Chapter 6. I/O Streams as an Introduction to Objects and Classes. Copyright 2014 Pearson Addison-Wesley. All rights reserved.

Transcription

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2 Chapter 6 I/O Streams as a Itroductio to Objects ad Classes

3 Overview 6.1 Streams ad Basic File I/O 6.2 Tools for Stream I/O 6.3 Character I/O Slide 6-3

4 6.1 Streams ad Basic File I/O

5 I/O Streams I/O refers to program iput ad output Iput is delivered to your program via a stream object Iput ca be from The keyboard A file Output is delivered to the output device via a stream object Output ca be to The scree A file Slide 6-5

6 Objects Objects are special variables that Have their ow special-purpose fuctios Set C++ apart from earlier programmig laguages Slide 6-6

7 Streams ad Basic File I/O Files for I/O are the same type of files used to store programs A stream is a flow of data. Iput stream: Data flows ito the program If iput stream flows from keyboard, the program will accept data from the keyboard If iput stream flows from a file, the program will accept data from the file Output stream: Data flows out of the program To the scree To a file Slide 6-7

8 ci Ad cout Streams ci Iput stream coected to the keyboard cout Output stream coected to the scree ci ad cout defied i the iostream library Use iclude directive: #iclude <iostream> You ca declare your ow streams to use with files. Slide 6-8

9 Why Use Files? Files allow you to store data permaetly! Data output to a file lasts after the program eds A iput file ca be used over ad over No typig of data agai ad agai for testig Create a data file or read a output file at your coveiece Files allow you to deal with larger data sets Slide 6-9

10 File I/O Readig from a file Takig iput from a file Doe from begiig to the ed (for ow) No backig up to read somethig agai (OK to start over) Just as doe from the keyboard Writig to a file Sedig output to a file Doe from begiig to ed (for ow) No backig up to write somethig agai( OK to start over) Just as doe to the scree Slide 6-10

11 Stream Variables Like other variables, a stream variable Must be declared before it ca be used Must be iitialized before it cotais valid data Iitializig a stream meas coectig it to a file The value of the stream variable ca be thought of as the file it is coected to Ca have its value chaged Chagig a stream value meas discoectig from oe file ad coectig to aother Slide 6-11

12 Streams ad Assigmet A stream is a special kid of variable called a object Objects ca use special fuctios to complete tasks Streams use special fuctios istead of the assigmet operator to chage values Slide 6-12

13 Declarig A Iput-file Stream Variable Iput-file streams are of type ifstream Type ifstream is defied i the fstream library You must use the iclude ad usig directives #iclude <fstream> usig amespace std; Declare a iput-file stream variable usig ifstream i_stream; Slide 6-13

14 Declarig A Output-file Stream Variable Ouput-file streams of are type ofstream Type ofstream is defied i the fstream library You must use these iclude ad usig directives #iclude <fstream> usig amespace std; Declare a iput-file stream variable usig ofstream out_stream; Slide 6-14

15 Coectig To A File Oce a stream variable is declared, coect it to a file Coectig a stream to a file is opeig the file Use the ope fuctio of the stream object i_stream.ope("ifile.dat"); Period Double quotes File ame o the disk Slide 6-15

16 Usig The Iput Stream Oce coected to a file, the iput-stream variable ca be used to produce iput just as you would use ci with the extractio operator Example: it oe_umber, aother_umber; i_stream >> oe_umber >> aother_umber; Slide 6-16

17 Usig The Output Stream A output-stream works similarly to the iput-stream ofstream out_stream; out_stream.ope("outfile.dat"); out_stream << "oe umber = " << oe_umber << "aother umber = " << aother_umber; Slide 6-17

18 Exteral File Names A Exteral File Name Is the ame for a file that the operatig system uses ifile.dat ad outfile.dat used i the previous examples Is the "real", o-the-disk, ame for a file Needs to match the amig covetios o your system Usually oly used i the stream's ope statemet Oce ope, referred to usig the ame of the stream coected to it. Slide 6-18

