L3_Conditionals.ppt

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1、3.IntroductiontoConditionalsBooleanexpressionsTheIf-ElseConstructAnd,or,notInsightThroughComputingWhatWeCannotDoSoFarIfthevalueofthearithmeticexpressionDice1+Dice2isseven,thenincreasethevalueofthevariableGamesWonbyone.Wecannotmakeacomputationcontingentuponoth

2、erthings.InsightThroughComputingTheIf-ElseConstructSolvesthisProblemWewillintroducethislanguagefeaturebysolvingproblemsaboutthebehaviorofagivenquadraticonagivenintervalL<=x<=R.InsightThroughComputingAssumeVariablesb,c,L,RareInitializedE.g.,b=input(‘Enterb’:)

3、c=input(‘Enterc’:)L=input(‘EnterL’:)R=input(‘EnterR’:)InsightThroughComputingLRTheSituationInsightThroughComputingWriteafragmentthatprints“yes”ifq(x)increasesacrosstheintervaland“no”ifitdoesnot.Problem1InsightThroughComputingLRNo!InsightThroughComputingLRYes

4、!Requirement:xc<=LInsightThroughComputingSolutionFragmentxc=-b/2;ifxc<=Ldisp(‘Yes’)elsedisp(‘No’)endInsightThroughComputingWriteafragmentthatprintsthemaximumvaluethatq(x)attainsontheinterval.Problem2InsightThroughComputingLRMaximumatLInsightThroughComputingL

5、RMaximumatRDependsonwhetherxcistotherightorleftoftheintervalmidpoint.InsightThroughComputingSolutionFragmentxc=-b/2;Mid=(L+R)/2;ifxc<=MidmaxVal=R^2+b*R+celsemaxVal=L^2+b*L+cendInsightThroughComputingWriteafragmentthatprints“yes”ifxcisintheintervaland“no”ifx

6、cisnotintheinterval.Problem3InsightThroughComputingLRNo!Becausexc

7、=xc<=RInsightThroughComputingSayingtheOppositexcisintheinterval[L,R]ifL<=xcandxc<=Rxcisnotintheinterval[L,R]ifxc

8、

9、(R

10、=-b/2;if(L<=xc)&&(xc<=R)disp(‘Yes’)elsedisp(‘No’)endInsightThroughComputingTheif-elseConstructifendelsebooleanexpressionCommandstoexecuteiftheexpressionifTRUECommandstoexecuteiftheexpress