Matlab预定义函数.ppt

1,Matlab与科学计算,计算机学院刘咏梅Email:liuyongmei,2,第四章MATLAB编程（预定义函数）,IntroductiontoMATLABMATLABBasicsofNumericalComputingMATLABProgramming(branchingandLoops)MATLABProgramming(PredefinedFunctions)MATLABGraphicandImageProcessingScientificComputingusingMATLAB,3,本章学习的目标：,IntroductiontoMatlabFunctionsbuiltinfunctionsanduserdefinedfunctionsOptionalArgumentsSharingDataUsingGlobalMemoryPreservingDataBetweenCallstoaFunctionFunctionFunctionsSubfunctions,4,Functions,FunctionsaremorecomplexthanscriptsFunctionshavetheirownlocalvariablesFunctionsreturnoutputasspecified,andcanacceptinputasspecified,5,Whyweusefunctions?,IndependentTestingofSub-Tasks:Eachtaskisanindependentunitthatcanbetestedseparately(unittesting).ReusableCode:Withinthesameprogram(thecodeisnotrepeated).Itcanbeusedinanotherprogram.Isolationfromunintendedsideeffects:Isolationoffunctionvariablesfromthemainprogramvariables.Theinputandtheoutputvariablesarespecifiedclearly.,6,SyntaxofMatlabFunctions,Userdefinedfunctionsbehavejustlikebuiltinfunctions.Allfunctionshaveasimilarsyntax,whethertheyarebuilt-infunctionsoruser-definedfunctions.NameInputOutput,A=cos(x),7,Howtocallfunctions?,Thefunctionscanbecalleddirectlyinthecommandwindow,orbyanyotherprogramorfunction.Veryimportant:iftheinputvariablesarechangedinthefunctionstheywillnotchangeinthemainprogram(PASS-BY-VALUESCHEME),8,BuiltinFunctions,Builtinfunctionsallowustoreusecomputercodeforcalculationsthatareperformedfrequently.Forexample,supposetherewerenobuiltinsinefunctioninMATLABandyouwilldoalotoftrigonometriccalculationsinvolvingthesineofanangle.WevealreadybecomefamiliarwithanumberofMATLABbuiltinfunctionslikesin(x)orplot(x,y)forexample.,9,SomeexamplesofBuiltinFunctions,Exponential(指数)exp(x)exponentialofx(ex)sqrt(x)squarerootofxLogarithmic(对数)log(x)naturallogarithm(lnx)log10(x)log2(x),10,SomeexamplesofBuiltinFunctions,Numeric(数值)fix(x)roundtonearestintegertowards0round(x)roundtonearestintegersign(x)+1,0or-1rem(x,y)Findsremainderofx/yComplex(复数)abs(x)absolutevalueangle(x)phaseangleincomplexplaneconj(x)conjugateofximag(x)imaginarypartofxreal(x)realpartofx,11,BuiltinFunctionsforvectors,max(x)returnslargestvalueinvectorxa,b=max(x)returnslargestvalueinaandindexwherefoundinbmax(x,y)xandyarraysofsamesize,returnsvectorofsamelengthwithlargervaluefromcorrespondingpositionsinxandymax(352,648)ans=658sametypeoffunctionsareavailableformin,12,BuiltinFunctionsforvectors,sort(x)sortinascendingordescendingordermean(x)averageormeanvaluemedian(x)medianvaluesum(x)sumofelementsofxprod(x)productofelementsofxcumsum(x)returnsvectorofsamesizewithcumulativesumofxi.e.x=4,2,3returns4,6,9cumprod(x)returnsvectorofsamesizewithcumulativeproductsinvectorofsamesizei.e.x=4,2,3returns4,8,24,13,Builtinfunctionsappliedtomatrices,Matrices(arrays)arestoredincolumnmajorformWhenbuiltinfunctionsforvectorsareappliedtoamatrixfunctionoperatesoncolumnsandreturnsarowvector,14,Builtinfunctionsappliedtomatrices,sum(012;345)ans=357prod(012;345)ans=0410cumsum(012;345)ans=012357,cumprod(012;345)ans=0120410sort(912;324)ans=312924,15,User-definedFunctions,YoucanuseuserdefinedfunctionsinyourprogramsjustasyouwoulduseMATLABsbuiltinfunctions.Userdefinedfunctionshelpyouwritemorecompactprogramsandwritecomplexprogramsintoshorteronesthatareeasiertodebugandgetrunning.,16,User-definedFunctions,User-definedfunctionsmuststartwithafunctiondefinitionline.ThedefinitionlinecontainsThewordfunctionAvariable(variables)thatdefinesthefunctionoutputAfunctionnameAvariable(variables)usedfortheinputargument,functionoutput