19 Closig a File After usig a file, it should be closed This discoects the stream from the file Close files to reduce the chace of a file beig corrupted if the program termiates abormally It is importat to close a output file if your program later eeds to read iput from the output file The system will automatically close files if you forget as log as your program eds ormally Display 6.1 Slide 6-19

20 Objects A object is a variable that has fuctios ad data associated with it i_stream ad out_stream each have a fuctio amed ope associated with them i_stream ad out_stream use differet versios of a fuctio amed ope Oe versio of ope is for iput files A differet versio of ope is for output files Slide 6-20

21 Member Fuctios A member fuctio is a fuctio associated with a object The ope fuctio is a member fuctio of i_stream i the previous examples A differet ope fuctio is a member fuctio of out_stream i the previous examples Slide 6-21

22 Objects ad Member Fuctio Names Objects of differet types have differet member fuctios Some of these member fuctios might have the same ame Differet objects of the same type have the same member fuctios Slide 6-22

23 Classes A type whose variables are objects, is a class ifstream is the type of the i_stream variable (object) ifstream is a class The class of a object determies its member fuctios Example: ifstream i_stream1, i_stream2; i_stream1.ope ad i_stream2.ope are the same fuctio but might have differet argumets Slide 6-23

24 Class Member Fuctios Member fuctios of a object are the member fuctios of its class The class determies the member fuctios of the object The class ifstream has a ope fuctio Every variable (object) declared of type ifstream has that ope fuctio Slide 6-24

25 Callig a Member Fuctio Callig a member fuctio requires specifyig the object cotaiig the fuctio The callig object is separated from the member fuctio by the dot operator Example: i_stream.ope("ifile.dat"); Callig object Member fuctio Dot operator Slide 6-25

26 Member Fuctio Callig Sytax Sytax for callig a member fuctio: Callig_object.Member_Fuctio_Name(Argumet_list); Slide 6-26

27 Errors O Opeig Files Opeig a file could fail for several reasos Commo reasos for ope to fail iclude The file might ot exist The ame might be typed icorrectly May be o error message if the call to ope fails Program executio cotiues! Slide 6-27

28 Catchig Stream Errors Member fuctio fail, ca be used to test the success of a stream operatio fail returs a boolea type (true or false) fail returs true if the stream operatio failed Slide 6-28

29 Haltig Executio Whe a stream ope fuctio fails, it is geerally best to stop the program The fuctio exit, halts a program exit returs its argumet to the operatig system exit causes program executio to stop exit is NOT a member fuctio Exit requires the iclude ad usig directives #iclude <cstdlib> usig amespace std; Slide 6-29

30 Usig fail ad exit Immediately followig the call to ope, check that the operatio was successful: i_stream.ope("stuff.dat"); if( i_stream.fail( ) ) { cout << "Iput file opeig failed.\"; exit(1) ; } Display 6.2 Slide 6-30

31 Techiques for File I/O Whe readig iput from a file Do ot iclude prompts or echo the iput The lies cout << "Eter the umber: "; ci >> the_umber; cout << "The umber you etered is " << the_umber; become just oe lie i_file >> the_umber; The iput file must cotai exactly the data expected Slide 6-31

32 Appedig Data (optioal) Output examples so far create ew files If the output file already cotais data, that data is lost To apped ew output to the ed a existig file use the costat ios::app defied i the iostream library: outstream.ope("importat.txt", ios::app); If the file does ot exist, a ew file will be created Display 6.3 Slide 6-32

33 File Names as Iput (optioal) Program users ca eter the ame of a file to use for iput or for output Program must use a variable that ca hold multiple characters A sequece of characters is called a strig Declarig a variable to hold a strig of characters: char file_ame[16]; file_ame is the ame of a variable Brackets eclose the maximum umber of characters + 1 The variable file_ame cotais up to 15 characters Slide 6-33

34 Usig A Character Strig char file_ame[16]; cout << "Eter the file_ame "; ci >> file_ame; ifstream i_stream; i_stream.ope(file_ame); if (i_stream.fail( ) ) { cout << "Iput file opeig failed.\"; exit(1); } Display 6.4 (1) Display 6.4 (2) Slide 6-34

