Ngôn ngữ lập trình C++ - Chương 7: Ra vào dữ liệu

eRecord( fstream& );
36 void outputLine( ostream&, const ClientData & );
37 int getAccount( const char * const );
38 
39 enum Choices { PRINT = 1, UPDATE, NEW, DELETE, END };
40 
41 int main()
42 {
43 // open file for reading and writing 
44 fstream inOutCredit( "credit.dat", ios::in | ios::out );
45 
46 // exit program if fstream cannot open file
47 if ( !inOutCredit ) {
48 cerr << "File could not be opened." << endl;
49 exit ( 1 );
50 
51 } // end if
52 
Mở file để đọc và ghi (cần
fstream object).
©2004 Trần Minh Châu.
FOTECH. VNU.
83
fig14_15.cpp
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53 int choice;
54 
55 // enable user to specify action
56 while ( ( choice = enterChoice() ) != END ) {
57 
58 switch ( choice ) {
59 
60 // create text file from record file
61 case PRINT:
62 printRecord( inOutCredit );
63 break;
64 
65 // update record
66 case UPDATE:
67 updateRecord( inOutCredit );
68 break;
69 
70 // create record
71 case NEW:
72 newRecord( inOutCredit );
73 break;
74 
75 // delete existing record
76 case DELETE:
77 deleteRecord( inOutCredit );
78 break;
79 
Hiện menu và trả về lựa chọn 
người dùng.
©2004 Trần Minh Châu.
FOTECH. VNU.
84
fig14_15.cpp
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80 // display error if user does not select valid choice
81 default:
82 cerr << "Incorrect choice" << endl;
83 break;
84 
85 } // end switch
86 
87 inOutCredit.clear(); // reset end-of-file indicator
88 
89 } // end while
90 
91 return 0;
92 
93 } // end main
94 
95 // enable user to input menu choice
96 int enterChoice()
97 {
98 // display available options
99 cout << "\nEnter your choice" << endl
100 << "1 - store a formatted text file of accounts" << endl
101 << " called \"print.txt\" for printing" << endl
102 << "2 - update an account" << endl
103 << "3 - add a new account" << endl
104 << "4 - delete an account" << endl
105 << "5 - end program\n? ";
©2004 Trần Minh Châu.
FOTECH. VNU.
85
fig14_15.cpp
(6 of 14)
106 
107 int menuChoice;
108 cin >> menuChoice; // receive choice from user
109 
110 return menuChoice;
111 
112 } // end function enterChoice
113 
114 // create formatted text file for printing
115 void printRecord( fstream &readFromFile )
116 {
117 // create text file 
118 ofstream outPrintFile( "print.txt", ios::out );
119 
120 // exit program if ofstream cannot create file
121 if ( !outPrintFile ) {
122 cerr << "File could not be created." << endl;
123 exit( 1 );
124 
125 } // end if
126 
127 outPrintFile << left << setw( 10 ) << "Account" << setw( 16 )
128 << "Last Name" << setw( 11 ) << "First Name" << right 
129 << setw( 10 ) << "Balance" << endl; 
130 
In ra print.txt. Trước 
tiên, in header của bảng.
©2004 Trần Minh Châu.
FOTECH. VNU.
86
fig14_15.cpp
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131 // set file-position pointer to beginning of record file
132 readFromFile.seekg( 0 ); 
133 
134 // read first record from record file
135 ClientData client;
136 readFromFile.read( reinterpret_cast( &client ),
137 sizeof( ClientData ) ); 
138 
139 // copy all records from record file into text file
140 while ( !readFromFile.eof() ) {
141 
142 // write single record to text file
143 if ( client.getAccountNumber() != 0 )
144 outputLine( outPrintFile, client );
145 
146 // read next record from record file 
147 readFromFile.read( reinterpret_cast( &client ),
148 sizeof( ClientData ) ); 
149 
150 } // end while
151 
152 } // end function printRecord
153 
Đến đầu file, đọc dữ liệu 
về tài khoản, và in bản 
ghi nếu nó không rỗng.
Lưu ý outputLine
lấy đối số là đối tượng
ostream object (lớp cơ 
sở của ofstream). Nó
có thể ghi ra file (như 
trong trường hợp này)
hoặc cout.
©2004 Trần Minh Châu.
FOTECH. VNU.
87
fig14_15.cpp
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154 // update balance in record
155 void updateRecord( fstream &updateFile )
156 {
157 // obtain number of account to update
158 int accountNumber = getAccount( "Enter account to update" );
159 
160 // move file-position pointer to correct record in file
161 updateFile.seekg( 
162 ( accountNumber - 1 ) * sizeof( ClientData ) ); 
163 
164 // read first record from file
165 ClientData client;
166 updateFile.read( reinterpret_cast( &client ),
167 sizeof( ClientData ) ); 
168 
169 // update record
170 if ( client.getAccountNumber() != 0 ) {
171 outputLine( cout, client );
172 
173 // request user to specify transaction
174 cout << "\nEnter charge (+) or payment (-): ";
175 double transaction; // charge or payment
176 cin >> transaction;
Đây là fstream (I/O) vì ta phải đọc balance cũ, 
cập nhật nó, và ghi balance mới.
©2004 Trần Minh Châu.
FOTECH. VNU.
