Advanced File Operations. Review of Files. Declaration Opening Using Closing. CS SJAllan Chapter 12 2

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1 Chapter 12 Advanced File Operations Review of Files Declaration Opening Using Closing CS SJAllan Chapter

2 Testing for Open Errors To see if the file is opened correctly, test as follows: in.open("cust.dat"); if (!in ) { cout << "Error opening file named in.\n"; exit(1); } // if Another way: out.open("cust.dat"); if ( out.fail() ) { cout << "Error opening file name out.\n"; exit(1); } // if CS SJAllan Chapter 12 3 Another Way to Test #include <assert.h> in.open("cust.dat"); assert( in ); CS SJAllan Chapter

3 Input/Output Flags File Access Flag ios::app ios::binary ios::in ios::out ios::trunc Meaning Append mode Binary mode Input mode Output mode Truncate mode CS SJAllan Chapter 12 5 Appending to a File Program #include <iostream> #include <fstream> using namespace std; int main ( void ) { fstream datafile; datafile.open("demofile.txt", ios::out); datafile << "Jones\n"; datafile << "Smith\n"; datafile.close( ); datafile.open("demofile.txt", ios::app); datafile << "Willis\n"; " datafile << "Davis\n"; datafile.close( ); return 0; } // main CS SJAllan Chapter

4 Appending to a File Output Jones Smith Willis Davis CS SJAllan Chapter 12 7 Detecting the End of a File The eof() member function reports when the end of a file has been encountered In C++, end of file doesn t mean the program is at the last piece of information in the file, but beyond it The eof() function returns true when there is no more information to be read CS SJAllan Chapter

5 Detecting end-of-file I never detect end-of-file using the eof function for ( ; in << value; ) { // Whatever we do with value } // for In the above loop, the test in << value returns a positive number if the read was successful and a 0 if it was unsuccessful CS SJAllan Chapter 12 9 Passing File Stream Objects to Functions Use pass by reference in the prototype and dheading bool getdata( ifstream &, int & ); Use the stream object in the call int score; ifstream gradefile( "grades.txt txt" ); if (!getdata( gradefile, score ) ) CS SJAllan Chapter

6 More Detailed Error Testing All stream objects have error state bits that indicate the condition of the stream ios::eofbit ios::failbit ios::hardfail ios::badbit ios::goodbit set when end of file detected set when operation failed set when error occurred and no recovery set when invalid operation attempted set when no other bits are set CS SJAllan Chapter Member Functions for Streams Function eof() fail() bad() good() clear() Description Returns true if the eofbit flag is set, otherwise it returns false. Returns true if the failbit or hardfail flags are set, otherwise returns false. Returns true if the badbit flag is set, otherwise returns false. Returns true if the goodbit flag is set, otherwise returns false. When called with no arguments, clears all the flags listed above. Can also be called with a specific flag as an argument. CS SJAllan Chapter

7 Working with Multiple Files Can have more than one file open at a time in a program These files may be input and/or output Need to define a stream object for each file CS SJAllan Chapter Binary Files Binary files contain data that is unformatted, and not necessarily stored as ASCII text A binary file is opened as file.open( "stuff.dat", ios::out ios::binary ); CS SJAllan Chapter

8 Binary Files Use read and write instead of <<, >> char ch; // read a character from a file in.read(&ch, sizeof ch ); Address of where to put the data being read. The read function expects to read chars // send a character to a file out.write( &ch, sizeof ch ); How many bytes to read from the file CS SJAllan Chapter Binary Files To read, write non-character data, you must use a typecast operator to treat the address of the data as a character address int num; in.read(reinterpret_cast<char *>( &num ), sizeof num ); Treat the address of num as the address of a char out.write(reinterpret_cast<char *>( &num ), sizeof num ); CS SJAllan Chapter

9 Using a Binary File Program #include <iostream> #include <fstream> using namespace std; int main ( void ) { fstream file("nums.dat", ios::out ios::binary ); int buffer[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}; cout << "Now writing the data to the file.\n"; file.write( reinterpret_cast<char*>(buffer), sizeof(buffer) ); file.close(); file.open( "nums.dat", ios::in ios::binary ); cout << "Now reading the data back into memory.\n"; file.read( reinterpret_case<char*>(buffer), sizeof(buffer ) ); for ( int count = 0; count < 10; count++ ) cout << buffer[count] << " "; file.close( ); return 0; } // main CS SJAllan Chapter Using a Binary File Output Now writing the data to the file. Now reading the data back into memory CS SJAllan Chapter

