Lecture 9 - itn.liu.seaidvi/courses/07/C++/lectures/lec9.pdf · TNCG18(C++): Lec 9 3 Files and Streams • When file opened –A stream object associated with it fstream file( "filename",

TNCG18(C++): Lec 9 1

Lecture 9

• File Processing– Streams– Stream I/O template hierarchy– Read and write sequential text files– Random access files

• Binary files

– Reading and writing binary files


Introduction• Storage of data

– Arrays variables are temporary– Files are permanent

• Magnetic disk, optical disk, tapes

• C++ views file as sequence of bytes– Ends with end-of-file marker

• C++ imposes no structure on file– Concept of "record" must be implemented by programmer

0 31 2 4 5 8 9

...

... n-1

end-of-file marker

6 7


Files and Streams

• When file opened– A stream object associated with itfstream file( "filename", fileOpenMode );

– cin, cout, etc. created when <iostream> included• Communication between program and file/device

0 31 2 4 5 8 9

...

... n-1

end-of-file marker

6 7

Write file position pointer

Read file position pointer

badbitfailbit

…

file


Files and Streams

• To perform file processing– #include <iostream> and <fstream>– Class templates

• basic_ifstream (input)• basic_ofstream (output)• basic_fstream (I/O)

– typedefs for specializations that allow char I/O for files• ifstream (char input)• ofstream (char output)• fstream (char I/O)


I/O Template Hierarchy• typedefs for specializations that allow char I/O

• istream specialization of basic_istream (char input)• ostream specialization of basic_ostream (char output)• cin is an instance of istream• cout is an instance of ostream

basic_fstream

basic_ios

basic_ifstream basic_ofstreambasic_iostream

basic_istream basic_ostream


An Example// display single recordvoid outputLine( ostream &output, const ClientData &record ){

output << left << setw( 10 ) << record.getAccountNumber()<< setw( 16 ) << record.getLastName()<< setw( 11 ) << record.getFirstName()<< setw( 10 ) << setprecision( 2 ) << right << fixed

<< showpoint << record.getBalance() << endl;

} // end function outputLine

ClientData client(1234, “Mark Junior”, -210);

ofstream ClientFile( "clients.dat", ios::out );

…

outputLine(cout, client);

outputLine(ClientFile, client);

ofstream is a derived class of ostream


Creating a Sequential-Access File• To open file

– Create a stream object• Creates "line of communication" from object to file

– Classes• ifstream (input only)• ofstream (output only)• fstream (I/O)

– Constructors take file name and file-open modeofstream outClientFile( "filename", fileOpenMode );

– To attach a file laterofstream outClientFile;outClientFile.open( "filename", fileOpenMode);


Creating a Sequential-Access File

• File-open modes

– ofstream opened for output by defaultofstream outClientFile( "clients.dat", ios::out );

ofstream outClientFile( "clients.dat");

fstream ClientFile( "clients.dat“, ios::out | ios::in);

Mode Description

ios::app Write all output to the end of the file.

ios::ate Open a file for output and move to the end of the file (normally used to append data to a file). Data can be written anywhere in the file.

ios::in Open a file for input.

ios::out Open a file for output.

ios::trunc Discard the file’s contents if it exists (this is also the default action for ios::out)

ios::binary Open a file for binary (i.e., non-text) input or output.


File Operations• Overloaded operator!()

– !outClientFile– Returns nonzero (true) if badbit or failbit set

• Opened non-existent file for reading, wrong permissions

• Overloaded operator void*()– Converts stream object to pointer– 0 when when failbit or badbit set, otherwise nonzero

• failbit set when EOF found

while ( cin >> myVariable );

• stream >> var; returns the stream object• Implicitly converts cin to pointer• Loops until EOF

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File Operations

• Writing to file (just like cout)– outClientFile << myVariable

• Closing file– outClientFile.close()

– Automatically closed when destructor called

• Example: Creating a text file of movie scoresname1 score1name2 score2name3 score3…

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Open a File

#include <iostream>#include <fstream>

//ofstream constructor opens file ofstream moviesData( “reviews.txt", ios::out );

//exit program if unable to create fileif ( !moviesData ) {

cerr << "File could not be opened" << endl;return;

}

Notice the the header files required for file I/O.

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Write Data to a Text File

char name[40];int score;

//read films name and score from cin, //then write them into file

while ( cin >> name >> score ) {moviesData << name << ' ' << score << endl;

}

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Reading Data from a Text File• Open the file

ifstream file( “fileName", ios::in );

• Test if the file was properly opened!file ! is overloaded

• Read the file until EOFwhile ( file >> myVariable ) { … }

– Stops when EOF found (gets value 0)– operator void*() converts a to pointer

• Testing if end-of-file reached– file.eof()

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Open Movies Data File#include <iostream>#include <fstream>

//ofstream constructor opens fileifstream moviesFile( "reviews.txt", ios::in );

//exit program if unable to open fileif ( !moviesFile ) {

cerr << "File could not be opened" << endl;return;

}

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Read Movies Data File


//read films name and score from file while( moviesFile >> name >> score ) {

cout << name << ": " << grade << endl;}

Read from file until EOF found.

