Javier Gonzalez-Sanchez | Compilers | 4 jgs 4 •(one error and stop) •PANIC MODE 5 •Implement error synchronization •ERROR RECOVERY What Next
5. jgs Error Recovery
6. Dr. Javier Gonzalez-Sanchez | Compilers | 6 jgs Error Recovery Error recovery is about ignoring tokens. ▪ How do we know which tokens should be ignored and how many? RULE What should be in place for me to enter? What should be in place for me to leave?
7. Dr. Javier Gonzalez-Sanchez | Compilers | 7 jgs Error Recovery What should be in place for me to enter? What should be in place for me to leave? T NT T NT What should be in place for me to enter?
8. Dr. Javier Gonzalez-Sanchez | Compilers | 8 jgs Parser | Error Recovery Rule FIRST set FOLLOW set PROGRAM { EOF BODY FIRST (PRINT) U FIRST (ASIGNMENT) U FIRST(VARIABLE) U FIRST (WHILE) U FIRST(IF) U FIRST (RETURN) } PRINT print ; ASSIGNMENT identifier ; VARIABLE int, float, boolean, void, char, string ; WHILE while } U FIRST(BODY) IF if } U FIRST(BODY) RETURN return ; EXPRESSION FIRST(X) ), ; X FIRST(Y) | U FOLLOW(EXPRESSION) Y ! U FIRST(R) & U FOLLOW(X) R FIRST(E) FOLLOW(Y) E FIRST (A) !=, ==, >, < U FOLLOW(R) A FIRST (B) -, + U FOLLOW(E) B - U FIRST (C) *, /, U FOLLOW(A) C integer, octal, hexadecimal, binary, true, false, string, char, float, identifier, ( FOLLOW(B)
9. jgs Calculating the First Set
10. Dr. Javier Gonzalez-Sanchez | Compilers | 10 jgs FIRST set Definition FIRST (a) is the set of tokens that can begin the construction a. Example <E> → <A> {+ <A>} <A> → <B> {* <B>} <B> → -<C> | <C> <C> → integer FIRST(E) = {-, integer} FIRST(A) = {-, integer} FIRST(B) = {-, integer} FIRST(C) = {integer}
11. Dr. Javier Gonzalez-Sanchez | Compilers | 11 jgs Calculate the FIRST set loop
12. Dr. Javier Gonzalez-Sanchez | Compilers | 12 jgs FIRST set PROGRAM FIRST(PROGRAM) = { "{"}
13. Dr. Javier Gonzalez-Sanchez | Compilers | 13 jgs FIRST set Define FIRST (BODY) FIRST(BODY) = FIRST (PRINT) U FIRST (ASSIGNMENT) U FIRST(VARIABLE) U FIRST(WHILE) U FIRST(IF) U FIRST(RETURN)
14. Dr. Javier Gonzalez-Sanchez | Compilers | 14 jgs FIRST set Define FIRST (C)
15. Dr. Javier Gonzalez-Sanchez | Compilers | 15 jgs FIRST set Define FIRST (A) Define FIRST (B)
16. Dr. Javier Gonzalez-Sanchez | Compilers | 16 jgs FIRST set <S> → <A><B><C> <S> → <F> <A> → <E><F>d <A> → a <B> → a<B>b <B> → ε <C> → c<C> <C> → d <E> → e<E> <E> → <F> <F> → <F>f <F> → ε
17. Dr. Javier Gonzalez-Sanchez | Compilers | 17 jgs FIRST set S → ABC S → F A → EFd A → a B → aBb B → ε C → cC C → d E → eE E → F F → Ff F → ε rule FIRST set - evolution S ø {a, ε} {a, ε, e, f} {a, ε, e, f, d} A ø {a} {a, e} {a, e, f, d} B ø {a, ε} C ø {c, d} E ø {e} {e, ε} {e, ε, f} F ø {ε} {ε, f}
18. jgs Test Yourselves Calculate FIRST set
19. Dr. Javier Gonzalez-Sanchez | Compilers | 19 jgs FIRST set | Exercise <X> → <A> | <A> a <A> → <B> | <B> b <B> → <C><D><E> | c d e | <C> c <D> d <E> e <C> → one <D> → two <E> → three
20. Dr. Javier Gonzalez-Sanchez | Compilers | 20 jgs FIRST set | Exercise <X> → <A> <X> → <A> a <A> → <B> <A> → <B> b <B> → <C><D><E> <B> → c d e <B> → <C> c <D> d <E> e <C> → one <D> → two <E> → three OPTION 1: FIRST(X) = {c, one} FIRST(A) = {c, one} FIRST(B) = {c, one} FIRST(C) = {one} FIRST(D) = {two} FIRST(E) = {three} OPTION 2: FIRST(X) = {c, one, ε} FIRST(A) = {b, c, one} FIRST(B) = {c, one} FIRST(C) = {one} FIRST(D) = {two} FIRST(E) = {three}
21. Dr. Javier Gonzalez-Sanchez | Compilers | 21 jgs FIRST set | Exercise <X> → <A> <X> → <A> a <A> → <B> <A> → <B> b <B> → <C><D><E> <B> → c d e <B> → <C> c <D> d <E> e <C> → one <C> → ε <D> → two <D> → ε <E> → three FIRST(X) = {c, one, three, two} FIRST(A) = {c, one, three, two} FIRST(B) = {c, one, three, two} FIRST(C) = {one, ε} FIRST(D) = {two, ε} FIRST(E) = {three}
22. Dr. Javier Gonzalez-Sanchez | Compilers | 22 jgs Next What about the FIST set for the rules in our Language?
23. Dr. Javier Gonzalez-Sanchez | Compilers | 23 jgs Homework Work on your Parser
24. Dr. Javier Gonzalez-Sanchez | Compilers | 24 jgs Questions
25. jgs Compilers Javier Gonzalez-Sanchez, Ph.D. jgonzalezs@up.edu.mx Spring 2026 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.