10 Creating Records with Structures Structures may be used to store fixed-length records to a file struct Info { char name [NAME_SIZE]; int age; char address1 [ADDR_SIZE]; char address2 [ADDR_SIZE]; char phone [PHONE_SIZE]; }; // Info Since structures can contain a mixture of data types, you should always use the ios::binary mode when opening a file to store them CS SJAllan Chapter Files and Structures Program #include <iostream> #include <fstream> #include <ctype> const int NAME_SIZE = 51, ADDR_SIZE = 51, PHONE_SIZE = 14; struct Info { char name [NAME_SIZE]; int age; char address1 [ADDR_SIZE]; char address2 [ADDR_SIZE]; char phone [PHONE_SIZE]; }; // Info CS SJAllan Chapter

11 Files and Structures Program (cont) int main ( void ) { ofstream people( "people.dat", ios::binary ); Info person; char again; if (!people ) { cout << "Error opening file. Program aborting.\n"; return; } // if do { cout << "Enter the following information about a << "person:\n"; " cout << "Name: "; CS SJAllan Chapter Files and Structures Program (cont) cin.getline( person.name, NAME_SIZE ); cout << "Age: "; cin >> person.age; cin.ignore( ); // skip over remaining newline. cout << "Address line 1: "; cin.getline( person.address1, ADDR_SIZE ); cout << "Address line 2: "; cin.getline( person.address2, ADDR_SIZE ); cout << "Phone: "; cin.getline(person.phone, PHONE_SIZE ); people.write( reinterpret_cast<(char *>( &person ), sizeof person ); cout << "Do you want to enter another record? "; cin >> again; cin.ignore(); } while ( toupper(again) == 'Y' ); // What is this? people.close(); return 0; } // main CS SJAllan Chapter

Files and Structures Output Enter the following information about a person: Name:

[Enter] Address line 2: Perth, SC 38754 [Enter] Phone: (803)555-1234 [Enter] Do you

Y [Enter] Enter the following information about a person: Name: Merideth Murney

Hazelwood, NC 28737 [Enter] Phone: (704)453-9999 [Enter] Do you N [Enter] CS 1410 -

12 Files and Structures Output Enter the following information about a person: Name: Charlie Baxter [Enter] Age: 42 [Enter] Address line 1: 67 Kennedy Bvd. [Enter] Address line 2: Perth, SC [Enter] Phone: (803) [Enter] Do you want to enter another record? Y [Enter] Enter the following information about a person: Name: Merideth Murney [Enter] Age: 22 [Enter] Address line 1: 487 Lindsay Lane [Enter] Address line 2: Hazelwood, NC [Enter] Phone: (704) [Enter] Do you want to enter another record? N [Enter] CS SJAllan Chapter Random Access Files Random Access means non-sequentially accessing information in a file. CS SJAllan Chapter

13 Random Access Member Functions seekg used with files opened for input (get) seekp used with files opened for output (put) CS SJAllan Chapter Flags for Random Access Mode Flags ios::beg ios::end ios::cur Description The offset is calculated from the beginning of the file. The offset is calculated from the end of the file. The offset is calculated from the current position. CS SJAllan Chapter

14 Examples Using seekp and seekg Statement file.seekp(32l, ios::beg); file.seekp(-10l, ios::end); file.seekp(120l, ios::cur); file.seekg(2l, ios::beg); file.seekg(-100l, ios::end); file.seekg(40l, ios::cur); file.seekg(0l, ios::end); How it Affects the Position Sets the write position to byte 32 from the beginning of the file. Sets the write position to byte 10 from the end of the file. Sets the write position to byte 120 from the current position. Sets the read position to byte 2 from the beginning of the file. Sets the read position to byte 100 from the end of the file. Sets the read position to bye 40 from the current position. Sets the read position to the end of the file. CS SJAllan Chapter Using seekg Program // letters.txt contains the letters of the alphabet #include <iostream> #include <fstream> using namespace std; int main ( void ) { ifstream file( "letters.txt" ); char ch; file.seekg( 5L, ios::beg ); file.get( ch ); cout << "Byte 5 from beginning: " << ch << endl; file.seekg( -10L, ios::end ); file.get(ch); cout << "Byte 10 from end: " << ch << endl; file.seekg( 3L, ios::cur ); file.get(ch); cout << "Byte 3 from current: " << ch << endl; file.close(); return 0; }// main CS SJAllan Chapter

