Programming Programming Principles IIPrinciples II

Lecture Notes 7Lecture Notes 7FilesFiles

Andreas Savva

22

FilesFiles A A filefile is a group of organized data stored in is a group of organized data stored in

secondary storage.secondary storage. Every file has a Every file has a filenamefilename – the name by which the – the name by which the

operating system refers to the contents of the file.operating system refers to the contents of the file.

Dear AnnI am now living in Cyprus where Igot married and bought a house.This is a great place to be. The weather is perfect and life is . . .

Letter.txt

33

Type of FilesType of Files The operating system usually distinguishes files in The operating system usually distinguishes files in

different categories:different categories: Text file (contains ASCII characters)Text file (contains ASCII characters) Executable (Contains code of a program)Executable (Contains code of a program) Binary file (contains arbitrary data)Binary file (contains arbitrary data)

Application programs usually store data in binary Application programs usually store data in binary files in a format understood only by the specific files in a format understood only by the specific application program e.g.application program e.g. MS Word documents (.doc)MS Word documents (.doc) MS Excel files (.xls)MS Excel files (.xls) GIF image files (.gif)GIF image files (.gif)

44

File Handle in C++File Handle in C++ In order to be able to handle a file we need to:In order to be able to handle a file we need to:

include the library include the library fstreamfstream.. declare an object to represent a file which will permit declare an object to represent a file which will permit

reading, writing, etc.reading, writing, etc. C++ provides the following classes to perform input C++ provides the following classes to perform input

and output from/to files:and output from/to files: ofstreamofstream: stream class to write in files: stream class to write in files ifstreamifstream: Stream class to read from files: Stream class to read from files fstreamfstream: Stream class to read and write from/to files: Stream class to read and write from/to files

#include <iostream>#include <fstream>using namespace std;int main() { fstream myFile; . . . return 0;}

55

Opening a FileOpening a File To open a file we must:To open a file we must:

determine the file name (e.g. data.txt)determine the file name (e.g. data.txt) create a new file handle (e.g. myFile)create a new file handle (e.g. myFile) use the handle to open the file use the handle to open the file myFile.open(filename, mode)myFile.open(filename, mode)

To open an existing file to read from, the To open an existing file to read from, the modemode must must be be ios::inios::in..

#include <iostream>#include <fstream>using namespace std;int main() {

fstream myFile;myFile.open(”data.txt”,ios::in);

. . . return 0;}

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Using Using fstreamfstream Objects Objects In using both In using both ifstreamifstream and and ofstreamofstream objects the objects the

mode, input or output, is implied by the object. Thus,mode, input or output, is implied by the object. Thus, ifstreamifstream objects can only be used for input objects can only be used for input ofstreamofstream objects can only be used for output objects can only be used for output fstreamfstream objects can be used for input or output but require objects can be used for input or output but require

an explicit mode designation.an explicit mode designation.

#include <iostream>#include <fstream>using namespace std;int main() {

ifstream inFile;ofstream outFile;fstream addFile;inFile.open(”in.txt”,ios::in);outFile.open(”out.txt”,ios::out);addFile.open(”new.txt”,ios::app);

. . .return 0;

}

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Opening a File ModeOpening a File Mode Mode is an optional parameter with a combination of the following flags:Mode is an optional parameter with a combination of the following flags:

fstream myFile;fstream myFile;myFile.open(”example.bin”, ios::out | ios::app | myFile.open(”example.bin”, ios::out | ios::app |

ios::bin);ios::bin);

Mode Description

ios::in Open a text file in input mode

ios::out Open a text file in output mode

ios::app Open a text file in append mode

ios::ate Go to the end of the open file

ios::binary Open a binary file in input mode (default is text file)

ios::trunc Delete file contents if file exists

All these flags can be combined using the bitwise operator OR (All these flags can be combined using the bitwise operator OR (||). For ). For example, the following will open a the file example, the following will open a the file example.binexample.bin in in binarybinary mode mode to to addadd data: data:

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Default ModesDefault Modes Each one of the Each one of the open() open() member functions of the classes member functions of the classes

ofstreamofstream, , ifstreamifstream and and fstreamfstream has a default mode that is has a default mode that is used if the file is opened without a second argument, i.e.used if the file is opened without a second argument, i.e.

