1、Chapter 4Procedural Abstraction and Functions That Return a ValueOverview4.1 Top-Down Design 4.2 Predefined Functions4.3 Programmer-Defined Functions4.4 Procedural Abstraction4.5 Local Variables4.6 Overloading Function NamesSlide 4- 34.1Top-Down DesignTop Down DesignTo write a programDevelop the alg

2、orithm that the program will useTranslate the algorithm into the programminglanguage Top Down Design (also called stepwise refinement)Break the algorithm into subtasksBreak each subtask into smaller subtasksEventually the smaller subtasks are trivial to implement in the programming languageSlide 4-

3、5Benefits of Top Down DesignSubtasks, or functions in C+, make programsEasier to understandEasier to changeEasier to writeEasier to testEasier to debugEasier for teams to developSlide 4- 64.2Predefined FunctionsPredefined FunctionsC+ comes with libraries of predefined functionsExample: sqrt function

4、 theRoot = sqrt(9.0); returns, or computes, the square root of a numberThe number, 9, is called the argumenttheRoot will contain 3.0Slide 4- 8Function Callssqrt(9.0) is a function callIt invokes, or sets in action, the sqrt functionThe argument (9), can also be a variable or an expressionA function

5、call can be used like any expressionbonus = sqrt(sales) / 10;Cout “The side of a square with area “ area “ is “ sqrt(area);Slide 4- 9Display 4.1Function Call SyntaxFunction_name (Argument_List)Argument_List is a comma separated list:(Argument_1, Argument_2, , Argument_Last)Example:side = sqrt(area);

6、cout “2.5 to the power 3.0 is “ pow(2.5, 3.0); Slide 4- 10Function Libraries Predefined functions are found in librariesThe library must be “included” in a programto make the functions availableAn include directive tells the compiler which library header file to include.To include the math library c

7、ontaining sqrt(): #include Newer standard libraries, such as cmath, also requirethe directive using namespace std;Slide 4- 11Other Predefined Functionsabs(x) - int value = abs(-8);Returns absolute value of argument xReturn value is of type intArgument is of type xFound in the library cstdlibfabs(x)

8、- double value = fabs(-8.0);Returns the absolute value of argument xReturn value is of type doubleArgument is of type doubleFound in the library cmathSlide 4- 12Display 4.2Random Number GenerationReally pseudo-random numbers1. Seed the random number generator only once#include #include srand(time(0)

9、;2. The rand() function returns a random integer that is greater than or equal to 0 and less than RAND_MAXrand();Slide 4- 13Random NumbersUse % and + to scale to the number range you wantFor example to get a random number from 1-6 to simulate rolling a six-sided die:int die = (rand() % 6) + 1;Can yo

10、u simulate rolling two dice?Generating a random number x where 10 x 21?Slide 4- 14Type CastingRecall the problem with integer division:int totalCandy = 9, numberOfPeople = 4;double candyPerPerson;candyPerPerson = totalCandy / numberOfPeople; candyPerPerson = 2, not 2.25!A Type Cast produces a value

11、of one type from another typestatic_cast(totalCandy) produces a doublerepresenting the integer value of totalCandySlide 4- 15Type Cast Exampleint totalCandy = 9, numberOfPeople = 4;double candyPerPerson;candyPerPerson = static_cast(totalCandy) / numberOfPeople;candyPerPerson now is 2.25!This would a

12、lso work: candyPerPerson = totalCandy / static_cast( numberOfPeople);This would not! candyPerPerson = static_cast( totalCandy / numberOfPeople);Slide 4- 16Integer division occurs before type castOld Style Type CastC+ is an evolving languageThis older method of type casting may be discontinued in fut

13、ure versions of C+candyPerPerson = double(totalCandy)/numberOfPeople; candyPerPerson = (double) totalCandy /numberOfPeople;Slide 4- 17Section 4.2 ConclusionSlide 4- 18Can youDetermine the value of d? double d = 11 / 2;Determine the value of pow(2,3)fabs(-3.5)sqrt(pow(3,2) 7 / abs(-2)ceil(5.8)floor(5

