Chapter 3. More Flow of Control. Copyright 2015 Pearson Education, Ltd.. All rights reserved.

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4 Flow Of Cotrol Flow of cotrol refers to the order i which program statemets are performed We have see the followig ways to specify flow of cotrol if-else-statemets while-statemets do-while-statemets New methods described i this chapter iclude switch-statemets for-statemets Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-4

6 Usig Boolea Expressios A Boolea Expressio is a expressio that is either true or false Boolea expressios are evaluated usig relatioal operatios such as = =, <, ad >= which produce a boolea value ad boolea operatios such as &&,, ad! which also produce a boolea value Type bool allows declaratio of variables that carry the value true or false Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-6

7 Evaluatig Boolea Expressios Boolea expressios are evaluated usig values from the Truth Tables i Display 3.1 For example, if y is 8, the expressio!( ( y < 3) ( y > 7) ) is evaluated i the followig sequece! ( false true )! ( true ) false Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-7

8 Order of Precedece If parethesis are omitted from boolea expressios, the default precedece of operatios is: Perform! operatios first Perform relatioal operatios such as < ext Perform && operatios ext Perform operatios last Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-8

9 Precedece Rules Items i expressios are grouped by precedece rules for arithmetic ad boolea operators Operators with higher precedece are performed first Biary operators with equal precedece are performed left to right Uary operators of equal precedece are performed right to left Display 3.2 Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-9

10 Precedece Rule Example The expressio (x+1) > 2 (x + 1) < -3 is equivalet to ( (x + 1) > 2) ( ( x + 1) < -3) Because > ad < have higher precedece tha ad is also equivalet to x + 1 > 2 x + 1 < - 3 Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-10

11 Evaluatig x + 1 > 2 x + 1 < - 3 Usig the precedece rules of Display 3.2 First apply the uary Next apply the +'s Now apply the > ad < Fially do the Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-11

12 Short-Circuit Evaluatio Some boolea expressios do ot eed to be completely evaluated if x is egative, the value of the expressio (x >= 0) && ( y > 1) ca be determied by evaluatig oly (x >= 0) C++ uses short-circuit evaluatio If the value of the leftmost sub-expressio determies the fial value of the expressio, the rest of the expressio is ot evaluated Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-12

13 Usig Short-Circuit Evaluatio Short-circuit evaluatio ca be used to prevet ru time errors Cosider this if-statemet if ((kids!= 0) && (pieces / kids >= 2) ) cout << "Each child may have two pieces!"; If the value of kids is zero, short-circuit evaluatio prevets evaluatio of (pieces / 0 >= 2) Divisio by zero causes a ru-time error Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-13

15 Problems with! The expressio (! time > limit ), with limit = 60, is evaluated as (!time) > limit If time is a it with value 36, what is!time? False! Or zero sice it will be compared to a iteger The expressio is further evaluated as 0 > limit false Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-15

16 Correctig the! Problem The itet of the previous expressio was most likely the expressio (! ( time > limit) ) which evaluates as (! ( false) ) true Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-16

17 Avoidig! Just as ot i Eglish ca make thigs ot udifficult to read, the! operator ca make C++ expressios difficult to uderstad Before usig the! operator see if you ca express the same idea more clearly without the! operator Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-17

18 Eumeratio Types (Optioal) A eumeratio type is a type with values defied by a list of costats of type it Example: eum MothLegth{JAN_LENGTH = 31, FEB_LENGTH = 28, MAR_LENGTH = 31, DEC_LENGTH = 31}; Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-18

19 Default eum Values If umeric values are ot specified, idetifiers are assiged cosecutive values startig with 0 eum Directio { NORTH = 0, SOUTH = 1, EAST = 2, WEST = 3}; is equivalet to eum Directio {NORTH, SOUTH, EAST, WEST}; Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-19

20 Eumeratio Values Uless specified, the value assiged a eumeratio costat is 1 more tha the previous costat eum MyEum{ONE = 17, TWO, THREE, FOUR = -3, FIVE}; results i these values ONE = 17, TWO = 18, THREE = 19, FOUR = -3, FIVE = -2 Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-20

21 Strog Eums C++11 itroduced a ew versio of eumeratio called strog eums or eum classes that avoids some problems of covetioal eums May ot wat a eum to act like a it Eums are global so you ca t have the same eum value twice Defie a strog eumas follows: Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 1-21

