CMSC 331 First Midterm Exam

Transcription

1 0 00/ 1 20/ 2 05/ 3 15/ 4 15/ 5 15/ 6 20/ 7 30/ 8 30/ 150/ 331 First Midterm Exm 7 October 2003 CMC 331 First Midterm Exm Nme: mple Answers tudent ID#: You will hve seventy-five (75) minutes to complete this closed book exm. Use the bcks of these pges if you need more room for your nswers. Describe ny ssumptions you mke in solving problem. We reserve the right to ssign prtil credit, nd to deduct points for nswers tht re needlessly wordy. 0. Wrm up (0 pts) How mny times in 24 hour dy do the minute nd hour hnds of clock exctly overlp? Assume tht the dy strts t the stroke of midnight nd goes up to, but doesn t include, the stroke of midnight 24 hours lter. 22 times. The hnds strt out overlpped. It tlks bit more thn 65 minutes before they overlp gin. After 12 hours, they overlp gin. o, in twelve hours they cn t line up 12 times (the time is more thn n hour), so it must be eleven times. Thus, the time between one overlp nd nother is 12/11 hours or 65 minutes nd 45 seconds. o, in 24 hour durtion, there re ( 24 / (12/11) )overlps, or True/Flse (20 pts) For ech of the following questions, circle T (true) or F (flse). Lrge lnguges re more orthogonl thn smll lnguges. FORTRAN introduced mny control structures, but few dt structures. Reserved words re specil words tht re llowed to be redefined by the progrmmer. In generl, Perl progrms re compiled nd interpreted every time they re executed. In generl, Jv progrms re compiled nd interpreted every time they re executed. LIP nd ML re often cited s exmples of impertive progrmming lnguges. Every ttribute grmmr cn be rewritten s BNF grmmr with possibly more non-terminls nd more rules. A grmmr G is mbiguous if there is more thn one derivtion for t lest one sentence in the lnguge defined by G. A grmmr G is mbiguous if there is more thn one prse tree for t lest one sentence in the lnguge defined by G. In n ttribute grmmr, node's synthesized ttributes cn be bsed on the vlues ssocited with tht node's children. In most progrmming lnguges tht hve infix opertors, +, -, * nd / re right ssocitive. A recursive descent prser for lnguge cn not be bsed directly on grmmr tht hs direct or indirect left recursion. Most modern progrmming lnguges use both dynmic nd sttic scoping for vribles. A vrible s lifetime is the time during which the vrible is bound to storge loction in memory. An LR prser lwys scns the input from left to right nd produces rightmost derivtion if one exists for the input string. The ide behind opertionl semntics is to define the mening of sttements in progrmming lnguge by trnslting them into sttements in nother lnguge. Axiomtic semntics is n pproch tht is often used to prove tht progrms correctly implement their requirements. Pge 1 of 6

2 331 First Midterm Exm 7 October 2003 The lw of consequences sys tht if sttement hs precondition P nd post-condition Q, we cn replce P with ny P tht implies P nd replce Q with ny Q tht is implied by Q. In generl, for grmmr to be good fit for bottom up prser like n LR prser, it must stisfy the pir-wise disjointness test. Every regulr expression cn be expressed s deterministic finite utomton (lso known s deterministic finite stte mchine). 2. Mtch lnguges to domins (5 pts) Write the letter corresponding with the ppliction domin ech progrmming lnguge ws designed to support in the spce provided. Ech letter should be used exctly once. B_ C E_ COBOL _D FORTRAN A_ LIP C JAVA (A) Artificil Intelligence (B) ystem progrmming (C) Internet progrmming (D) cientific pplictions (E) Business pplictions 3. Attributes of implementtion techniques (15 pts) Here re four techniques tht cn be used to implement progrmming lnguge. For ech one, briefly describe the significnt ttributes tht strongly fvor or disfvor the pproch. You might consider ttributes such s debugging ese, execution speed, trnsltion speed, compctness, portbility, etc. () direct execution on hrdwre (b) compiltion to low level lnguge such s mchine lnguge (c) compiltion to n intermedite lnguge (e.g., byte code ) tht is then interpreted in softwre (d) direct interprettion in softwre () is n pproch seldom used for higher level progrmming lnguges. To some degree, it hs been used for some specilized lnguges such s Lisp nd Prolog. This pproch would be expected to give the fstest execution speed s well s no trnsltion speed nd compct representtion (i.e., the source code itself). Wek points will be debugging nd portbility the lnguge would run only on the given hrdwre. (b) is the pproch tken by lnguges like C, which is compiled down to mchine lnguge. The execution speed should be very fst, but the trnsltion time might be long, the portbility of the trnsltion (the object code) will not be portble, nd debugging the executble will be primitive. (c) is the pproch used for mny lnguges tody, including Jv. Execution speed is ok, but not s good s in (b). Object code size is resonble between source code nd mchine lnguge. Debugging is probbly not so good. Portbility is big win. (d) is the pproch trditionlly used in lnguges like Lisp, Prolog nd Bsic. Trnsltion time is miniml, compctness is high, portbility is high nd debugging ese is very good. The only negtive is execution speed. Pge 2 of 6