Class Default mode parameter

ofstream

ios::out

ifstream ios::in

fstream ios::in | ios::out The three classes have a constructor that takes the same The three classes have a constructor that takes the same

parameters as parameters as open() open() and automatically calls it. and automatically calls it.

ofstream myFile;ofstream myFile;myFile.open(”data.txt”);myFile.open(”data.txt”);

ofstream myFile(”data.txt”, ios::out);ofstream myFile(”data.txt”, ios::out);

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Opening a FileOpening a File File streams opened in File streams opened in binary mode binary mode perform input and output perform input and output

operations independently of any format considerations.operations independently of any format considerations. Non-binary files are known as Non-binary files are known as text filestext files, and some translations , and some translations

may occur due to formatting of some special characters (like may occur due to formatting of some special characters (like newline and carriage return characters).newline and carriage return characters).

The method The method is_open() is_open() with no arguments (returns a with no arguments (returns a boolbool value) can be called to check whether or not a file stream has value) can be called to check whether or not a file stream has successfully opened a file.successfully opened a file.

ofstream myFile(”data.txt”);if (myFile.is_open()){

// ok, proceed with output }else cout << ”Unable to open file”;

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Reading from a Text-FileReading from a Text-File

#include <iostream>#include <fstream>using namespace std;

int main() { ifstream inFile(”data.txt”);

if (inFile.is_open()) { char ch; int k; char s[50];

inFile >> ch; inFile >> k >> s;

cout << k << s << ch; } return 0;}

3458 Jurassic Park

data.txt

458Jurassic3

34A58B Jurassic Park

data.txt

4A58B3

Ignores spaces and

control characters

Data output and input operations on text-files are performed Data output and input operations on text-files are performed in the same way as with in the same way as with coutcout and and cincin..

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Reading MethodsReading Methods There exist a variety of methods for reading There exist a variety of methods for reading

streams (files). streams (files). Read a single character, including spaces and control Read a single character, including spaces and control

characters:characters:

ch = myFile.get();ch = myFile.get(); Read a line of text or up to 20 characters into array s[] :Read a line of text or up to 20 characters into array s[] :

myFile.getline(s,20);myFile.getline(s,20); Read a line of text or up to 20 characters or until character Read a line of text or up to 20 characters or until character

’A’ occurred into array s[] :’A’ occurred into array s[] :

myFile.getline(s,20,’A’);myFile.getline(s,20,’A’); cincin is also a stream object; we can use the same is also a stream object; we can use the same

operations:operations:

ch = cin.get();ch = cin.get();

cin.getline(s,20);cin.getline(s,20);

cin.getline(s,20,’A’);cin.getline(s,20,’A’);

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Exceptions and ErrorsExceptions and Errors With file operations there may be a variety of With file operations there may be a variety of

problems, i.e.problems, i.e. File not found during opening.File not found during opening. File cannot be opened (already opened exclusively by File cannot be opened (already opened exclusively by

other program).other program). End of file reached.End of file reached. Disk full during writing.Disk full during writing.

Detecting when a problem occurred:Detecting when a problem occurred:

!myFile!myFilemyFile.fail()myFile.fail()

myFile.open("data.txt",ios::in);

if (!myFile) // an error has occurred

cout << ”Could not open file”;

. . .

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Checking State FlagsChecking State Flags A number of A number of boolbool member functions exist to check for member functions exist to check for

specific states of a stream:specific states of a stream:

Method Description

bad() Returns true if a reading or writing operation fails. For example, if there is an attempt to write in a file that was open for input, or if there is no available memory on the device that there is an attempt to write in a file.

fail() Returns true in the same cases as bad(), but also in the case that a format error happens. For example if a character is extracted while an integer is expected.

eof() Returns true if a file opened for input has reached the end.

good() It is the most generic state flag – it returns false in the same cases in which calling any of the previous functions would return true.