22 Usig Strog Eums To use our strog eums: Days d = Days::Tue; Weather w = Weather::Su; The variables d ad w are ot itegers so we ca t treat them as such. Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 1-22

23 Sectio 3.1 Coclusio Ca you Write a fuctio defiitio for a fuctio amed i_order that takes three argumets of type it? The fuctio returs true if the argumets are i ascedig order; otherwise, it returs false. Determie the value of these Boolea expressios? Assume cout = 0 ad limit = 10 (cout == 0) && (limit < 20)!(cout == 12) (limit < 0) && ((limit /cout) > 7) Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-23

25 Multiway Braches A brachig mechaism selects oe out of a umber of alterative actios The if-else-statemet is a brachig mechaism Brachig mechaisms ca be a subpart of aother brachig mechaism A if-else-statemet ca iclude aother if-else-statemet as a subpart Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-25

26 Nested Statemets A statemet that is a subpart of aother statemet is a ested statemet Whe writig ested statemets it is ormal to idet each level of estig ideted Example: Display 3.3 if (cout < 10) if ( x < y) cout << x << " is less tha " << y; else cout << y << " is less tha " << x; Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-26

27 Nested if-else Statemets Use care i estig if-else-statemets Example: To desig a if-else statemet to war a driver whe fuel is low, but tells the driver to bypass pit stops if the fuel is close to full. Other wise there should be o output. Pseudocode: if fuel gauge is below ¾ the: if fuel gauge is below ¼ the: issue a warig otherwise (gauge > ¾) the: output a statemet sayig do't stop Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-27

28 First Try Nested if's Traslatig the previous pseudocode to C++ could yield (if we are ot careful) if (fuel_gauge_readig < 0.75) if (fuel_gauge_readig < 0.25) cout << "Fuel very low. Cautio!\"; else cout << "Fuel over 3/4. Do't stop ow!\"; This would compile ad ru, but does ot produce the desired results The compiler pairs the "else" with the earest previous "if" Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-28

29 Braces ad Nested Statemets Braces i ested statemets are like parethesis i arithmetic expressios Braces tell the compiler how to group thigs Use braces aroud substatemets Display 3.4 demostrates the use of braces i ested if-else-statemets Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-29

30 Multi-way if-else-statemets A if-else-statemet is a two-way brach Three or four (or more) way braches ca be desiged usig ested if-else-statemets Example: The umber guessig game with the umber stored i variable umber, the guess i variable guess. How do we give hits? Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-30

31 Number Guessig The followig ested statemets implemet the hits for our umber guessig game if (guess> umber) cout << "Too high."; else if (guess < umber) cout << "Too low."); else if (guess == umber) cout << "Correct!"; Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-31

32 Idetig Nested if-else Notice how the code o the previous slide crept across the page leavig less ad less space Use this alterative for idetig several ested if-else-statemets: if (guess> umber) cout << "Too high."; else if (guess < umber) cout << "Too low."); else if (guess == umber) cout << "Correct!"; Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-32

33 The Fial if-else-statemet Whe the coditios tested i a if-else-statemet are mutually exclusive, the fial if-else ca sometimes be omitted. The previous example ca be writte as if (guess> umber) cout << "Too high."; else if (guess < umber) cout << "Too low."); else // (guess == umber) cout << "Correct!"; Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-33

34 Nested if-else Sytax A Multiway if-else statemet is writte as if(boolea_expressio_1) Statemet_1 else if ( Boolea_Expressio_2) Statemet_2 else if (Boolea_Expressio_) Statemet _ else Statemet_For_All_Other_Possibilities Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-34

35 Program Example: State Icome Tax Write a program for a state that computes tax accordig to the rate schedule: No tax o first $15,000 of icome 5% tax o each dollar from $15,001 to $25,000 10% tax o each dollar over $25,000 Display 3.5 (1) Display 3.5 (2) Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-35

36 Refiig if-else-statemets Notice that the lie else if (( et_icome > && et_icome < = 25000)) ca be replaced with else if (et_icome <= 25000) The computer will ot get to this lie uless it is already determied that et_icome > Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-36