3 progrm min; 331 First Midterm Exm 7 October 2003 vr x, y : integer; 4. ttic nd dynmic scope (15 pts) Consider the progrm in the box to the right. Assuming dynmic scoping, wht is the output of: ttement 1: 1,2 ttement 2: 1,200 ttement 3: 10,200 ttement 4: 1,200 Assuming sttic scoping, wht is the output of: ttement 1: 1,2 ttement 2: 1,200 ttement 3: 10,200 ttement 4: 1,200 procedure sub1; vr y : integer; procedure sub2; print x, y; //sttement 1 end; // sub2 x = 1; y = 2; sub2; end; // sub1 procedure sub3; vr x : integer; x = 10; print x, y; //sttement 3 end; // sub3 x = 100; y = 200; sub1; print x, y; //sttement 2 sub3; print x, y; //sttement 4 end; // min 5. Ambiguous grmmrs (15 pts) Which of the following grmmrs G1 nd G2 is mbiguous. G1: <> ::= x <> z x <> y <> z G2: <> ::= x <> x <> y <> () (5 pts) Which of these two grmmrs is mbiguous: G1 G2 (circle one) (b) (10 pts) how tht this grmmr is mbiguous by giving two different prse trees for single sentence in the lnguge defined by the grmmr. x X y The sentence xxxy hs two prse trees. As the following digrm shows. x x X y x Pge 3 of 6

4 331 First Midterm Exm 7 October Opertors (20 pts) Given the following BNF grmmr for lnguge with two infix opertors represented by % ::= <br> <br> ::= <br > % ::= ( ) b c A. (5 pts) Give prse tree for the following string: b % % ( b ) B. (5 pts) Circle the ssocitivity of opertor: left right neither C. (5 pts) Circle the ssocitivity of the % opertor: left right neither D. (5 pts) Which opertor hs higher precedence: neither c <br> <br> % <br> c <br> b <br> c Pge 4 of 6

5 331 First Midterm Exm 7 October pecifying lnguge (30 pts) This problem sks you to define simple lnguge using deterministic finite utomton (DFA), regulr expression (RE) nd context free grmmr (CFG). The lnguge is tht consisting of single emil ddress. We won't cover the full syntx of vlid emil ddresses specified by IETF RFC 822, but just simple ones like foo@br.com nd 331@cs.umbc.edu. Here's n English description of the kinds of emil ddresses we wnt to recognize: An emil ddress is UERNAME followed by the "@" chrcter followed by HOT. A UERNAME is one or more lphnumeric chrcters. A HOT is composed of two or more DOMAINs seprted by the chrcter ".". A DOMAIN is sequence of one or more lphnumeric chrcters. To mke it esier, ssume tht ALPHA represents ny lphbetic chrcter nd DIGIT represents ny. () (10 pts) Drw DFA for this lnguge. Mrk the ccepting sttes with double circle. Lbel ech trnsition rrow with either n input chrcter or one of the specil tokens ALPHA or DIGIT. (b) (10 pts) Write RE to recognize this lnguge using Lex's nottion, gin ssuming tht ALPHA represents ny lphbetic chrcter (e.g., [-za-z] nd DIGIT represents ny (e.g., [0-9]). (c) (10 pts) Recst your lnguge s CFG using the non-terminl s the strt symbol nd whtever other non-terminls you wnt. Agin, tret DIGIT nd ALPHA s tokens, so you need not expnd them This exmple shows DFA for the RE 0?[12] which might lso be described using the CFG: -> 0A A A -> 1 RE: (lph (lph )+. ((lph )+)+ CFG: Emil -> Host Usernme -> Alphnums Host -> Alphnums. Alphnums Alphnums. Host Alphnums -> Alphnum Alphnum Alphnums Alphnum -> lph Pge 5 of 6

6 331 First Midterm Exm 7 October LR prsing (30 pts) Consider n LR prser for the following simple expression grmmr, producing the tble to the right. 1: E -> E+T 2: E -> T 3: T -> T*F 4: T -> F 5: F -> (E) 6: F -> id () (1pt) How mny sttes re there? eleven (b) (1pt) Wht does the $ represent? End of input (c) (1pt) In n ction 5, wht does the 5 represent? hift nd go to stte five (d) (1pt) In the ction R2, wht does the 2 represent? Reduce using the second grmmr rule (e) (1pt) Wht does the ction ccept represent? The input is in the lnguge nd prsed ok (f) (15 pts) Complete the following tble which shows the first five steps of n LR prser prsing the input string id*id*id TACK INPUT ACTION 0 id*id*id$ hift 5 0 id 5 *id*id$ Reduce 6, use goto(0,f) 0 F 3 *id*id$ Reduce 4, use goto(0,t) 0 T 2 *id*id$ hift 7 0 T 2 * 7 id*id$ hift 5 (g) (10 pts) For the sententil form T*F*id, give the phrses, simple phrses nd hndle. This sententil form corresponds to the prse tree: (E (T (T (T * F)) * (F (id)))) Phrses: imple phrses: Hndle: T*F*id, T*F, id T*F, id T*F Pge 6 of 6