Chapter 4. Procedural Abstraction and Functions That Return a Value. Copyright 2015 Pearson Education, Ltd.. All rights reserved.

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3 Overview 4.1 Top-Dow Desig 4.2 Predefied Fuctios 4.3 Programmer-Defied Fuctios 4.4 Procedural Abstractio 4.5 Local Variables 4.6 Overloadig Fuctio Names Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-3

5 Top Dow Desig To write a program Develop the algorithm that the program will use Traslate the algorithm ito the programmig laguage Top Dow Desig (also called stepwise refiemet) Break the algorithm ito subtasks Break each subtask ito smaller subtasks Evetually the smaller subtasks are trivial to implemet i the programmig laguage Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-5

6 Beefits of Top Dow Desig Subtasks, or fuctios i C++, make programs Easier to uderstad Easier to chage Easier to write Easier to test Easier to debug Easier for teams to develop Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-6

8 Predefied Fuctios C++ comes with libraries of predefied fuctios Example: sqrt fuctio the_root = sqrt(9.0); returs, or computes, the square root of a umber The umber, 9, is called the argumet the_root will cotai 3.0 Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-8

9 Fuctio Calls sqrt(9.0) is a fuctio call It ivokes, or sets i actio, the sqrt fuctio The argumet (9), ca also be a variable or a expressio A fuctio call ca be used like ay expressio bous = sqrt(sales) / 10; Cout << The side of a square with area << area << is << sqrt(area); Display 4.1 Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-9

10 Fuctio Call Sytax Fuctio_ame (Argumet_List) Argumet_List is a comma separated list: (Argumet_1, Argumet_2,, Argumet_Last) Example: side = sqrt(area); cout << 2.5 to the power 3.0 is << pow(2.5, 3.0); Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-10

11 Fuctio Libraries Predefied fuctios are foud i libraries The library must be icluded i a program to make the fuctios available A iclude directive tells the compiler which library header file to iclude. To iclude the math library cotaiig sqrt(): #iclude <cmath> Newer stadard libraries, such as cmath, also require the directive usig amespace std; Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-11

12 Other Predefied Fuctios abs(x) --- it value = abs(-8); Returs absolute value of argumet x Retur value is of type it Argumet is of type x Foud i the library cstdlib fabs(x) --- double value = fabs(-8.0); Returs the absolute value of argumet x Retur value is of type double Argumet is of type double Foud i the library cmath Display 4.2 Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-12

13 Radom Number Geeratio Really pseudo-radom umbers 1. Seed the radom umber geerator oly oce #iclude <cstdlib> #iclude <ctime> srad(time(0)); 2. The rad() fuctio returs a radom iteger that is greater tha or equal to 0 ad less tha RAND_MAX rad(); Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 1-13

14 Radom Numbers Use % ad + to scale to the umber rage you wat For example to get a radom umber from 1-6 to simulate rollig a six-sided die: it die = (rad() % 6) + 1; Ca you simulate rollig two dice? Geeratig a radom umber x where 10 < x < 21? Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 1-14

15 Type Castig Recall the problem with iteger divisio: it total_cady = 9, umber_of_people = 4; double cady_per_perso; cady_per_perso = total_cady / umber_of_people; cady_per_perso = 2, ot 2.25! A Type Cast produces a value of oe type from aother type static_cast<double>(total_cady) produces a double represetig the iteger value of total_cady Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-15

16 Type Cast Example it total_cady = 9, umber_of_people = 4; double cady_per_perso; cady_per_perso = static_cast<double>(total_cady) / umber_of_people; cady_per_perso ow is 2.25! This would also work: cady_per_perso = total_cady / static_cast<double>( umber_of_people); This would ot! cady_per_perso = static_cast<double>( total_cady / umber_of_people); Iteger divisio occurs before type cast Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-16

17 Old Style Type Cast C++ is a evolvig laguage This older method of type castig may be discotiued i future versios of C++ cady_per_perso = double(total_cady)/umber_of_people; cady_per_perso = (double) total_cady /umber_of_people; Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-17