37 The switch-statemet The switch-statemet is a alterative for costructig multi-way braches The example i Display 3.6 determies output based o a letter grade Grades 'A', 'B', ad 'C' each have a brach Grades 'D' ad 'F' use the same brach If a ivalid grade is etered, a default brach is used Display 3.6 (1) Display 3.6 (2) Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-37

38 switch-statemet Sytax switch (cotrollig expressio) { case Costat_1: statemet_sequece_1 break; case Costat_2: Statemet_Sequece_2 break;... case Costat_: Statemet_Sequece_ break; default: Default_Statemet_Sequece } Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-38

39 The Cotrollig Statemet A switch statemet's cotrollig statemet must retur oe of these types A bool value A eum costat A iteger type A character The value retured is compared to the costat values after each "case" Whe a match is foud, the code for that case is used Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-39

40 The break Statemet The break statemet eds the switch-statemet Omittig the break statemet will cause the code for the ext case to be executed! Omittig a break statemet allows the use of multiple case labels for a sectio of code case 'A': case 'a': cout << "Excellet."; break; Rus the same code for either 'A' or 'a' Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-40

41 The default Statemet If o case label has a costat that matches the cotrollig expressio, the statemets followig the default label are executed If there is o default label, othig happes whe the switch statemet is executed It is a good idea to iclude a default sectio Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-41

42 Switch-statemets ad Meus Nested if-else statemets are more versatile tha a switch statemet Switch-statemets ca make some code more clear A meu is a atural applicatio for a switchstatemet Display 3.7 (1) Display 3.7 (2) Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-42

43 Fuctio Calls i Braches Switch ad if-else-statemets allow the use of multiple statemets i a brach Multiple statemets i a brach ca make the switch or if-else-statemet difficult to read Usig fuctio calls (as show i Display 3.7) istead of multiple statemets ca make the switch or if-else-statemet much easier to read Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-43

44 Blocks Each brach of a switch or if-else statemet is a separate sub-task If the actio of a brach is too simple to warrat a fuctio call, use multiple statemets betwee braces A block is a sectio of code eclosed by braces Variables declared withi a block, are local to the block or have the block as their scope. Variable ames declared i the block ca be reused outside the block Display 3.8 (1) Display 3.8 (2) Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-44

45 Statemet Blocks A statemet block is a block that is ot a fuctio body or the body of the mai part of a program Statemet blocks ca be ested i other statemet blocks Nestig statemet blocks ca make code difficult to read It is geerally better to create fuctio calls tha to est statemet blocks Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-45

46 Scope Rule for Nested Blocks If a sigle idetifier is declared as a variable i each of two blocks, oe withi the other, the these are two differet variables with the same ame Oe of the variables exists oly withi the ier block ad caot be accessed outside the ier block The other variable exists oly i the outer block ad caot be accessed i the ier block Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-46

47 Sectio 3.2 Coclusio Ca you Give the output of this code fragmet? { it x = 1; cout << x << edl; { cout << x << edl; it x = 2; cout << x << edl; } cout << x << edl; Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-47

49 More About C++ Loop Statemets A loop is a program costructio that repeats a statemet or sequece of statemets a umber of times The body of the loop is the statemet(s) repeated Each repetitio of the loop is a iteratio Loop desig questios: What should the loop body be? How may times should the body be iterated? Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-49

50 while ad do-while A importat differece betwee while ad do-while loops: A while loop checks the Boolea expressio at the begiig of the loop A while loop might ever be executed! A do-while loop checks the Boolea expressio at the ed of the loop A do-while loop is always executed at least oce Review while ad do-while sytax i Display 3.9 Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-50

51 The Icremet Operator We have used the icremet operator i statemets such as umber++; to icrease the value of umber by oe The icremet operator ca also be used i expressios: it umber = 2; it value_produced = 2 * (umber++); (umber++) first returs the value of umber (2) to be multiplied by 2, the icremets umber to three Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-51

52 umber++ vs ++umber (umber++) returs the curret value of umber, the icremets umber A expressio usig (umber++) will use the value of umber BEFORE it is icremeted (++umber) icremets umber first ad returs the ew value of umber A expressio usig (++umber) will use the value of umber AFTER it is icremeted Number has the same value after either versio! Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-52

53 ++ Comparisos it umber = 2; it value_produced = 2 * (umber++); cout << value_produced << " " << umber; displays 4 3 it umber = 2; it value_produced = 2* (++umber); cout << value_produced << " " umber; displays 6 3 Display 3.10 Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-53