15 Using seekg Output Byte 5 from beginning: f Byte 10 from end: q Byte 3 from current: u CS SJAllan Chapter Warning If a program has read to the end of file, you must call the file stream object s clear member function before calling seekg or seekp This clears the file stream object s eof flag fio.clear(); CS SJAllan Chapter

16 Other Member Functions tellp returns a long integer that is the current byte number of the file s write position tellg returns a long integer that is the current byte number of the file s read position CS SJAllan Chapter Opening a File for Both Input and Output You may perform input and output on an fstream file without closing it and reopening it For example: fstream file( "data.dat", ios::in ios::out ); CS SJAllan Chapter

17 Creating a Inventory File Program #include <iostream> #include <fstream> using namespace std; const int DESCSIZE = 31, NUM_RECORDS = 5;; struct Invtry { char desc[descsize]; int qty; float price; }; // Invtry int main ( void ) { ofstream inventory( "invtry.dat", ios::binary ); Invtry record = { "", 0, 0.0 }; for ( int count = 0; count < NUM_RECORDS; count++ ) { cout << "Now writing record " << count << endl; inventory.write( reinterpret_cast<char *>&record, sizeof record ); } // for inventory.close(); return 0; } // main CS SJAllan Chapter Display Inventory File Program #include <iostream> #include <fstream> using namespace std; const int DESCSIZE = 31; struct Invtry { char desc [DESCSIZE]; int qty; float price; }; // Invtry int main ( void ) { ifstream inventory( "invtry.dat", ios::binary ); Invtry record = { "", 0, 0.0 }; CS SJAllan Chapter

18 Display Inventory File Program inventory.read( reinterpret_cast<char *>( &record ), sizeof record ); for ( ;!inventory.eof(); ) { cout << "Description: "; cout << record.desc << endl; cout << "Quantity: "; cout << record.qty << endl; cout << "Price: "; cout << record.price << endl << endl; inventory.read( reinterpret_cast<char *>&record, } // for inventory.close(); return 0; } // main sizeof record ); CS SJAllan Chapter Display Inventory File Output Description: Quantity: 0 Price: 0.0 Description: Quantity: 0 Price: 0.0 Description: Quantity: 0 Price: 0.0 Description: Quantity: 0 Price: 0.0 Description: Quantity: 0 Price: 0.0 CS SJAllan Chapter

19 Edit Inventory Program #include <iostream> #include <fstream> using namespace std; const int DESCSIZE = 31; struct Invtry { char desc [DESCSIZE]; int qty; float price; }; // Invtry int main ( void ) { CS SJAllan Chapter Edit Inventory Program (cont) fstream inventory( "invtry.dat", ios::in ios::out ios::binary ); Invtry record; long recnum; cout << "Which record do you want to edit?"; cin >> recnum; inventory.seekg( recnum * sizeof(record), ios::beg ); inventory.read( reinterpret_cast<char *>&record, sizeof(record) ); cout << "Description: "; cout << record.desc << endl; cout << "Quantity: "; cout << record.qty << endl; cout << "Price: "; cout << record.price << endl; cout << "Enter the new data:\n"; cout << "Description: "; CS SJAllan Chapter

20 Edit Inventory Program (cont) cin.ignore(); cin.getline( record.desc, DESC_SIZE ); cout << "Quantity: "; cin >> record.qty; cout << "Price: "; cin >> record.price; inventory.seekp( recnum * sizeof(record), ios::beg ); inventory.write( reintepret_cast<char *>&record, sizeof(record) ); inventory.close(); return 0; } // main CS SJAllan Chapter

Chapter 12 File Operations. Starting Out with C++, 3 rd Edition

Chapter 12 File Operations. Starting Out with C++, 3 rd Edition Chapter 12 File Operations 1 12.1 What is a File? A file is a collection on information, usually stored on a computer s disk. Information can be saved to files and then later reused. 2 12.2 File Names