18 Sectio 4.2 Coclusio Ca you Determie the value of d? double d = 11 / 2; Determie the value of pow(2,3) fabs(-3.5) sqrt(pow(3,2)) 7 / abs(-2) ceil(5.8) floor(5.8) x + y Covert the followig to C++ Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. b + 2 b 4ac 2a x y + 7 Slide 4-18

20 Programmer-Defied Fuctios Two compoets of a fuctio defiitio Fuctio declaratio (or fuctio prototype) Shows how the fuctio is called Must appear i the code before the fuctio ca be called Sytax: Type_retured Fuctio_Name(Parameter_List); //Commet describig what fuctio does ; Fuctio defiitio Describes how the fuctio does its task Ca appear before or after the fuctio is called Sytax: Type_retured Fuctio_Name(Parameter_List) { //code to make the fuctio work } Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-20

21 Fuctio Declaratio Tells the retur type Tells the ame of the fuctio Tells how may argumets are eeded Tells the types of the argumets Tells the formal parameter ames Formal parameters are like placeholders for the actual argumets used whe the fuctio is called Formal parameter ames ca be ay valid idetifier Example: double total_cost(it umber_par, double price_par); // Compute total cost icludig 5% sales tax o // umber_par items at cost of price_par each Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-21

22 Fuctio Defiitio Provides the same iformatio as the declaratio Describes how the fuctio does its task Example: fuctio header double total_cost(it umber_par, double price_par) { cost double TAX_RATE = 0.05; //5% tax double subtotal; subtotal = price_par * umber_par; retur (subtotal + subtotal * TAX_RATE); } fuctio body Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-22

23 The Retur Statemet Eds the fuctio call Returs the value calculated by the fuctio Sytax: retur expressio; expressio performs the calculatio or expressio is a variable cotaiig the calculated value Example: retur subtotal + subtotal * TAX_RATE; Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-23

24 The Fuctio Call Tells the ame of the fuctio to use Lists the argumets Is used i a statemet where the retured value makes sese Example: double bill = total_cost(umber, price); Display 4.3 Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-24

25 Fuctio Call Details The values of the argumets are plugged ito the formal parameters (Call-by-value mechaism with call-by-value parameters) The first argumet is used for the first formal parameter, the secod argumet for the secod formal parameter, ad so forth. The value plugged ito the formal parameter is used i all istaces of the formal parameter i the fuctio body Display 4.4 Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-25

26 Alterate Declaratios Two forms for fuctio declaratios List formal parameter ames List types of formal parmeters, but ot ames First aids descriptio of the fuctio i commets Examples: double total_cost(it umber_par, double price_par); double total_cost(it, double); Fuctio headers must always list formal parameter ames! Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-26

27 Order of Argumets Compiler checks that the types of the argumets are correct ad i the correct sequece. Compiler caot check that argumets are i the correct logical order Example: Give the fuctio declaratio: char grade(it received_par, it mi_score_par); it received = 95, mi_score = 60; Display 4.5 (1) Display 4.5 (2) cout << grade( mi_score, received); Produces a faulty result because the argumets are ot i the correct logical order. The compiler will ot catch this! Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-27

28 Fuctio Defiitio Sytax Withi a fuctio defiitio Variables must be declared before they are used Variables are typically declared before the executable statemets begi At least oe retur statemet must ed the fuctio Each brach of a if-else statemet might have its ow retur statemet Display 4.6 Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-28

29 Placig Defiitios A fuctio call must be preceded by either The fuctio s declaratio or The fuctio s defiitio If the fuctio s defiitio precedes the call, a declaratio is ot eeded Placig the fuctio declaratio prior to the mai fuctio ad the fuctio defiitio after the mai fuctio leads aturally to buildig your ow libraries i the future. Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-29