=poly(x),Theabovelineofcodesdefinesafunctionnamedpoly.,17,Asimpleexampleofuser-definedfunctions,18,YoucanuseyouruserdefinedfunctionsfromtheCommandWindoworfromaM-fileprogram.,19,Whenxisavector,Considerthefollowing,A=1,2,5;%Aisa3-elementrowvectory=2*poly(A)y=1482982Inthestatementy=2*poly(A)above,theuserdefinedfunctionpolyiscalledwhichevaluatesthepolynomialateachpointintheinputrowvector.Then,eachvalueismultipliedby2,resultingintherowvectorcalledyhavingthevalues14,82,982.,20,CommentsforFunctions,Thecommentlinesimmediatelyafterthefirstlinearereturnedwhenyouquerythehelpfunction,21,22,Userdefinedfunctionsmay,Havemorethanoneinputvariable,functions=f(x,y,z).Thisfunctionhasthreeinputvariables.Havemorethanoneoutputvariable,functionout1,out2=f(x)isafunctionwithoneinputvariableandtwooutputvariables.Inputandoutputvariablescanbescalars,vectorsormatrices.Withinthebodyofthefunction,thereneedstobeoneassignmentstatementforeachoutputvariable.,23,LocalVariables,Anyvariablesusedwithinafunctionarecalledlocalvariables.ThesevariablesdonotappearintheWorkspaceWindowandarenotavailableforthemainprogramtoaccess.Theoutputvariablenamesusedinthefunctionstatementandinthebodyofthefunctionarealsolocalvariables.Theinputvariablesarelocalvariablesalso.,24,LocalVariables,VariablesdefinedinanM-filefunction,onlyhavemeaninginsidethatprogramTheonlywaytocommunicatebetweenfunctionsandtheworkspace,isthroughthefunctioninputandoutputarguments,25,x,y,a,andoutputarelocalvariablestothegfunction,Whenthegfunctionisexecuted,theonlyvariablecreatedisdeterminedinthecommandwindow(orscriptM-fileusedtoexecuteaprogram),ansistheonlyvariablecreated,26,OptionalArguments,Forexample:plotfunctionx=0:pi/100:2*pi;y1=sin(2*x);y2=2*cos(2*x);plot(x,y1,x,y2);,x=0:pi/100:2*pi;y1=sin(2*x);y2=2*cos(2*x);plot(x,y1);holdon;plot(x,y2);,27,OptionalArguments,max(x)returnslargestvalueinvectorxa,b=max(x)returnslargestvalueinaandindexwherefoundinbmax(x,y)xandyarraysofsamesize,returnsvectorofsamelengthwithlargervaluefromcorrespondingpositionsinxandysametypeoffunctionsareavailableformin,28,OptionalArguments(InputandOutput),Optionaloutput,Optionalinput,29,Determinethenumberofinputandoutputvariables,Twobuiltinfunctionsletyoudeterminethenumberofinputandoutputvariablesinanyfunction,forfunctions=f(x)nargin(f)returnsinourcase,ans=1sincefhasoneinputargument,andnargout(f)alsoreturnsans=1inourcasebecausefhasonlyoneoutputvariable.,nargin:determinesthenumberofinputargumentsnargout:determinesthenumberofoutputarguments,30,GlobalVariables,Remembervariablenamesusedwithinafunctionarelocalvariables,theydonotappearintheWorkspacewindowofthemainprogram.ItispossibletodefineaGlobalVariablethatisthencommontoboththemainprogramandthefunction.Itisrecommendednottodothis.Althoughitispossibletodefineglobalvariables,itisabadidea!,31,SharingDataUsingGlobalMemory,Globalmemoryisaspecialmemorythatcanbeaccessedfromanyworkspace.Syntax:globalvar1var2var3Remark:Eachglobalvariablemustbedeclaredtobeglobalbeforeitisusedforthefirsttimeinafunction.Bettertobethefirststatementinthefunctionaftertheinitialcomments.,32,PreservingDataBetweenCallstoaFunction,Eachtimethefunctionisfinished,dataintheworkspaceisdestroyed.Tomakesomevariablespersistentforthenextfunctioncalls,usethepersistcommand.Specialtypeofmemorythatcanbeaccessedonlyfromthesamefunction.Syntax:persistentvar1var2var3,33,PreservingDataBetweenCallstoaFunction,Exercise:runningaveragesandstandarddeviations.Usermustenterthenumberofvaluesindataset,andthenentervaluebyvalue.Eachtimeheentersavaluetheprogramcallsthefunction