CMSI 386 Homework 5 solution


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1. Frank Rubin used the following exam ple (rewritten here in C)to argue in favorofa goto statem ent:

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< n ; i + + ) { / * f o r e a c h r o w * / f o r ( j = 0 ; j < n ; j + + ) { / * f o r e a c h e n t r y i n t h e r o w * / } i f ( A [ i ] [ j ] ) g o t o n e x t ; / * i f n o n - z e r o g o o n t o t h e n e x t r o w * / f i r s t _ z e r o _ r o w = i ; / * w e n t a l l t h r o u g h t h e r o w , y o u g o t i t ! * / b r e a k ; / * g e t o u t o f t h e w h o l e t h i n g * / } n e x t : ; / * f i r s t _ z e r o _ r o w i s n o w s e t * / The intentofthe code is to setfirst_zero_row to the index ofthe firstall-zero row,ifany,ofan n × n m atrix,or-1 ifno such row exists.Do you find the exam ple convincing? Is there a good structured alternative in C? In any language? G ive answers in the form ofa shortessay.Include a good introductory section,a background section describing views on the goto statem entthroughouthistory,a beefy section analyzing alternatives to Rubin's problem ,and a good concluding section.Talk aboutsolutions in atleastthree languages. 2. Foreach ofthe following sets ofstrings,write functions orm ethods in two languages chosen from {Python, JavaScript,Java,Ruby,Clojure,C++}to return whethera given string is in the set,using regularexpression m atching.You willlikely find,forsom e ofthe problem s,thatgiving a finite state m achine forthe setis helpful. You m ay include a diagram ofyourm achine in yoursubm ission butitwillnotaffectyourgrade. a. Strings ofcharacters beginning and ending with a double quote character,thatdo notcontain control characters,and forwhich the backslash is used to escape the nextcharacter.(These are sim ilarto,but notexactly the sam e as,string literals in C.) b. The paren-starform ofcom m ents in Pascal:strings beginning with (* and ending with *) thatdo not contain *).Note com m ents "don'tnest". c. Num bers in JSO N. d. Allnon-em pty sequences ofletters otherthan "read","red",and "real". 3. W rite a tail-recursive function to com pute the m inim um value ofan array in Python,C,JavaScript,and G o. O bviously these languages probably already have a m in-value-in-array function in a standard library,butthe purpose ofthis problem is foryou to dem onstrate yourunderstanding oftailrecursion.Yoursolution m ustbe in the classic functionalprogram m ing style,thatis,itm ustbe stateless.Use param eters,notnonlocal variables,to accum ulate values. 4. Here's som e code in som e language thatlooks exactly like C++.It's sortoflike G o,also,exceptthe pointer types are kind ofbackwards.Itis defining two m utually recursive types,A and B. 1/4 1/30/2014 CM SI386:Programming Languages:Homework #5 s t r u c t A { B * x ; i n t y ; } ; s t r u c t B { A * x ; i n t y ; } ; Suppose the rules forthis language stated thatthis language used structuralequivalence fortypes.How would you feelifyou were a com pilerand had to typecheck an expression in which an A was used as a B? W hatproblem m ightyou run into? 5. W rite a program in C++,JavaScript,Python,Ruby,Scala,orClojure thatdeterm ines the orderin which subroutine argum ents are evaluated. 6. Considerthe following (erroneous)program in C: v o i d f o o ( ) { i n t i ; } p r i n t f ( " % d " , i + + ) ; i n t m a i n ( ) { i n t j ; } f o r ( j = 1 ; j < = 1 0 ; j + + ) f o o ( ) ; Localvariable i in subroutine foo is neverinitialized.O n m any system s,however,the program willdisplay repeatable behavior,printing 0 1 2 3 4 5 6 7 8 9 .Suggestan explanation.Also explain why the behavioron othersystem s m ightbe different,ornondeterm inistic. 