BSCS Fall Mid Term Examination December 2012

Transcription

1 PUNJAB UNIVERSITY COLLEGE OF INFORMATION TECHNOLOGY University of the Punjab Sheet No.: Invigilator Sign: BSCS Fall 2009 Date: Mid Term Examination December 2012 Student ID: Section: Morning / Afternoon Student Name: Student Signature: Course Code: CS 420 Course Title: Compiler Construction Maximum Time: 90 Minutes Maximum Marks: 185 Instructions There is only one instruction: We have zero tolerance to cheating. F grade will be given to a cheater without any prior warning. Question # Total Total Marks Obtained Examiner Signature: Marks Out of 185: DO NOT OPEN UNTIL YOU ARE TOLD TO DO SO Page 1 of 10

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3 Midterm Examination CS420: Compiler Construction Instructor: Prof. Dr. Faisal Aslam Total Marks: 185 Mark each question as true (T) or false (F). Marks: The input of lexical analyzer is source program. 2. The output of lexical analyzer is a tree. 3. The input of intermediate code optimizer is an intermediate representation. 4. The input of semantic analyzer is a list of tokens. 5. Usually a program using JIT executes faster than a compiled program 6. Regular languages are more powerful than languages expressed using NFA. 7. All languages expressed using NFAs can also be expressed using DFAs. 8. LL(k) parsers are used for bottom up parsing. 9. LR(k) parsers read input from left to right. 10. A NFA can have many start states. 11. A CFG is more powerful (i.e. can express more languages) than a Regular language. 12. LL(k) parsers can accept more languages than LR(k) parsers. 13. LR(k) parsers can use CFG with left recursion. 14. LR(k) parsers can use ambiguous grammars. 15. DFA can have multiple final states. 16. DFA transition table will never have a null/empty entry. 17. Java bytecode is language dependent but machine independent. 18. Interpreter is usually easier to port on a new machine as compared to a compiler. 19. Each NFA can be expressed using a CFG 20. Context sensitive grammars are less powerful than context free grammars. 21. Recursive descent parsing is used for top down parsing. 22. LL(k) parser can be based on either rightmost or leftmost derivations. 23. The Java code a = = b + c has an error that a lexical analyzer cannot find. 24. The Java code a = = b + c has an error that syntax analyzer cannot find. 25. Compiler's Front-end is machine dependent. Page 3 of 10

4 Q1: Write a regular expression for all possible strings of 0 s and 1 s with at most single 1. (e.g , , and ) Q2: Write a NFA for the regular expression ab? ((b a) + c) + Q3: Write transition table of the NFA created in Q2. Show only first five states. a b c ε Q4: Show that the following grammar is ambiguous using the input string aaa. (Hint: you will need to create two parse trees) R R R R R R R R * R ( R ) R a R b Page 4 of 10

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6 Q5: Find FIRST and FOLLOW of each non-terminal. E abt T (L)T [E]T a ε L ER R, ER b Marks 15 FIRST(E) = FOLLOW(E) = FIRST(T) = FOLLOW(T) = FIRST(L) = FOLLOW(L) = FIRST(R) = FOLLOW(R) = Q 6: Create LL(1) parsing table for the CFG of Q5. Marks 15 a b ( ) [ ], $ E T L R Q7: Given the following parsing table (M) use the LL(1) parsing algorithm to validate the input (id+id). Show next 7 stack state and input processed at each stage. Marks 15 id + * ( ) $ E TE' TE' E' +TE' ε ε T FT' FT' T' ε *FT' ε ε F id (E) Stack 1 $ (id+id) 2 $E (id+id) 3 $E'T (id+id) 4 $E'T'F (id+id) 5 6 Input Pointer (remove input characters as pointer moves ahead) Page 6 of 10

7 Q8: Remove indirect left recursion from the following CFG. A Ba b B Cd e C Df g D Df Aa Cg Marks 15 Q9: Transform the following grammar by apply left factoring. Z X X Y aec Y aed f Y a b Page 7 of 10

8 Q10: Above is a finite automaton for LR(k) parser. Show only next 5 steps to verify the input string (id+id) using LR(k) parser. Stack Step # States Symbols Input Action Handle 1 0 $ (id+id) shift to $( id+id) shift to $(id +id) reduce using 3 id Q11: Write CFG for the given NFA. Page 8 of 10

9 Q12: Given the following stack of the LR(k) construct parse tree. Also write separately the input and the start symbol. LR(k) Stack Trace Start Symbol: Input Sentence: Parse Tree: x F T T* T*( T*(y T*(F T*(T T*(E T*(E) T*F T E Marks Page 9 of 10

10 Q13: Mark the last five values (nodes) added in the following tree created using recursive descent parsing. No marks will be given if you have marked more than five values. (For example you can mark "Program, Block, Var and y") Q14: Subset construction algorithm is used to create DFA from the NFA. Show next five function and subset s created using the NFA of Q11. function subset 1 ε-closure(q0) {q0} Page 10 of 10