14、.8)Convert the following to C+4.3Programmer-Defined FunctionsProgrammer-Defined FunctionsTwo components of a function definitionFunction declaration (or function prototype)Shows how the function is calledMust appear in the code before the function can be calledSyntax:Type_returned Function_Name(Para

15、meter_List); ment describing what function doesFunction definitionDescribes how the function does its taskCan appear before or after the function is calledSyntax: Type_returned Function_Name(Parameter_List) /code to make the function work Slide 4- 20;Function DeclarationTells the return typeTells th

16、e name of the functionTells how many arguments are neededTells the types of the argumentsTells the formal parameter namesFormal parameters are like placeholders for the actualarguments used when the function is calledFormal parameter names can be any valid identifierExample:double totalCost(int numb

17、erPar, double pricePar);/ Compute total cost including 5% sales tax on/ numberPar items at cost of pricePar eachSlide 4- 21Function DefinitionProvides the same information as the declaration Describes how the function does its taskExample:double totalCost(int numberPar, double pricePar) const double

18、 TAX_RATE = 0.05; /5% tax double subtotal; subtotal = pricePar * numberPar; return (subtotal + subtotal * TAX_RATE);Slide 4- 22function headerfunction bodyThe Return StatementEnds the function callReturns the value calculated by the functionSyntax: return expression; expression performs the calculat

19、ion orexpression is a variable containing the calculated valueExample: return subtotal + subtotal * TAX_RATE;Slide 4- 23The Function CallTells the name of the function to useLists the argumentsIs used in a statement where the returned valuemakes senseExample:double bill = totalCost(number, price);Sl

20、ide 4- 24Display 4.3Function Call DetailsThe values of the arguments are plugged into the formal parameters (Call-by-value mechanism with call-by-value parameters)The first argument is used for the first formal parameter, the second argument for the secondformal parameter, and so forth.The value plu

21、gged into the formal parameter is usedin all instances of the formal parameter in the function bodySlide 4- 25Display 4.4Alternate DeclarationsTwo forms for function declarationsList formal parameter namesList types of formal parmeters, but not namesFirst aids description of the function in comments

22、 Examples: double totalCost(int numberPar, double pricePar);double totalCost(int, double);Function headers must always list formal parameter names!Slide 4- 26Order of ArgumentsCompiler checks that the types of the argumentsare correct and in the correct sequence.Compiler cannot check that arguments

23、are in thecorrect logical orderExample: Given the function declaration: char grade(int received_par, int minScore_par);int received = 95, minScore = 60;cout =10) & ( rate =10) & ( rate 20) | (rate = 0);Slide 4- 31Section 4.3 ConclusionCan youWrite a function declaration and a function definitionfor

24、a function that takes three arguments, all of typeint, and that returns the sum of its three arguments?Describe the call-by-value parameter mechanism?Write a function declaration and a function definition for a function that takes one argument of type int and one argument of type double, and that re

25、turns a value of type double that is the average of the two arguments?Slide 4- 324.4Procedural AbstractionProcedural AbstractionThe Black Box AnalogyA black box refers to something that we know how to use, but the method of operation is unknownA person using a program does not need to knowhow it is

26、codedA person using a program needs to know what theprogram does, not how it does itFunctions and the Black Box AnalogyA programmer who uses a function needs to know what the function does, not how it does itA programmer needs to know what will be produced if the proper arguments are put into the bo

27、xSlide 4- 34Information HidingDesigning functions as black boxes is an example of information hidingThe function can be used without knowing howit is codedThe function body can be “hidden from view”Slide 4- 35Function Implementationsand The Black BoxDesigning with the black box in mind allows usTo c

28、hange or improve a function definition withoutforcing programmers using the function to changewhat they have doneTo know how to use a function simply by reading the function declaration and its commentSlide 4- 36Display 4.7Procedural Abstraction and C+Procedural Abstraction is writing and using func

29、tions as if they were black boxesProcedure is a general term meaning a “function like”set of instructionsAbstraction implies that when you use a function asa black box, you abstract away the details of the code in the function bodySlide 4- 37Procedural Abstraction and FunctionsWrite functions so the