54 The Decremet Operator The decremet operator (--) decreases the value of the variable by oe it umber = 8; it value_produced = umber--; cout << value_produced << " " << umber; displays 8 7 Replacig "umber--" with "--umber" displays 7 7 Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-54

55 The for-statemet A for-statemet (for-loop) is aother loop mechaism i C++ Desiged for commo tasks such as addig umbers i a give rage Is sometimes more coveiet to use tha a while loop Does ot do aythig a while loop caot do Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-55

56 for/while Loop Compariso sum = 0; = 1; while( <= 10) // add the umbers 1-10 { sum = sum + ; ++; } sum = 0; for ( = 1; <= 10; ++) //add the umbers 1-10 sum = sum + ; Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-56

57 For Loop Dissectio The for loop uses the same compoets as the while loop i a more compact form for ( = 1; <= 10; ++) Iitializatio Actio Update Actio Boolea Expressio Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-57

58 for Loop Alterative A for loop ca also iclude a variable declaratio i the iitializatio actio for (it = 1; < = 10; ++) This lie meas Create a variable,, of type it ad iitialize it with 1 Cotiue to iterate the body as log as <= 10 Icremet by oe after each iteratio For-loop sytax ad while loop compariso are foud i Display 3.11 Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-58

59 for-loop Details Iitializatio ad update actios of for-loops ofte cotai more complex expressios Here are some samples for ( = 1; < = 10; = + 2) for( = 0 ; > -100 ; = -7) for(double x = pow(y,3.0); x > 2.0; x = sqrt(x) ) Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-59

60 The for-loop Body The body of a for-loop ca be A sigle statemet A compoud statemet eclosed i braces Example: Display 3.13 for(it umber = 1; umber >= 0; umber--) { // loop body statemets } shows the sytax for a for-loop with a multi-statemet body Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-60

61 The Empty Statemet A semicolo creates a C++ statemet Placig a semicolo after x++ creates the statemet x++; Placig a semicolo after othig creates a empty statemet that compiles but does othig cout << "Hello" << edl; ; cout << "Good Bye"<< edl; Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-61

62 Extra Semicolo Placig a semicolo after the paretheses of a for loop creates a empty statemet as the body of the loop Example: for(it cout = 1; cout <= 10; cout++); cout << "Hello\"; prits oe "Hello", but ot as part of the loop! The empty statemet is the body of the loop cout << "Hello\"; is ot part of the loop body! Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-62

63 Local Variable Stadard ANSI C++ stadard requires that a variable declared i the for-loop iitializatio sectio be local to the block of the for-loop Fid out how your compiler treats these variables! If you wat your code to be portable, do ot deped o all compilers to treat these variables as local to the for-loop! Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-63

64 Which Loop To Use? Choose the type of loop late i the desig process First desig the loop usig pseudocode Traslate the pseudocode ito C++ The traslatio geerally makes the choice of a appropriate loop clear While-loops are used for all other loops whe there might be occassios whe the loop should ot ru Do-while loops are used for all other loops whe the loop must always ru at least oce Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-64

65 Choosig a for-loop for-loops are typically selected whe doig umeric calculatios, especially whe usig a variable chaged by equal amouts each time the loop iterates Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-65

66 Choosig a while-loop A while-loop is typically used Whe a for-loop is ot appropriate Whe there are circumstaces for which the loop body should ot be executed at all Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-66

67 Choosig a do-while Loop A do-while-loop is typically used Whe a for-loop is ot appropriate Whe the loop body must be executed at least oce Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-67

68 The break-statemet There are times to exit a loop before it eds If the loop checks for ivalid iput that would rui a calculatio, it is ofte best to ed the loop The break-statemet ca be used to exit a loop before ormal termiatio Be careful with ested loops! Usig break oly exits the loop i which the break-statemet occurs Display 3.14 Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-68

69 Sectio 3.3 Coclusio Ca you Determie the output of the followig? for(it cout = 1; cout < 5; cout++) cout << (2 * cout) << " " ; Determie which type of loop is likely to be best for Summig a series such as 1/2 + 1/3 + 1/ /10? Readig a list of exam scores for oe studet? Testig a fuctio to see how it performs with differet values of its argumets Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-69