30 bool Retur Values A fuctio ca retur a bool value Such a fuctio ca be used where a boolea expressio is expected Makes programs easier to read if (((rate >=10) && ( rate < 20)) (rate == 0)) is easier to read as if (appropriate (rate)) If fuctio appropriate returs a bool value based o the the expressio above Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-30

31 Fuctio appropriate To use fuctio appropriate i the if-statemet if (appropriate (rate)) { } appropriate could be defied as bool appropriate(it rate) { retur (((rate >=10) && ( rate < 20)) (rate == 0)); } Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-31

32 Sectio 4.3 Coclusio Ca you Write a fuctio declaratio ad a fuctio defiitio for a fuctio that takes three argumets, all of type it, ad that returs the sum of its three argumets? Describe the call-by-value parameter mechaism? Write a fuctio declaratio ad a fuctio defiitio for a fuctio that takes oe argumet of type it ad oe argumet of type double, ad that returs a value of type double that is the average of the two argumets? Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-32

34 Procedural Abstractio The Black Box Aalogy A black box refers to somethig that we kow how to use, but the method of operatio is ukow A perso usig a program does ot eed to kow how it is coded A perso usig a program eeds to kow what the program does, ot how it does it Fuctios ad the Black Box Aalogy A programmer who uses a fuctio eeds to kow what the fuctio does, ot how it does it A programmer eeds to kow what will be produced if the proper argumets are put ito the box Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-34

35 Iformatio Hidig Desigig fuctios as black boxes is a example of iformatio hidig The fuctio ca be used without kowig how it is coded The fuctio body ca be hidde from view Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-35

36 Fuctio Implemetatios ad The Black Box Desigig with the black box i mid allows us To chage or improve a fuctio defiitio without forcig programmers usig the fuctio to chage what they have doe To kow how to use a fuctio simply by readig the fuctio declaratio ad its commet Display 4.7 Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-36

37 Procedural Abstractio ad C++ Procedural Abstractio is writig ad usig fuctios as if they were black boxes Procedure is a geeral term meaig a fuctio like set of istructios Abstractio implies that whe you use a fuctio as a black box, you abstract away the details of the code i the fuctio body Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-37

38 Procedural Abstractio ad Fuctios Write fuctios so the declaratio ad commet is all a programmer eeds to use the fuctio Fuctio commet should tell all coditios required of argumets to the fuctio Fuctio commet should describe the retured value Variables used i the fuctio, other tha the formal parameters, should be declared i the fuctio body Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-38

39 Formal Parameter Names Fuctios are desiged as self-cotaied modules Differet programmers may write each fuctio Programmers choose meaigful ames for formal parameters Formal parameter ames may or may ot match variable ames used i the mai part of the program It does ot matter if formal parameter ames match other variable ames i the program Remember that oly the value of the argumet is plugged ito the formal parameter Display 4.8 Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-39

40 Case Study Buyig Pizza What size pizza is the best buy? Which size gives the lowest cost per square ich? Pizza sizes give i diameter Quatity of pizza is based o the area which is proportioal to the square of the radius Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-40

41 Buyig Pizza Problem Defiitio Iput: Diameter of two sizes of pizza Cost of the same two sizes of pizza Output: Cost per square ich for each size of pizza Which size is the best buy Based o lowest price per square ich If cost per square ich is the same, the smaller size will be the better buy Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-41

42 Buyig Pizza Problem Aalysis Subtask 1 Get the iput data for each size of pizza Subtask 2 Compute price per ich for smaller pizza Subtask 3 Compute price per ich for larger pizza Subtask 4 Determie which size is the better buy Subtask 5 Output the results Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-42

43 Buyig Pizza Fuctio Aalysis Subtask 2 ad subtask 3 should be implemeted as a sigle fuctio because Subtask 2 ad subtask 3 are idetical tasks The calculatio for subtask 3 is the same as the calculatio for subtask 2 with differet argumets Subtask 2 ad subtask 3 each retur a sigle value Choose a appropriate ame for the fuctio We ll use uitprice Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-43