30、 declaration and commentis all a programmer needs to use the functionFunction comment should tell all conditions required of arguments to the functionFunction comment should describe the returnedvalueVariables used in the function, other than the formal parameters, should be declared in the function

31、 bodySlide 4- 38Formal Parameter NamesFunctions are designed as self-contained modulesDifferent programmers may write each functionProgrammers choose meaningful names for formal parametersFormal parameter names may or may not match variable names used in the main part of the programIt does not matte

32、r if formal parameter names match other variable names in the programRemember that only the value of the argument is plugged into the formal parameterSlide 4- 39Display 4.8Case Study Buying PizzaWhat size pizza is the best buy?Which size gives the lowest cost per square inch?Pizza sizes given in dia

33、meterQuantity of pizza is based on the area whichis proportional to the square of the radiusSlide 4- 40Buying Pizza Problem DefinitionInput:Diameter of two sizes of pizzaCost of the same two sizes of pizzaOutput:Cost per square inch for each size of pizzaWhich size is the best buyBased on lowest pri

34、ce per square inchIf cost per square inch is the same, the smaller sizewill be the better buySlide 4- 41Buying Pizza Problem AnalysisSubtask 1 Get the input data for each size of pizzaSubtask 2Compute price per inch for smaller pizzaSubtask 3Compute price per inch for larger pizzaSubtask 4Determine

35、which size is the better buySubtask 5Output the resultsSlide 4- 42Buying Pizza Function AnalysisSubtask 2 and subtask 3 should be implementedas a single function becauseSubtask 2 and subtask 3 are identical tasksThe calculation for subtask 3 is the same as the calculation for subtask 2 with differen

36、t argumentsSubtask 2 and subtask 3 each return a single valueChoose an appropriate name for the functionWell use unitpriceSlide 4- 43Buying Pizza unitprice Declarationdouble unitprice(int diameter, int double price);/Returns the price per square inch of a pizza/The formal parameter named diameter is

37、 the /diameter of the pizza in inches. The formal / parameter named price is the price of the/ pizza.Slide 4- 44Buying Pizza Algorithm DesignSubtask 1Ask for the input values and store them in variablesdiameterSmalldiameterLargepriceSmallpriceLargeSubtask 4Compare cost per square inch of the two piz

38、zas usingthe less than operator Subtask 5Standard output of the resultsSlide 4- 45Buying Pizza unitprice AlgorithmSlide 4- 46Subtasks 2 and 3 are implemented as calls tofunction unitpriceunitprice algorithmCompute the radius of the pizzaComputer the area of the pizza using Return the value of (price

39、 / area)Buying Pizza unitprice PseudocodePseudocodeMixture of C+ and englishAllows us to make the algorithm more precise without worrying about the details of C+ syntaxunitprice pseudocoderadius = one half of diameter;area = * radius * radiusreturn (price / area)Slide 4- 47Buying Pizza The Calls of

40、unitpriceMain part of the program implements calls of unitprice asdouble unitPriceSmall, unitPriceLarge;unitPriceSmall = unitprice(diameterSmall, priceSmall);unitPriceLarge = unitprice(diameterLarge, priceLarge);Slide 4- 48Buying Pizza First try at unitpricedouble unitprice (int diameter, double pri

41、ce) const double PI = 3.14159; double radius, area; radius = diameter / 2; area = PI * radius * radius; return (price / area);Oops! Radius should include the fractional partSlide 4- 49Buying Pizza Second try at unitpricedouble unitprice (int diameter, double price) const double PI = 3.14159; double

42、radius, area; radius = diameter / static_cast(2) ; area = PI * radius * radius; return (price / area);Now radius will include fractional partsradius = diameter / 2.0 ; / This would also workSlide 4- 50Display 4.10 (1)Display 4.10 (2)Program TestingPrograms that compile and run can still produce erro

43、rsTesting increases confidence that the programworks correctlyRun the program with data that has known outputYou may have determined this output with pencil and paperor a calculatorRun the program on several different sets of dataYour first set of data may produce correct results inspite of a logica