35 Sectio 6.1 Coclusio Ca you Write a program that uses a stream called fi which will be coected to a iput file ad a stream called fout which will be coected to a output file? How do you declare fi ad fout? What iclude directive, if ay, do you ee to place i your program file? Name at least three member fuctios of a iostream object ad give examples of usage of each? Slide 6-35

36 6.2 Tools for Streams I/O

37 Tools for Stream I/O To cotrol the format of the program's output We use commads that determie such details as: The spaces betwee items The umber of digits after a decimal poit The umeric style: scietific otatio for fixed poit Showig digits after a decimal poit eve if they are zeroes Showig plus sigs i frot of positive umbers Left or right justifyig umbers i a give space Slide 6-37

38 Formattig Output to Files Format output to the scree with: cout.setf(ios::fixed); cout.setf(ios::showpoit); cout.precisio(2); Format output to a file usig the out-file stream amed out_stream with: out_stream.setf(ios::fixed); out_stream.setf(ios::showpoit); out_stream.precisio(2); Slide 6-38

39 out_stream.precisio(2); precisio is a member fuctio of output streams After out_stream.precisio(2); Output of umbers with decimal poits will show a total of 2 sigificat digits e e OR will show 2 digits after the decimal poit e e-4 Calls to precisio apply oly to the stream amed i the call Slide 6-39

40 setf(ios::fixed); setf is a member fuctio of output streams setf is a abbreviatio for set flags A flag is a istructio to do oe of two optios ios::fixed is a flag After out_stream.setf(ios::fixed); All further output of floatig poit umbers Will be writte i fixed-poit otatio, the way we ormally expect to see umbers Calls to setf apply oly to the stream amed i the call Slide 6-40

41 setf(ios::showpoit); After out_stream.setf(ios::showpoit); Output of floatig poit umbers Will show the decimal poit eve if all digits after the decimal poit are zeroes Display 6.5 Slide 6-41

42 Creatig Space i Output The width fuctio specifies the umber of spaces for the ext item Applies oly to the ext item of output Example: To prit the digit 7 i four spaces use out_stream.width(4); out_stream << 7 << edl; Three of the spaces will be blak 7 7 (ios::right) (ios::left) Slide 6-42

43 Not Eough Width? What if the argumet for width is too small? Such as specifyig cout.width(3); whe the value to prit is The etire item is always output If too few spaces are specified, as may more spaces as eeded are used Slide 6-43

44 Usettig Flags Ay flag that is set, may be uset Use the usetf fuctio Example: cout.usetf(ios::showpos); causes the program to stop pritig plus sigs o positive umbers Slide 6-44

45 Maipulators A maipulator is a fuctio called i a otraditioal way Maipulators i tur call member fuctios Maipulators may or may ot have argumets Used after the isertio operator (<<) as if the maipulator fuctio call is a output item Slide 6-45

46 The setw Maipulator setw does the same task as the member fuctio width setw calls the width fuctio to set spaces for output Example: cout << "Start" << setw(4) << 10 << setw(4) << setw(6) << 30; produces: Start Two Spaces Four Spaces Slide 6-46

47 The setprecisio Maipulator setprecisio does the same task as the member fuctio precisio Example: cout.setf(ios::fixed); cout.setf(ios::showpoit); cout << "$" << setprecisio(2) << 10.3 << edl << "$" << 20.5 << edl; produces: $10.30 $20.50 setprecisio settig stays i effect util chaged Slide 6-47

48 Maipulator Defiitios The maipulators setw ad setprecisio are defied i the iomaip library To use these maipulators, add these lies #iclude <iomaip> usig amespace std; Slide 6-48

49 Stream Names as Argumets Streams ca be argumets to a fuctio The fuctio's formal parameter for the stream must be call-by-referece Example: void make_eat(ifstream& messy_file, ofstream& eat_file); Slide 6-49

50 The Ed of The File Iput files used by a program may vary i legth Programs may ot be able to assume the umber of items i the file A way to kow the ed of the file is reached: The boolea expressio (i_stream >> ext) Reads a value from i_stream ad stores it i ext True if a value ca be read ad stored i ext False if there is ot a value to be read (the ed of the file) Slide 6-50

