CSCI1101& 2nd&Test&Solutions& April&17,&2015&! 1.!!The!following!statements!are!executed:!! x=[3.5, -9, 2.0, 6.7, 4, 2]

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1 CSCI1101& 2nd&Test&Solutions& April&17,&2015& 1.Thefollowingstatementsareexecuted: x=[3.5, -9, 2.0, 6.7, 4, 2] y=((3, 7 ), (4,2), (2,3)) z = {1:['one','un','uno','yi'], 2:['two','deux','dos','er'], 3:['three','trois','tres', 'san']} Answerthefollowingquestionsaboutx,yandz.Insomeparts,ifthequestionasks youwhathappenswhenaparticularstatementisexecuted,therightanswermight be'itresultsinanerror'. (a)whatisthetypeofy?tuple (b)whatissum(x[:2])?f5.5frequentincorrectanswer:f3.5,becauseyouadded x[0]+x[1]+x[2]. (c)whatarelen(z), len(z[2])andlen(z[2][2])?(becareful)3,4,3the admonitiontobecarefulwasforthelastpart.sincezisadictionary,z[2]refersto theiteminthedictionarywithkey2,andsincez[2]isalist,z[2][2]referstothe thirditeminthisdictionary,whichis'dos',astringoflength3.hadzbeendefined withsquareinsteadofcurlybraces,itwouldhavebeenalist,andz[2][2]would referto'tres',withlength4. (d)whatisthevalueofxafterthefollowingstatementisexecuted? x=x+3erroryoucan'taddalistandanint.x+[3]wouldhavebeenokay,andhave value=[3.5, -9, 2.0, 6.7, 4, 2,3] (e)whatisthevalueofxafterthefollowingstatementisexecuted? x.append(3) [3.5, -9, 2.0, 6.7, 4, 2,3] Note that it is the value of x that changes, the expression x.append(3) has no value. (f)whatisthevalueofyafterexecutingthefollowingstatement?(beverycareful) y += (5,6) ((3, 7), (4, 2), (2,3),5,6) Adding two tuples just concatenates the elements of the tuples, but of course this is designed to make you write the incorrect answer ((3, 7), (4, 2), (2,3),(5,6)). (I would feel bad about this trick question if it did not come packaged with a sign

2 that in effect said 'Trick Question') If you really wanted to get ((3,7),(4,2),(2,3),(5,6)) you would write y += ((5,6),) (g)whatisthevalueofyafterexecutingthefollowingstatement? y[2]= (6,2) error Tuples, like strings, are immutable, and you cannot directly assign to one of its components. Assumeforthenextseveralquestionthatx,yandzareasgivenatthebeginningof thisproblem(inotherwords,thatthestatementsdescribedinthequestionsabove havenotbeenexecuted). (h)whatarethetypeandvalueofthefollowingexpression? [z[w[0]][2] for w in y if w[0] in z] type:listvalue:['tres','dos'] Afewpeoplegavethewrongtype,butevenifyoucan'tdecipherthelist comprehension,youshouldknowthatlistcomprehensionalwaysgivesyoualist. Sincetheonlyelementswofywithw[0]havingthevalueofakeyinzare(3,7)and (2,3),thisisequivalentto [z[3][2],z[2][2]],whichis['tres','dos'],thoughiaccepted['dos','tres']without penalty. 2.Considerthefunctionprob2definedbelow: def prob2(nums): u=[(int(math.sqrt(x)),x) for x in nums] v=[y for y in u if y[0]**2==y[1]] return [y[1] for y in v] Supposewecallprob2(x),wherexisthelist [11,11,17,9,3,5,8,11,9,14,14,2,3,13,13,6,1,8,7,6] (a)whatarethefirstthreeelementsofthelistucreatedinthefirststatementofthe function? (3,11),(3,11),(4,17)

3 (b)whatarethefirstthreeelementsofthelistvcreatedinthesecondstatementof thefunction? (3,9),(3,9),(1,1) Asitturnsout,thesearetheonlythreeelementsofv. (c)whatisthevaluereturnedbythefunction? [9,9,1] (d)giveasuccinctdescriptionofwhatprob2(nums)returnsingeneral,when numsisalistofintegers. Itreturnsalistoftheperfectsquaresinnums. SomepeoplegavemethebrainFdumponthisproblem,describingeverystepofthe programbeforegettingtothepoint. Thetextbookdescribesanumberofcommonpatternsforprocessinglists,with nameslikedecorate,filter,undecorate,map,reduce.thisonemightbecalled decorateffilterfundecorate,orsimplyacomplicatedfilteroperation.wecouldalso doitwithasinglelistcomprehension [yforyinnumsifint(math.sqrt(y))**2==y] 3.Writeafunction def columnsums(v) wherevisalistoflistsofintegers(i.e.,atwofdimensionalarray).forexample [[2,F1,3],[4,1,6],[1,1,0],[0,F4,2]] whichwethinkofasrepresentingthearray F4 2 Yourfunctionshouldreturnalistofthesumsofthesuccessivecolumns,sointhe exampleabove,thereturnvalueshouldbe[7,f1,11].yourfunctionshouldwork witharraysofanywidthandheight,butyoumayassumethatitisonlycalledwith arraysthatareproperlyformed(inthesensethatalltherowshavethesame numberofelements).