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Closing a FileClosing a File After completing the use of the file, it should be After completing the use of the file, it should be

closed so that it resources become available again.closed so that it resources become available again. Closing a file:Closing a file:

myFile.close();myFile.close(); Once closing the file, the file becomes available to Once closing the file, the file becomes available to

be opened by other processes and the stream be opened by other processes and the stream object can be used to open another file.object can be used to open another file.

In case that an object is destructed before is closed In case that an object is destructed before is closed the destructor calls the member function the destructor calls the member function close().close().

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Reaching the End of FileReaching the End of File The method The method eof()eof() determines if the end of the file is reached. determines if the end of the file is reached.

// the following program reads lines and displays// them on the screen until the end of the file.#include <iostream>#include <fstream>using namespace std;int main() { fstream myFile; char str[256]; myFile.open("data.txt", ios::in); if (myFile.is_open()) { while(!myFile.eof()) { myFile.getline(str,256); // get a line cout << str << endl; // display line } myFile.close(); } else cout << ”Unable to open file”; return 0;}

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Using method Using method good()good() The method The method good() good() can be used instead of the method can be used instead of the method eof()eof() . .

// the following program reads lines and displays// them on the screen until the end of the file.#include <iostream>#include <fstream>using namespace std;int main() { fstream myFile; char str[256]; myFile.open("data.txt", ios::in); if (myFile.is_open()) { while(myFile.good()) { myFile.getline(str,256); // get a line cout << str << endl; // display line } myFile.close(); } else cout << ”Unable to open file”; return 0;}

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Writing to a FileWriting to a File Similar to opening for reading, but use Similar to opening for reading, but use

ios::outios::out when opening: when opening:

myFile.open("data.txt", ios::out);myFile.open("data.txt", ios::out);

Consequences to file:Consequences to file: If the file does not exist, it is created.If the file does not exist, it is created. If it exists its data is overwritten.If it exists its data is overwritten.

Writing to a file:Writing to a file:

myFile << ”Hello” << 12 << ’C’ << endl;myFile << ”Hello” << 12 << ’C’ << endl;

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Copying a File to Another FileCopying a File to Another File#include <iostream>#include <fstream>using namespace std;int main() { fstream fin, fout; char str[256];

fin.open("data.txt", ios::in); if (fin.is_open()) { fout.open("newdata.txt", ios::out);

while(!fin.eof()) { fin.getline(str, 256); // get a line fout << str << endl; // put a line } } fin.close(); fout.close(); return 0;}

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Appending to a FileAppending to a File

Data can be appended at the end of the Data can be appended at the end of the file by using file by using ios::appios::app when opening. when opening.

Existing data will not be overwritten.Existing data will not be overwritten.

myFile.open("data.txt", ios::app);myFile.open("data.txt", ios::app);

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Constant Constant EOFEOF#include <iostream>#include <fstream>using namespace std;int main() { fstream fin, fout; char ch;

fin.open("data.txt", ios::in); if (!fin) cout << "File not found"; else { fout.open("newdata.txt", ios::out);

ch = fin.get(); while(ch != EOF) { if (ch >= ’A’ && ch <= ’Z’) fout << ’?’; else fout << ch; ch = fin.get(); } fin.close(); fout.close(); } return 0;}

My name is George andI live in London.

data.txt

?y name is ?eorge and? live in ?ondon.

newdata.txt

Character denoting the end

of file

2121

Stream Pointers: Stream Pointers: getget and and putput

All i/o stream objects have, at least, one internal stream All i/o stream objects have, at least, one internal stream pointer:pointer: ifstreamsifstreams like like istreamistream, has a pointer known as , has a pointer known as get pointerget pointer that that

points to the element to be read in the next input operation.points to the element to be read in the next input operation. ofstreamsofstreams like like ostreamostream, has a pointer known as , has a pointer known as put pointer put pointer that that

points to the location where the next element will be written.points to the location where the next element will be written. These internal stream pointers that point to the reading These internal stream pointers that point to the reading

or writing locations within a stream can be manipulated or writing locations within a stream can be manipulated using the following member functions:using the following member functions: tellg() tellg() and and tellp() tellp() – These two functions return a value of the – These two functions return a value of the

member type member type pos_typepos_type, which is an integer data type, , which is an integer data type, representing the current position of the respective pointer.representing the current position of the respective pointer.