51 Ed of File Example To calculate the average of the umbers i a file double ext, sum = 0; it cout = 0; while(i_stream >> ext) { sum = sum + ext; cout++; } double average = sum / cout; Slide 6-51

52 Stream Argumets ad Namespaces Usig directives have bee local to fuctio defiitios i the examples so far Whe parameter type ames are i a amespace A usig directive must be outside the fuctio so C++ will uderstad the parameter type ames such as ifstream Easy solutio is to place the usig directive at the begiig of the file May experts do ot approve as this does ot allow usig multiple amespaces with ames i commo Slide 6-52

53 Program Example The program i Display 6.6 Takes iput from rawdata.dat Writes output to the scree ad to eat.dat Formattig istructios are used to create a eater layout Numbers are writte oe per lie i a field width of 12 Each umber is writte with 5 digits after the decimal poit Each umber is writte with a plus or mius sig Uses fuctio make_eat that has formal parameters for the iput-file stream ad output-file stream Display 6.6 (1) Display 6.6 (2) Display 6.6 (3) Slide 6-53

54 Sectio 6.2 Coclusio Ca you Show the output produced whe the followig lie is executed? cout << "*" << setw(3) << << "*" edl; Describe the effect of each of these flags? Ios::fixed ios::scietific ios::showpoit ios::right ios::right ios::showpos Slide 6-54

55 6.3 Character I/O

56 Character I/O All data is iput ad output as characters Output of the umber 10 is two characters '1' ad '0' Iput of the umber 10 is also doe as '1' ad '0' Iterpretatio of 10 as the umber 10 or as characters depeds o the program Coversio betwee characters ad umbers is usually automatic Slide 6-56

57 Low Level Character I/O Low level C++ fuctios for character I/O Perform character iput ad output Do ot perform automatic coversios Allow you to do iput ad output i ayway you ca devise Slide 6-57

58 Member Fuctio get Fuctio get Member fuctio of every iput stream Reads oe character from a iput stream Stores the character read i a variable of type char, the sigle argumet the fuctio takes Does ot use the extractio operator (>>) which performs some automatic work Does ot skip blaks Slide 6-58

59 Usig get These lies use get to read a character ad store it i the variable ext_symbol char ext_symbol; ci.get(ext_symbol); Ay character will be read with these statemets Blak spaces too! '\' too! (The ewlie character) Slide 6-59

60 get Sytax iput_stream.get(char_variable); Examples: char ext_symbol; ci.get(ext_symbol); ifstream i_stream; i_stream.ope("ifile.dat"); i_stream.get(ext_symbol); Slide 6-60

61 More About get Give this code: char c1, c2, c3; ci.get(c1); ci.get(c2); ci.get(c3); ad this iput: AB CD c1 = 'A' c2 = 'B' c3 = '\' ci >> c1 >> c2 >> c3; would place 'C' i c3 (the ">>" operator skips the ewlie character) Slide 6-61

62 The Ed of The Lie To read ad echo a lie of iput Look for '\' at the ed of the iput lie: cout<<"eter a lie of iput ad I will " << "echo it.\"; char symbol; do { ci.get(symbol); cout << symbol; } while (symbol!= '\'); All characters, icludig '\' will be output Slide 6-62

63 '\ ' vs "\ " '\' A value of type char Ca be stored i a variable of type char "\" A strig cotaiig oly oe character Caot be stored i a variable of type char I a cout-statemet they produce the same result Slide 6-63

64 Member Fuctio put Fuctio put Member fuctio of every output stream Requires oe argumet of type char Places its argumet of type char i the output stream Does ot do allow you to do more tha previous output with the isertio operator ad cout Slide 6-64

65 put Sytax Output_stream.put(Char_expressio); Examples: cout.put(ext_symbol); cout.put('a'); ofstream out_stream; out_stream.ope("outfile.dat"); out_stream.put('z'); Slide 6-65

66 Member Fuctio putback The putback member fuctio places a character i the iput stream putback is a member fuctio of every iput stream Useful whe iput cotiues util a specific character is read, but you do ot wat to process the character Places its argumet of type char i the iput stream Character placed i the stream does ot have to be a character read from the stream Slide 6-66