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Read Movies Data File• Assume that the file looks as follows

Mystic River: 4Lost: 1The Reader: 5Pulp Fiction: 5No Country for Old Man: 5…

moviesFile.getline(name, 40, ':');moviesFile >> score;while ( !moviesFile.eof() ) {

cout << name << ": " << score << endl;moviesFile.ignore();moviesFile.getline(name, 40, ':');moviesFile >> score;

}

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Read Movies Data File• Assume that the file looks as follows

Mystic River: 4Lost: 1The Reader: 5Pulp Fiction: 5No Country for Old Man: 5…while ( !moviesFile.eof() ) {

moviesFile.getline(name, 40, ':');

moviesFile >> score;cout << name << ": " << score << endl;

moviesFile.ignore();

}

The loop is execute once, if the file is emptyDoesn’t work!!!

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Testing with eof()

• moviesFile.eof()– First read then test if end-of-file– If EOF is reached

• getline() and >> give no error• eofbit is set• eof() returns true

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File Position Pointers

– Number of next byte to read/write– Functions to reposition pointer

• seekg (seek get for istream class)• seekp (seek put for ostream class)• Classes have "get" and "put" pointers

– seekg and seekp take offset and reference position• Offset: number of bytes relative to a reference position• Reference positions (ios::beg default)

– ios::beg - relative to beginning of stream– ios::cur - relative to current position– ios::end - relative to end

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File Position Pointers• Examples

– fileObject.seekg(0)• Goes to front of file (location 0) because ios::beg is default

– fileObject.seekg(n)• Goes to nth byte from beginning

– fileObject.seekg(n, ios::cur)• Goes n bytes forward

– fileObject.seekg(n, ios::end)• Goes n bytes back from end

– fileObject.seekg(0, ios::end)

• Goes to last byte– seekp similar

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File Position Pointers

• To find pointer location– tellg and tellplong location;location = fileObject.tellg()

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Example

• Assume you need to scan a file twice from the beginning

//read file from beginning to EOF…// reset eofbit for next inputfile.clear();

// move to beginning of filefile.seekg( 0 );

//read again file from beginning to EOF…

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Updating Sequential-Access Files• Updating sequential files

– Risk overwriting other data– Example: change name "White" to "Worthington"

• Old data300 White 0.00 400 Jones 32.87

• Insert new data

– Formatted text different from internal representation

300 White 0.00 400 Jones 32.87

300 Worthington 0.00ones 32.87

300 Worthington 0.00

Data gets overwritten

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Random-Access Files

• Instant access– Want to locate record quickly

• Airline reservations, ATMs

– Sequential files must search through each one

• Random-access files are solution– Instant access– Insert record without destroying other data

• As if an empty record was previously created

– Update/delete items without changing other data

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Random-Access Files

• C++ imposes no structure on files– Programmer must create random-access files– Simplest way: fixed-length records

• Calculate position in file from record size and key• What if records are not fixed length?

0 200 300 400 500

byte offsets}

} } } } } }

100

100bytes

100bytes

100bytes

100bytes

100bytes

100bytes

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Random-Access File vs Text File• "1234567" (char*) vs 1234567 (int)

– char* takes 8 bytes (1 for each character + ‘\0’)– int takes fixed number of bytes (perhaps 4)

• 123 same size in bytes as 1234567

• << operator and write()– outFile << number

• Outputs number (int) as a char*• Variable number of bytes

– outFile.write( const char*, int );

• Outputs raw bytes• Takes pointer to memory location and number of bytes to write

– Copies data directly from memory into file

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file.write( const char*, int );

int i = 5567;int*p = &i;file.seekp(8);file.write( ( char* ) p , sizeof( int ) );

pint i;

disk fileMain memory

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Creating a Random-Access File

• ExampleoutFile.write( (char*)( &number ), sizeof( number ) );

– &number is an int*• Convert to char*

– sizeof(number)• Size of number (an int) in bytes

– read function similar (more later)

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Writing a Binary File with Movies Datatypedef struct {


} Movie;

Movie film;cin.getline(film.name, 40, ':');cin >> film.score;

ofstream moviesDat( "movies.dat", ios::binary )

//exit program if ofstream could not open fileif ( !moviesDat ) {

cerr << "File could not be opened." << endl;return;

}

moviesDat.write((char*)( &film ), sizeof(film));

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Writing Data Randomly to a Random-Access File

• Use seekp to write to exact location in file– Where does the first record begin?

• Byte 0

– The second record?• Byte 0 + sizeof(object)

– Any record?• (Recordnum - 1) * sizeof(object)

– Example: write a new movie data on the ith position

moviesData.seekp( (i-1) * sizeof( Movie ) );moviesDat.write((char*)( &film ), sizeof(film));

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Reading Data Sequentially from a Binary File

• read - similar to write– Reads raw bytes from file into memory– inFile.read( (char*)( &number ), sizeof( int ) );

• &number: location of the data• sizeof(int): how many bytes to read

– Do not use inFile >> number with raw bytes• >> converts number to text format• e.g. 123 does not occupy same number of bytes as 1234

Lecture 9 - itn.liu.seaidvi/courses/07/C++/lectures/lec9.pdf · TNCG18(C++): Lec 9 3 Files and Streams • When file opened –A stream object associated with it fstream file( "filename",

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