72 Sums ad Products A commo task is readig a list of umbers ad computig the sum Pseudocode for this task might be: sum = 0; repeat the followig this_may times ci >> ext; sum = sum + ext; ed of loop This pseudocode ca be implemeted with a for-loop as show o the ext slide Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-72

73 for-loop for a sum The pseudocode from the previous slide is implemeted as it sum = 0; for(it cout=1; cout <= this_may; cout++) { ci >> ext; sum = sum + ext; } sum must be iitialized prior to the loop body! Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-73

74 Repeat "this may times" Pseudocode cotaiig the lie repeat the followig "this may times" is ofte implemeted with a for-loop A for-loop is geerally the choice whe there is a predetermied umber of iteratios Example: for(it cout = 1; cout <= this_may; cout++) Loop_body Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-74

75 for-loop For a Product Formig a product is very similar to the sum example see earlier it product = 1; for(it cout=1; cout <= this_may; cout++) { ci >> ext; product = product * ext; } product must be iitialized prior to the loop body Notice that product is iitialized to 1, ot 0! Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-75

76 Edig a Loop The are four commo methods to termiate a iput loop List headed by size Whe we ca determie the size of the list beforehad Ask before iteratig Ask if the user wats to cotiue before each iteratio List eded with a setiel value Usig a particular value to sigal the ed of the list Ruig out of iput Usig the eof fuctio to idicate the ed of a file Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-76

77 List Headed By Size The for-loops we have see provide a atural implemetatio of the list headed by size method of edig a loop Example: it items; cout << "How may items i the list?"; ci >> items; for(it cout = 1; cout <= items; cout++) { it umber; cout << "Eter umber " << cout; ci >> umber; cout << edl; // statemets to process the umber } Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-77

78 Ask Before Iteratig A while loop is used here to implemet the ask before iteratig method to ed a loop sum = 0; cout << "Are there umbers i the list (Y/N)?"; char as; ci >> as; while (( as = 'Y') (as = 'y')) { //statemets to read ad process the umber cout << "Are there more umbers(y/n)? "; ci >> as; } Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-78

79 List Eded With a Setiel Value A while loop is typically used to ed a loop usig the list eded with a setiel value method cout << "Eter a list of oegative itegers.\" << "Place a egative iteger after the list.\"; sum = 0; ci >> umber; while (umber > 0) { //statemets to process the umber ci >> umber; } Notice that the setiel value is read, but ot processed Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-79

80 Ruig Out of Iput The while loop is typically used to implemet the ruig out of iput method of edig a loop ifstream ifile; ifile.ope("data.dat"); while (! ifile.eof( ) ) { // read ad process items from the file // File I/O covered i Chapter 6 } ifile.close( ); Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-80

81 Geeral Methods To Cotrol Loops Three geeral methods to cotrol ay loop Cout cotrolled loops Ask before iteratig Exit o flag coditio Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-81

82 Cout Cotrolled Loops Cout cotrolled loops are loops that determie the umber of iteratios before the loop begis The list headed by size is a example of a cout cotrolled loop for iput Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-82

83 Exit o Flag Coditio Loops ca be eded whe a particular flag coditio exists A variable that chages value to idicate that some evet has take place is a flag Examples of exit o a flag coditio for iput List eded with a setiel value Ruig out of iput Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-83

84 Exit o Flag Cautio Cosider this loop to idetify a studet with a grade of 90 or better it = 1; grade = compute_grade(); while (grade < 90) { ++; grade = compute_grade(); } cout << "Studet umber " << << " has a score of " << grade << edl; Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-84

85 The Problem The loop o the previous slide might ot stop at the ed of the list of studets if o studet has a grade of 90 or higher It is a good idea to use a secod flag to esure that there are still studets to cosider The code o the followig slide shows a better solutio Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-85

86 The Exit O Flag Solutio This code solves the problem of havig o studet grade at 90 or higher it =1; grade = compute_grade(); while (( grade < 90) && ( < umber_of_studets)) { // same as before } if (grade > 90) // same output as before else cout << "No studet has a high score."; Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-86

87 Nested Loops The body of a loop may cotai ay kid of statemet, icludig aother loop Whe loops are ested, all iteratios of the ier loop are executed for each iteratio of the outer loop Give serious cosideratio to makig the ier loop a fuctio call to make it easier to read your program Display 3.15 show two versios of a program with ested loops Display 3.15 Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-87