seekg(position)seekg(position) and and seekp(position) seekp(position) – These functions change the – These functions change the position of the respective pointers. The pointer is changed to the position of the respective pointers. The pointer is changed to the absolute position absolute position (counting from the beginning of the file). (counting from the beginning of the file). Parameter position is of member type Parameter position is of member type pos_typepos_type (integer). (integer).

2222

Stream PointersStream Pointers

#include <iostream>#include <fstream>using namespace std;

int main() { ifstream fin(”data.txt”); if (fin.fail()) cout << "File not found"; else { char ch; while(ch = fin.get(), ch != EOF) { cout << ch; if (fin.tellg()==fstream::pos_type(7)) fin.seekg(18); } fin.close(); } return 0;}

My name is George andI live in London.

data.txt

My nameandI live in London

2323

Overloaded Methods forOverloaded Methods forchanging the Stream Pointerschanging the Stream Pointers

There is an overloaded function for both member functions There is an overloaded function for both member functions seekg seekg and and seekp:seekp:

direction

Description

ios::beg offset counted from the beginning of the stream

ios::cur offset counted from the current position of the stream pointer

ios::end offset counted from the end of the stream

The position of the get or put pointer is set to an offset value The position of the get or put pointer is set to an offset value relative torelative to some specific point determined by the parameter some specific point determined by the parameter directiondirection. Parameter . Parameter offsetoffset is of the member type is of the member type off_typeoff_type, , which is an integer type and which is an integer type and directiondirection is of type is of type seekdirseekdir which which is an enumerated type that determines the point from where is an enumerated type that determines the point from where offset is counted from, and that can take any of the following offset is counted from, and that can take any of the following values:values:

seekg( offset, direction )seekg( offset, direction )seekp( offset, direction )seekp( offset, direction )

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Changing the Stream PointersChanging the Stream PointersExamplesExamples

Code Description

seekg(0, ios::end);

Pointer get will be set at the end of the stream

seekg(-2 ,ios::end);

Pointer get will be set two positions before the end of the stream

seekg(-1, ios::cur);

Pointer get will be set one position before its current position

seekp(2, ios::beg);

Pointer put will be set two position after the beginning of the stream

myFile.seekg(0, ios::end);myFile.seekg(0, ios::end);long size = myFile.tellg();long size = myFile.tellg();cout << ”Size is ” << size << ” bytes”;cout << ”Size is ” << size << ” bytes”;

Display the size of the file:Display the size of the file:

2525

Unget CharacterUnget Character

int main() { ifstream fin("data.txt"); if (fin.fail()) cout << "File not found"; else { char ch, s[100]; ch=fin.get(); fin.unget(); if (ch == 'a') { fin >> s; cout <<s<<endl; } while (ch=fin.get(), ch!=EOF) { if (ch == ' ') { ch = fin.get(); if (ch == 'a') { fin.seekg(-1,ios::cur); fin >> s; cout << s << endl; } } } fin.close(); } return 0;}

i traveled abroad to amsterdam and i wasvery happy to go aroundand see this amazing city

data.txt

abroadamsterdamandaroundandamazing

Display the words starting with ‘a’:Display the words starting with ‘a’:

fin.unget()fin.unget();;

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Binary FilesBinary Files In binary files, to input and output data with the extraction and insertion operators (<< and >>) and functions like getline is In binary files, to input and output data with the extraction and insertion operators (<< and >>) and functions like getline is

not efficient, since there is no need to format data, and data my not use the separation codes used by text-files to separate not efficient, since there is no need to format data, and data my not use the separation codes used by text-files to separate elements (like spaces, newlines, etc.)elements (like spaces, newlines, etc.)