67 putback Example The followig code reads up to the first blak i the iput stream fi, ad writes the characters to the file coected to the output stream fout fi.get(ext); while (ext!= ' ') { fout.put(ext); fi.get(ext); } fi.putback(ext); The blak space read to ed the loop is put back ito the iput stream Slide 6-67

68 Program Example Checkig Iput Icorrect iput ca produce worthless output Use iput fuctios that allow the user to re-eter iput util it is correct, such as Echoig the iput ad askig the user if it is correct If the iput is ot correct, allow the user to eter the data agai Slide 6-68

69 Checkig Iput: get_it The get_it fuctio see i Display 6.7 obtais a iteger value from the user get_it prompts the user, reads the iput, ad displays the iput After displayig the iput, get_it asks the user to cofirm the umber ad reads the user's respose usig a variable of type character The process is repeated util the user idicates with a 'Y' or 'y' that the umber etered is correct Slide 6-69

70 Checkig Iput: ew_lie The ew_lie fuctio see i Display 6.7 is called by the get_it fuctio ew_lie reads all the characters remaiig i the iput lie but does othig with them, essetially discardig them ew_lie is used to discard what follows the first character of the the user's respose to get_lie's "Is that correct? (yes/o)" The ewlie character is discarded as well Display 6.7 (1) Display 6.7 (2) Slide 6-70

71 Iheritace ad Output ostream is the class of all output streams cout is of type ostream ofstream is the class of output-file streams The ofstream class is a child class of ostream More about this i Chapter 10 This fuctio ca be called with ostream or ofstream argumets void say_hello(ostream& ay_out_stream) { ay_out_stream << "Hello"; } Slide 10-71

72 Program Example: Aother ew_lie Fuctio The ew_lie fuctio from Display 6.7 oly works with ci This versio works for ay iput stream void ew_lie(istream& i_stream) { char symbol; do { i_stream.get(symbol); } while (symbol!= '\'); } Slide 10-72

73 Program Example: Callig ew_lie The ew versio of ew_lie ca be called with ci as the argumet ew_lie(ci); If the origial versio of ew_lie is kept i the program, this call produces the same result ew_lie( ); New_lie ca also be called with a iput-file stream as a argumet ew_lie(fi); Slide 10-73

74 Default Argumets It is ot ecessary to have two versios of the ew_lie fuctio A default value ca be specified i the parameter list The default value is selected if o argumet is available for the parameter The ew_lie header ca be writte as ew_lie (istream & i_stream = ci) If ew_lie is called without a argumet, ci is used Slide 10-74

75 Multiple Default Argumets Whe some formal parameters have default values ad others do ot All formal parameters with default values must be at the ed of the parameter list Argumets are applied to the all formal parameters i order just as we have see before The fuctio call must provide at least as may argumets as there are parameters without default values Slide 10-75

76 Default Argumet Example void default_args(it arg1, it arg2 = -3) { cout << arg1 << ' ' << arg2 << edl; } default_args ca be called with oe or two parameters default_args(5); //output is 5-3 default_args(5, 6); //output is 5 6 Slide 10-76

77 Checkig Iput: Check for Yes or No? get_it cotiues to ask for a umber util the user respods 'Y' or 'y' usig the do-while loop do { // the loop body } while ((as!='y') &&(as!= 'y') ) Why ot use ((as =='N') (as == '') )? User must eter a correct respose to cotiue a loop tested with ((as =='N') (as == '') ) What if they mis-typed "Bo" istead of "No"? User must eter a correct respose to ed the loop tested with ((as!='y') &&(as!= 'y') ) Slide 6-77

78 Mixig ci >> ad ci.get Be sure to deal with the '\' that eds each iput lie if usig ci >> ad ci.get "ci >>" reads up to the '\' The '\' remais i the iput stream Usig ci.get ext will read the '\' The ew_lie fuctio from Display 6.7 ca be used to clear the '\' Slide 6-78

79 '\' Example The Code: cout << "Eter a umber:\"; it umber; ci >> umber; cout << "Now eter a letter:\"; char symbol; ci.get(symbol); The Dialogue: Eter a umber: 21 Now eter a letter: A The Result: umber = 21 symbol = '\' Slide 6-79