88
fig14_15.cpp
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177 
178 // update record balance
179 double oldBalance = client.getBalance();
180 client.setBalance( oldBalance + transaction );
181 outputLine( cout, client );
182 
183 // move file-position pointer to correct record in file
184 updateFile.seekp( 
185 ( accountNumber - 1 ) * sizeof( ClientData ) ); 
186 
187 // write updated record over old record in file
188 updateFile.write( 
189 reinterpret_cast( &client ),
190 sizeof( ClientData ) ); 
191 
192 } // end if
193 
194 // display error if account does not exist
195 else
196 cerr << "Account #" << accountNumber 
197 << " has no information." << endl;
198 
199 } // end function updateRecord
200 
©2004 Trần Minh Châu.
FOTECH. VNU.
89
fig14_15.cpp
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201 // create and insert record
202 void newRecord( fstream &insertInFile )
203 {
204 // obtain number of account to create
205 int accountNumber = getAccount( "Enter new account number" );
206 
207 // move file-position pointer to correct record in file
208 insertInFile.seekg( 
209 ( accountNumber - 1 ) * sizeof( ClientData ) );
210
211 // read record from file
212 ClientData client;
213 insertInFile.read( reinterpret_cast( &client ),
214 sizeof( ClientData ) ); 
215
216 // create record, if record does not previously exist
217 if ( client.getAccountNumber() == 0 ) {
218 
219 char lastName[ 15 ];
220 char firstName[ 10 ];
221 double balance;
Đây là fstream vì ta đọc 
thử để xem đã có sẵn một bản 
ghi rỗng hay chưa, nếu chưa, 
ta ghi một bản ghi mới.
©2004 Trần Minh Châu.
FOTECH. VNU.
90
fig14_15.cpp
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222 
223 // user enters last name, first name and balance
224 cout << "Enter lastname, firstname, balance\n? ";
225 cin >> setw( 15 ) >> lastName;
226 cin >> setw( 10 ) >> firstName;
227 cin >> balance;
228 
229 // use values to populate account values
230 client.setLastName( lastName );
231 client.setFirstName( firstName );
232 client.setBalance( balance );
233 client.setAccountNumber( accountNumber );
234 
235 // move file-position pointer to correct record in file
236 insertInFile.seekp( ( accountNumber - 1 ) * 
237 sizeof( ClientData ) ); 
238 
239 // insert record in file 
240 insertInFile.write( 
241 reinterpret_cast( &client ),
242 sizeof( ClientData ) ); 
243 
244 } // end if
245 
©2004 Trần Minh Châu.
FOTECH. VNU.
91
fig14_15.cpp
(12 of 14)
246 // display error if account previously exists
247 else
248 cerr << "Account #" << accountNumber
249 << " already contains information." << endl;
250 
251 } // end function newRecord
252 
253 // delete an existing record
254 void deleteRecord( fstream &deleteFromFile )
255 {
256 // obtain number of account to delete
257 int accountNumber = getAccount( "Enter account to delete" );
258 
259 // move file-position pointer to correct record in file
260 deleteFromFile.seekg( 
261 ( accountNumber - 1 ) * sizeof( ClientData ) ); 
262 
263 // read record from file
264 ClientData client;
265 deleteFromFile.read( reinterpret_cast( &client ),
266 sizeof( ClientData ) ); 
267 
là fstream vì ta đọc để
kiểm tra xem account có tồn 
tại không. Nếu có, ta ghi dữ
liệu rỗng (xóa nó). Nếu 
không, không cần xóa.
©2004 Trần Minh Châu.
FOTECH. VNU.
92
fig14_15.cpp
(13 of 14)
268 // delete record, if record exists in file
269 if ( client.getAccountNumber() != 0 ) {
270 ClientData blankClient;
271 
272 // move file-position pointer to correct record in file
273 deleteFromFile.seekp( ( accountNumber - 1 ) * 
274 sizeof( ClientData ) ); 
275 
276 // replace existing record with blank record 
277 deleteFromFile.write( 
278 reinterpret_cast( &blankClient ),
279 sizeof( ClientData ) ); 
280 
281 cout << "Account #" << accountNumber << " deleted.\n";
282 
283 } // end if
284 
285 // display error if record does not exist
286 else
287 cerr << "Account #" << accountNumber << " is empty.\n";
288 
289 } // end deleteRecord
290 
©2004 Trần Minh Châu.
FOTECH. VNU.
93
fig14_15.cpp
(14 of 14)
291 // display single record
292 void outputLine( ostream &output, const ClientData &record )
293 {
294 output << left << setw( 10 ) << record.getAccountNumber()
295 << setw( 16 ) << record.getLastName().data()
296 << setw( 11 ) << record.getFirstName().data()
297 << setw( 10 ) << setprecision( 2 ) << right << fixed 
298 << showpoint << record.getBalance() << endl;
299 
300 } // end function outputLine
301 
302 // obtain account-number value from user
303 int getAccount( const char * const prompt )
304 {
305 int accountNumber;
306 
307 // obtain account-number value
308 do {
309 cout << prompt << " (1 - 100): ";
310 cin >> accountNumber;
311 
312 } while ( accountNumber 100 );
313 
314 return accountNumber;
315 
316 } // end function getAccount
outputLine rất mềm dẻo, và có
thể ghi ra ostream object bất kỳ
(chẳng hạn 1 file hoặc cout).