88 Debuggig Loops Commo errors ivolvig loops iclude Off-by-oe errors i which the loop executes oe too may or oe too few times Ifiite loops usually result from a mistake i the Boolea expressio that cotrols the loop Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-88

89 Fixig Off By Oe Errors Check your compariso: should it be < or <=? Check that the iitializatio uses the correct value Does the loop hadle the zero iteratios case? Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-89

90 Fixig Ifiite Loops Check the directio of iequalities: < or >? Test for < or > rather tha equality (==) Remember that doubles are really oly approximatios Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-90

91 More Loop Debuggig Tips Be sure that the mistake is really i the loop Trace the variable to observe how the variable chages Tracig a variable is watchig its value chage durig executio May systems iclude utilities to help with this cout statemets ca be used to trace a value Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-91

92 Debuggig Example The followig code is supposed to coclude with the variable product cotaiig the product of the umbers 2 through 5 it ext = 2, product = 1; while (ext < 5) { ext++; product = product * ext; } Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-92

93 Tracig Variables Add temporary cout statemets to trace variables it ext = 2, product = 1; while (ext < 5) { ext++; product = product * ext; cout << "ext = " << ext << "product = " << product << edl; } Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-93

94 First Fix The cout statemets added to the loop show us that the loop ever multiplied by 2 Solve the problem by movig the statemet ext++ it ext = 2, product = 1; while (ext < 5) { product = product * ext; ext++; } cout << "ext = " << ext << "product = " << product << edl; There is still a problem! Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-94

95 Secod Fix Re-testig the loop shows us that ow the loop ever multiplies by 5 The fix is to use <= istead of < i our compariso it ext = 2, product = 1; while (ext <= 5) { product = product * ext; ext++; } Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-95

96 Loop Testig Guidelies Every time a program is chaged, it must be retested Chagig oe part may require a chage to aother Every loop should at least be tested usig iput to cause: Zero iteratios of the loop body Oe iteratio of the loop body Oe less tha the maximum umber of iteratios The maximum umber of iteratos Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-96

97 Startig Over Sometimes it is more efficiet to throw out a buggy program ad start over The ew program will be easier to read The ew program is less likely to be as buggy You may develop a workig program faster tha if you repair the bad code The lessos leared i the buggy code will help you desig a better program faster Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-97

98 Chapter 3.4 Coclusio Ca you Describe how to trace a variable? List possible solutios to a off-by-oe error? Determie the umber of fece posts eeded for a 100 meter log fece? Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-98

118 Display 3.15 Back Next //DISPLAY 3.15 Explicitly Nested Loops //Determies the total umber of gree-ecked vulture eggs //couted by all coservatioists i the coservatio district. #iclude <iostream> usig amespace std; it mai() { cout << "This program tallies coservatioist reports\" << "o the gree-ecked vulture.\" << "Each coservatioist's report cosists of\" << "a list of umbers. Each umber is the cout of\" << "the eggs observed i oe " << "gree-ecked vulture est.\" << "This program the tallies " << "the total umber of eggs.\"; it umber_of_reports; cout << "How may coservatioist reports are there? "; ci >> umber_of_reports; it grad_total = 0, subtotal, cout; for (cout = 1; cout <= umber_of_reports; cout++) { cout << edl << "Eter the report of " << "coservatioist umber " << cout << edl; } cout << "Eter the umber of eggs i each est.\" << "Place a egative iteger at the ed of your list.\"; subtotal = 0; it ext; ci >> ext; while (ext >=0) { subtotal = subtotal + ext; ci >> ext; } cout << "Total egg cout for coservatioist " << " umber " << cout << " is " << subtotal << edl; grad_total = grad_total + subtotal; } cout << edl << "Total egg cout for all reports = " << grad_total << edl; retur 0; Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Slide 3-118

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

Chapter 4. Procedural Abstraction and Functions That Return a Value. Copyright 2015 Pearson Education, Ltd.. All rights reserved. Chapter 4 Procedural Abstractio ad Fuctios That Retur a Value Copyright 2015 Pearso Educatio, Ltd.. All rights reserved. Overview 4.1 Top-Dow Desig 4.2 Predefied Fuctios 4.3 Programmer-Defied Fuctios 4.4