44 Buyig Pizza uitprice Declaratio double uitprice(it diameter, it double price); //Returs the price per square ich of a pizza //The formal parameter amed diameter is the //diameter of the pizza i iches. The formal // parameter amed price is the price of the // pizza. Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-44

45 Buyig Pizza Algorithm Desig Subtask 1 Ask for the iput values ad store them i variables Subtask 4 diameter_small diameter_large price_small price_large Compare cost per square ich of the two pizzas usig the less tha operator Subtask 5 Stadard output of the results Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-45

46 Buyig Pizza uitprice Algorithm Subtasks 2 ad 3 are implemeted as calls to fuctio uitprice uitprice algorithm Compute the radius of the pizza π r 2 Computer the area of the pizza usig Retur the value of (price / area) Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-46

47 Buyig Pizza uitprice Pseudocode Pseudocode Mixture of C++ ad eglish Allows us to make the algorithm more precise without worryig about the details of C++ sytax uitprice pseudocode radius = oe half of diameter; area = π * radius * radius retur (price / area) Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-47

48 Buyig Pizza The Calls of uitprice Mai part of the program implemets calls of uitprice as double uit_price_small, uit_price_large; uit_price_small = uitprice(diameter_small, price_small); uit_price_large = uitprice(diameter_large, price_large); Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-48

49 Buyig Pizza First try at uitprice double uitprice (it diameter, double price) { cost double PI = ; double radius, area; } radius = diameter / 2; area = PI * radius * radius; retur (price / area); Oops! Radius should iclude the fractioal part Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-49

50 Buyig Pizza Secod try at uitprice double uitprice (it diameter, double price) { cost double PI = ; double radius, area; } radius = diameter / static_cast<double>(2) ; area = PI * radius * radius; retur (price / area); Display 4.10 (1) Display 4.10 (2) Now radius will iclude fractioal parts radius = diameter / 2.0 ; // This would also work Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-50

51 Program Testig Programs that compile ad ru ca still produce errors Testig icreases cofidece that the program works correctly Ru the program with data that has kow output You may have determied this output with pecil ad paper or a calculator Ru the program o several differet sets of data Your first set of data may produce correct results i spite of a logical error i the code Remember the iteger divisio problem? If there is o fractioal remaider, iteger divisio will give apparetly correct results Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-51

52 Use Pseudocode Pseudocode is a mixture of Eglish ad the programmig laguage i use Pseudocode simplifies algorithm desig by allowig you to igore the specific sytax of the programmig laguage as you work out the details of the algorithm If the step is obvious, use C++ If the step is difficult to express i C++, use Eglish Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-52

53 Sectio 4.4 Coclusio Ca you Describe the purpose of the commet that accompaies a fuctio declaratio? Describe what it meas to say a programmer should be able to treat a fuctio as a black box? Describe what it meas for two fuctios to be black box equivalet? Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-53

55 Local Variables Variables declared i a fuctio: Are local to that fuctio, they caot be used from outside the fuctio Have the fuctio as their scope Variables declared i the mai part of a program: Are local to the mai part of the program, they caot be used from outside the mai part Have the mai part as their scope Display 4.11 (1) Display 4.11 (2) Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-55

56 Global Costats Global Named Costat Available to more tha oe fuctio as well as the mai part of the program Declared outside ay fuctio body Declared outside the mai fuctio body Declared before ay fuctio that uses it Example: cost double PI = ; double volume(double); it mai() { } PI is available to the mai fuctio ad to fuctio volume Display 4.12 (1) Display 4.12 (2) Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-56

57 Global Variables Global Variable -- rarely used whe more tha oe fuctio must use a commo variable Declared just like a global costat except cost is ot used Geerally make programs more difficult to uderstad ad maitai Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-57