44、l error in the codeRemember the integer division problem? If there is no fractional remainder, integer division will give apparently correct resultsSlide 4- 51Use PseudocodePseudocode is a mixture of English and the programming language in usePseudocode simplifies algorithm design by allowing you to

45、 ignore the specific syntax of the programming language as you work out the details of the algorithmIf the step is obvious, use C+If the step is difficult to express in C+, use English Slide 4- 52Section 4.4 ConclusionCan youDescribe the purpose of the comment that panies a function declaration?Desc

46、ribe what it means to say a programmer should be able to treat a function as a black box?Describe what it means for two functions to be black box equivalent?Slide 4- 534.5Local VariablesLocal VariablesVariables declared in a function:Are local to that function, they cannot be used from outside the f

47、unctionHave the function as their scopeVariables declared in the main part of a program:Are local to the main part of the program, they cannot be used from outside the main partHave the main part as their scopeSlide 4- 55Display 4.11 (1)Display 4.11 (2)Global ConstantsGlobal Named ConstantAvailable

48、to more than one function as well as themain part of the programDeclared outside any function bodyDeclared outside the main function body Declared before any function that uses itExample: const double PI = 3.14159; double volume(double); int main() PI is available to the main function and to functio

49、n volumeSlide 4- 56Display 4.12 (1)Display 4.12 (2)Global VariablesGlobal Variable - rarely used when morethan one function must use a common variableDeclared just like a global constant except const is not usedGenerally make programs more difficult to understand and maintainSlide 4- 57Formal Parame

50、tersare Local VariablesFormal Parameters are actually variables that arelocal to the function definitionThey are used just as if they were declared in the function bodyDo NOT re-declare the formal parameters in the function body, they are declared in the functiondeclarationThe call-by-value mechanis

51、mWhen a function is called the formal parameters are initialized to the values of thearguments in the function callSlide 4- 58Display 4.13 (1)Display 4.13 (2)Block ScopeLocal and global variables conform to the rules of Block ScopeThe code block (generally defined by the ) where an identifier like a

52、 variable is declared determines the scope of the identifierBlocks can be nestedSlide 4- 59Display 4.14Namespaces RevisitedThe start of a file is not always the best place for using namespace std;Different functions may use different namespacesPlacing using namespace std; inside the starting brace o

53、f a functionAllows the use of different namespaces in different functionsMakes the “using” directive local to the functionSlide 4- 60Display 4.15 (1)Display 4.15 (2)Example: Factorialn! Represents the factorial functionn! = 1 x 2 x 3 x x nThe C+ version of the factorial function found in Display 3.1

54、4Requires one argument of type int, nReturns a value of type intUses a local variable to store the current productDecrements n each time it does another multiplication n * n-1 * n-2 * * 1Slide 4- 61Display 4.164.6Overloading Function NamesOverloading Function NamesC+ allows more than one definition

55、for the same function nameVery convenient for situations in which the “same”function is needed for different numbers or typesof argumentsOverloading a function name means providing more than one declaration and definition using the same function nameSlide 4- 63Overloading Examplesdouble ave(double n

56、1, double n2) return (n1 + n2) / 2);double ave(double n1, double n2, double n3) return ( n1 + n2 + n3) / 3);Compiler checks the number and types of argumentsin the function call to decide which function to use cout ave( 10, 20, 30); uses the second definitionSlide 4- 64Overloading DetailsOverloaded

57、functionsMust have different numbers of formal parameters AND / OR Must have at least one different type of parameterMust return a value of the same typeSlide 4- 65Display 4.17Overloading ExampleRevising the Pizza Buying programRectangular pizzas are now offered!Change the input and add a function t

58、o compute the unit price of a rectangular pizzaThe new function could be named unitprice_rectangularOr, the new function could be a new (overloaded) version of the unitprice function that is already usedExample: double unitprice(int length, int width, double price) double area = length * width; return (price / area); Slide 4- 66Display 4.18 (1 3)Automatic Type ConversionGiven the definition double mpg(double miles, double gallons) return (miles / gallons)