7. Considerthe following pseudocode: v a r x = 1 0 0 ; f u n c t i o n s e t X ( n ) { x = n ; } f u n c t i o n p r i n t X ( ) { c o n s o l e . l o g ( x ) ; } f u n c t i o n f o o ( S , P , n ) { v a r x ; i f ( n = = = 1 | | n = = = 3 ) { s e t X ( n ) ; } e l s e { S ( n ) ; } i f ( n = = = 1 | | n = = = 2 ) { p r i n t X ( ) ; } e l s e { P ( ) ; } } s e t X ( 0 ) ; f o o ( s e t X , p r i n t X , 1 ) ; p r i n t X ( ) ; s e t X ( 0 ) ; f o o ( s e t X , p r i n t X , 2 ) ; p r i n t X ( ) ; s e t X ( 0 ) ; f o o ( s e t X , p r i n t X , 3 ) ; p r i n t X ( ) ; s e t X ( 0 ) ; f o o ( s e t X , p r i n t X , 4 ) ; p r i n t X ( ) ; Assum e thatthe language uses dynam ic scoping.W hatdoes the program printifthe language uses shallow binding? W hatdoes itprintwith deep binding? W hy? 8. In som e im plem entations ofan old language called Fortran IV,the following code would printa 3.Can you suggestan explanation? (Hint:Fortran passes by reference.)M ore recentversions ofthe Fortran language don'thave this problem .How can itbe thattwo versions ofthe sam e language can give differentresults even though param eters are officially passed "the sam e way"? Note thatknowledge ofFortran is notrequired for this problem . c a l l f o o ( 2 ) p r i n t * 2 s t o p e n d s u b r o u t i n e f o o ( x ) x = x + 1 e n d r e t u r n 2/4 1/30/2014 CM SI386:Programming Languages:Homework #5 9. Considerthe problem ofdeterm ining whethertwo trees have the sam e fringe:the sam e setofleaves in the sam e order,regardless ofinternalstructure.An obvious way to solve this problem is to write a function fringe thattakes a tree as argum entand returns an ordered listofits leaves.Then we can say d e f s a m e _ f r i n g e ( t 1 , t 2 ) : r e t u r n f r i n g e ( t 1 ) = = f r i n g e ( t 2 ) a. W rite a straightforward version offringe in Python orJavaScript. b. G iven yourstraightforward code in part(a),how efficientis same_fringe when the trees differin their firstfew leaves? c. Ifyou answered part(b)correctly,you know thatwhatyou need forefficiency is laziness.W rite an efficient(lazy)version ofsame_fringe in Python. d. W rite an efficient(lazy)version ofsame_fringe in JavaScriptorG o. 10. Explain whatis printed under(a)callby value,(b)callby value-result,(c)callby reference,(d)callby nam e. x = 1 ; y = [ 2 , 3 , 4 ] ; s u b f ( a , b ) { b + + ; a = x + 1 ; } f ( y [ x ] , x ) ; p r i n t x , y ; 11. I've written a sim ple JavaScriptqueue type thatdoes notuse encapsulation.Can we achieve encapsulation using the m odule system in node.js? Ifso,im plem entit.Ifnot,state why not. 12. Itis certainly possible to m ake a Person class,then subclasses ofPerson fordifferentjobs,like M anager, Em ployee,Student,M onitor,Advisor,Teacher,O fficerand so on.Butthis is a bad idea,even though the IS-A testpasses.W hy is this a bad idea and how should this society ofclasses be built? 13. W rite in Java,Python,JavaScript,and C++,a m odule with a function called nextOdd (ornext_odd or next-odd depending on the nam ing conventions ofthe language's culture.The firsttim e you callthis subroutine you getthe value 1.The nexttim e,you geta 3,then 5,then 7,and so on.Show a snippetofcode thatuses this subroutine from outside the m odule.Is itpossible to m ake this m odule hack-proof? In other words,once you com pile this m odule,can you be sure thatm alicious code can'tdo som ething to disruptthe sequence ofvalues resulting from successive calls to this function? 14. W hathappens to the im plem entation ofa class ifwe "redefine"a field in a subclass? Forexam ple,suppose we have: c l a s s F o o { p u b l i c i n t a ; p u b l i c S t r i n g b ; } . . . c l a s s B a r e x t e n d s F o o { p u b l i c f l o a t c ; p u b l i c i n t b ; } Does the representation ofa Bar objectcontain one b field ortwo? Iftwo,are both accessible,oronly one? Underwhatcircum stances? AnswerforC++,Java,and Python.