4 HereareTHREEdifferentsolutions.Theideaofthefirstoneistotraversethearray goingdowneachcolumninturn,accumulatingthecolumnsumandappendingitto thenewlist. Theideaofthesecondistosimultaneouslycomputethesumineachcolumn, initiallysettingarowof0's,thentraversingthearrayrowbyrowandaddingaswe goalong. Thethirdsolutionisthebasicallythesameideaasthefirst,usinglist comprehensionattwolevels. def columnsum1(v): height = len(v) width = len(v[0]) result=[] for col in range(width): accumsum=0 for row in range(height): accumsum+=v[row][col] result.append(accumsum) return result def columnsum2(v): height = len(v) width = len(v[0]) result=[0]*width for row in range(height): for col in range(width): result[col]+=v[row][col] return result def columnsum3(v): height = len(v) width = len(v[0]) return[sum([v[row][col] for row in range(height)]) for col in range(width)]

5 def binsearch(thelist,target): count = 0 low = 0 high = len(thelist)-1 while low<= high: mid=(low+high)/2 if thelist[mid] == target: return mid elif thelist[mid]<target: low=mid+1 else: #if you're here, then thelist[mid]>target high = mid-1 return -1 Figure&1:&&The&original&binary&search& def binsearch_new(thelist,target): low = 0 high = len(thelist)-1 while low <= high: mid=(low+high)/2 if thelist[mid] == target: return (thelist[mid],thelist[mid+1]) elif thelist[mid]<target: low=mid+1 else: high = mid-1 if low>mid: return (thelist[mid],thelist[low]) else: return (thelist[high],thelist[mid]) Figure&2:&The&modified&binary&search&

6 4.Displayedonthenextpageistheoriginalversionofthebinarysearchfunction forsearchingasortedlist,aspresentedinclass,alongwithamodifiedversion.as youmayrecall,theoriginalimplementationreturnstheindexofthetargetinthe list,ifthetargetitemispresent,andreturnsf1otherwise. Inthemodifiedversionthereturnvalueshavebeenaltered.Whattheresulting functiondoesisactuallyquiteuseful,butitsuffersfromaseriousdefect,asyou shoulddiscoverinpart(c)below. Supposethemodifiedfunctioniscalledwith [1,8,27,64,125,216,343,729,1000] asthefirstargument. (a)whatisthereturnvalueifthefunctioniscalledwith343asthesecond argument? (343,729) Intheeventtheitemisfound,thisjustreturnsthetupleconsistingoftheitemand thefollowingitem. (b)...with600asthesecondargument?? (343,729) Lookcarefullyatwhathappenswiththevalueslow,highandmidduringthesearch for600.atsomepointwehavelow=6,high=7withthetargettrappedbetween elements6and7ofthelist.inthenextstepwesetmidto6.sincethevalueistoo low,wesetlowto7.sonowlow,midandhighareall7,andweexittheloopwith high=6,low=mid=7,sothefunctionreturns(thelist[high],thelist[mid]),or (343,729). (c)...with1500asthesecondargument? error Eventuallywegetlow=high=mid=8,andsincewehavethelist[mid]<target,weset low=9andexittheloop.sincelow>mid,thefunctiontriestoreturn (thelist[mid],thelist[low]),butthelist[low]isoutoftherangeofindicesofthelist.

7 5.WerepresentacollectionofBostonFarearestaurantsasaPythondictionarydi. Hereisatypicalentry(therestaurantisrealFFFthedatacomesfromYelp). "Sandrine's" : ('Cambridge','French','$$$',3.5) Thatis,thekeyofeachentryisthenameoftherestaurant,andthevaluefieldisa tupleconsistingof:thelocation,thetypeoffood,anexpenserating(the3dollar signs),andanaverageofreviewerratings(3.5stars). (a)myfavoriterestaurantismom's)eat)at)joe's.iwouldliketoseeifothersagree withme,soiwanttowriteapythonstatementorstatementsthatwillprintoutthe starratingofthisrestaurant.thecodeyouwriteshouldnotcauseanpythonf generatederrormessageonthechancethatthisrestaurantisnotinthedatabase, butinsteadprint'restaurantnotfound'. if"mom'seatatjoe's"indi printdi["mom'seatatjoe's"][3] else: print'restaurantnotfound' Moststudentsoverthoughtthisone,andtriedtogivemeafunctionthatworkedfor ageneralrestaurantpassedasaparameter,butionlywantedabrieffragmentof code. (b)iwanttoimpresssomeonebytakingthemtoafancyitalianplaceinthenorth End.(Idon'tcareiftherestaurantisgood,Ijustwantmyguesttoseemespendinga lotofmoney.)writeastatementorstatementsthatprintsalistofthenamesofall therestaurantswithlocation'northend',foodtype'italian',andatleast4dollar signs.this)list)should)be)sorted)in)alphabetical)order. s=[rforrindiifdi[r][0]=='northend'anddi[r][1]=='italian'andlen(di[r][2])>=4] s.sort() prints Manystudentsassumed4starswasthehighestpossibleandwrote==inplaceof>=. Otherswrote"'NorthEnd'indi[r]"inplaceof"di[r]=='NorthEnd'".That'sok,butif therewereadistrictinthecitycalled'italian',itmightnotworkandsomedidnot usethelenfunctionandwrotedi[r][2]>='$$$$',whichactuallyworks Someforgotthats.sort()doesnothaveavalue,andthusprints.sort()doesnot work.