58 Formal Parameters are Local Variables Formal Parameters are actually variables that are local to the fuctio defiitio They are used just as if they were declared i the fuctio body Do NOT re-declare the formal parameters i the fuctio body, they are declared i the fuctio declaratio The call-by-value mechaism Whe a fuctio is called the formal parameters are iitialized to the values of the argumets i the fuctio call Display 4.13 (1) Display 4.13 (2) Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-58

59 Block Scope Local ad global variables coform to the rules of Block Scope The code block (geerally defied by the { }) where a idetifier like a variable is declared determies the scope of the idetifier Blocks ca be ested Display 4.14 Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 1-59

60 Namespaces Revisited The start of a file is ot always the best place for usig amespace std; Differet fuctios may use differet amespaces Placig usig amespace std; iside the startig brace of a fuctio Allows the use of differet amespaces i differet fuctios Makes the usig directive local to the fuctio Display 4.15 (1) Display 4.15 (2) Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-60

61 Example: Factorial! Represets the factorial fuctio! = 1 x 2 x 3 x x The C++ versio of the factorial fuctio foud i Display 3.14 Requires oe argumet of type it, Returs a value of type it Uses a local variable to store the curret product Decremets each time it does aother multiplicatio * -1 * -2 * * 1 Display 4.16 Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-61

63 Overloadig Fuctio Names C++ allows more tha oe defiitio for the same fuctio ame Very coveiet for situatios i which the same fuctio is eeded for differet umbers or types of argumets Overloadig a fuctio ame meas providig more tha oe declaratio ad defiitio usig the same fuctio ame Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-63

64 Overloadig Examples double ave(double 1, double 2) { retur ((1 + 2) / 2); } double ave(double 1, double 2, double 3) { retur (( ) / 3); } Compiler checks the umber ad types of argumets i the fuctio call to decide which fuctio to use cout << ave( 10, 20, 30); uses the secod defiitio Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-64

65 Overloadig Details Overloaded fuctios Must have differet umbers of formal parameters AND / OR Must have at least oe differet type of parameter Must retur a value of the same type Display 4.17 Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-65

66 Overloadig Example Revisig the Pizza Buyig program Rectagular pizzas are ow offered! Chage the iput ad add a fuctio to compute the uit price of a rectagular pizza The ew fuctio could be amed uitprice_rectagular Or, the ew fuctio could be a ew (overloaded) versio of the uitprice fuctio that is already used Example: double uitprice(it legth, it width, double price) { double area = legth * width; retur (price / area); } Display 4.18 (1 3) Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-66

67 Automatic Type Coversio Give the defiitio double mpg(double miles, double gallos) { retur (miles / gallos); } what will happe if mpg is called i this way? cout << mpg(45, 2) << miles per gallo ; The values of the argumets will automatically be coverted to type double (45.0 ad 2.0) Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-67

68 Type Coversio Problem Give the previous mpg defiitio ad the followig defiitio i the same program it mpg(it goals, it misses) // returs the Measure of Perfect Goals { retur (goals misses); } what happes if mpg is called this way ow? cout << mpg(45, 2) << miles per gallo ; The compiler chooses the fuctio that matches parameter types so the Measure of Perfect Goals will be calculated Do ot use the same fuctio ame for urelated fuctios Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-68

69 Sectio 4.6 Coclusio Ca you Describe Top-Dow Desig? Describe the types of tasks we have see so far that could be implemeted as C++ fuctios? Describe the priciples of The black box Procedural abstractio Iformatio hidig Defie local variable? Overload a fuctio ame? Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 4-69

Chapter Procedural Abstraction and Functions That Return a Value. Overview. Top-Down Design. Benefits of Top Down Design.

Chapter Procedural Abstraction and Functions That Return a Value. Overview. Top-Down Design. Benefits of Top Down Design. Chapter 4 Procedural Abstraction and Functions That Return a Value Overview 4.1 Top-Down Design 4.2 Predefined Functions 4.3 Programmer-Defined Functions 4.4 Procedural Abstraction 4.5 Local Variables