80 A Fix To Remove '\' cout << "Eter a umber:\"; it umber; ci >> umber; cout << "Now eter a letter:\"; char symbol; ci >>symbol; Slide 6-80

81 Aother '\' Fix cout << "Eter a umber:\"; it umber; ci >> umber; ew_lie( ); // From Display 6.7 cout << "Now eter a letter:\"; char symbol; ci.get(symbol); Slide 6-81

82 Detectig the Ed of a File Member fuctio eof detects the ed of a file Member fuctio of every iput-file stream eof stads for ed of file eof returs a boolea value True whe the ed of the file has bee reached False whe there is more data to read Normally used to determie whe we are NOT at the ed of the file Example: if (! i_stream.eof( ) ) Slide 6-82

83 Usig eof This loop reads each character, ad writes it to the scree i_stream.get(ext); while (! i_stream.eof( ) ) { cout << ext; i_stream.get(ext); } (! I_stream.eof( ) ) becomes false whe the program reads past the last character i the file Slide 6-83

84 The Ed Of File Character Ed of a file is idicated by a special character i_stream.eof( ) is still true after the last character of data is read i_stream.eof( ) becomes false whe the special ed of file character is read Slide 6-84

85 How To Test Ed of File We have see two methods while ( i_stream >> ext) while (! i_stream.eof( ) ) Which should be used? I geeral, use eof whe iput is treated as text ad usig a member fuctio get to read iput I geeral, use the extractio operator method whe processig umeric data Slide 6-85

86 Program Example: Editig a Text File The program of Display 6.8 Reads every character of file cad.dat ad copies it to file cplusad.dat except that every 'C' is chaged to "C++" i cplusad.dat Preserves lie breaks i cad.dat get is used for iput as the extractio operator would skip lie breaks get is used to preserve spaces as well Uses eof to test for ed of file Display 6.8 (1) Display 6.8 (2) Slide 6-86

87 Character Fuctios Several predefied fuctios exist to facilitate workig with characters The cctype library is required #iclude <cctype> usig amespace std; Slide 6-87

88 The toupper Fuctio toupper returs the argumet's upper case character toupper('a') returs 'A' toupper('a') retur 'A' Slide 6-88

89 toupper Returs A it Characters are actually stored as a iteger assiged to the character toupper ad tolower actually retur the iteger represetig the character cout << toupper('a'); //prits the iteger for 'A' char c = toupper('a'); //places the iteger for 'A' i c cout << c; //prits 'A' cout << static_cast<char>(toupper('a')); //works too Slide 6-89

90 The isspace Fuctio isspace returs true if the argumet is whitespace Whitespace is spaces, tabs, ad ewlies isspace(' ') returs true Example: if (isspace(ext) ) cout << '-'; else cout << ext; Prits a '-' if ext cotais a space, tab, or ewlie character See more character fuctios i Display 6.9 (1) Display 6.9 (2) Slide 6-90

91 Sectio 6.3 Coclusio Ca you Write code that will read a lie of text ad echo the lie with all the uppercase letters deleted? Describe two methods to detect the ed of a iput file: Describe whitespace? Slide 6-91

92 Chapter 6 -- Ed Slide 6-92

93 Display 6.1 Back Next Slide 6-93

94 Display 6.2 Back Next Slide 6-94

95 Display 6.3 Back Next Slide 6-95

96 Display 6.4 (1/2) Back Next Slide 6-96

97 Display 6.4 (2/2) Back Next Slide 6-97

98 Display 6.5 Back Next Slide 6-98

99 Display 6.6 (1/3) Back Next Slide 6-99

100 Display 6.6 (2/3) Back Next Slide 6-100

101 Display 6.6 (3/3) Back Next Slide 6-101

102 Display 6.7 (1/2) Back Next Slide 6-102

103 Display 6.7 (2/2) Back Next Slide 6-103

104 Display 6.8 (1/2) Back Next Slide 6-104

105 Display 6.8 (2/2) Back Next Slide 6-105

106 Display 6.9 (1/2) Back Next Slide 6-106

107 Display 6.9 (2/2) Back Next Slide 6-107