For binary files, file streams include two member functions for input and output.For binary files, file streams include two member functions for input and output.

where where memory_blockmemory_block is the address of an array of bytes and is the address of an array of bytes and sizesize is an integer. is an integer.

read( read( char*char* memory_block, memory_block, intint size ) size ) – ifstream, fstream– ifstream, fstreamwrite( write( char*char* memory_block, memory_block, intint size ) size ) – ofstream, – ofstream,

fstreamfstream

2727

Writing Structures to Binary Files Writing Structures to Binary Files class human {public: human(){} human(char s[], int x) { strcpy(name,s); age = x; }private: char name[50]; int age;};

void write_to_file(){ char name[50]; int age; cout << "Enter name and age: "; cin >> name >> age; human person(name,age); ofstream myFile("data.bin", ios::app | ios::binary); myFile.write((char*)&person, sizeof(human)); myFile.close();}

2828

Reading Structures from Binary Reading Structures from Binary Files Files

void read_from_file(human people[], unsigned long &n) { ifstream myFile("data.bin", ios::binary); if (myFile.fail()) cout << "File not found"; else { myFile.seekg(0,ios::end); n = myFile.tellg()/sizeof(human); myFile.seekg(0); for (unsigned long i=0; i<n; i++) myFile.read((char*)&people[i], sizeof(human)); myFile.close(); }}

void read_from_file(human people[], unsigned long &n) { ifstream myFile("data.bin", ios::binary); if (myFile.fail()) cout << "File not found"; else { myFile.seekg(0,ios::end); n = myFile.tellg()/sizeof(human); myFile.seekg(0); myFile.read((char*)people, sizeof(human)*n); myFile.close(); }}

Reading records one-

by-one

Reading records all at

once

2929

Buffers and SynchronizationBuffers and Synchronization File streams are associated to an internal buffer of type File streams are associated to an internal buffer of type streambufstreambuf. This . This

buffer is a memory block that acts as an intermediary between the stream buffer is a memory block that acts as an intermediary between the stream and the physical file. For example, with an and the physical file. For example, with an ofstreamofstream, each time the member , each time the member function function putput (which writes a single character) is called, the character is not (which writes a single character) is called, the character is not written directly to the physical file with which the stream is associated. written directly to the physical file with which the stream is associated. Instead, the character is inserted in the stream’s intermediate buffer.Instead, the character is inserted in the stream’s intermediate buffer.

When the buffer is flushed, all the data contained in it is written to the When the buffer is flushed, all the data contained in it is written to the physical medium (if it is an output stream) or freed (if it is an input stream). physical medium (if it is an output stream) or freed (if it is an input stream). This process is called This process is called synchronizationsynchronization and takes place under the following and takes place under the following circumstances:circumstances: When the file is closedWhen the file is closed: before closing a file all buffers that have not yet been : before closing a file all buffers that have not yet been

flushed are synchronized and all pending data is written or read to the physical flushed are synchronized and all pending data is written or read to the physical medium.medium.

When the buffer is fullWhen the buffer is full: Buffers have a certain size. When the buffer is full it : Buffers have a certain size. When the buffer is full it automatically synchronized.automatically synchronized.

Explicitly, with manipulatorsExplicitly, with manipulators: When certain manipulators are used on streams, : When certain manipulators are used on streams, an explicit synchronization takes place. These manipulators are: an explicit synchronization takes place. These manipulators are: flushflush and and endlendl..

Explicitly, with member function Explicitly, with member function sync()sync(): calling stream’s member function : calling stream’s member function sync(), which takes no parameters, causes an immediate synchronization. This sync(), which takes no parameters, causes an immediate synchronization. This function returns an int value equal to -1 if the stream has no associated buffer or function returns an int value equal to -1 if the stream has no associated buffer or in the case failure. Otherwise (if successful) it returns 0.in the case failure. Otherwise (if successful) it returns 0.