Cs1123 10 file operations

File Operations
(CS1123)
By
Dr. Muhammad Aleem,
Department of Computer Science,
Mohammad Ali Jinnah University, Islamabad
2013

What is a File?
• A file is a collection of information, usually stored
on a computer’s disk. Information can be saved to
files and then later reused.
• All files are assigned a name that is used for
identification purposes by the operating system
and the user.
– E.g.,
MyProg.cpp, Marks.txt, etc.

Basic File Operations
• Opening a file
• Writing to a file
• Reading from a file
• Close a file
• …

Setting Up a Program for File Input/Output
• Before file I/O can be performed, a C++ program must
be set up properly.
• File access requires the inclusion of fstream.h
• A stream is a general name given to a flow of data.
• So far, we’ve used the cin and cout stream objects.
• Different streams are used to represent different kinds
of data flow. For example: the ifstream represents data
flow from input disk files.

File I/O (files are attached to streams)
• ifstream – input from the file
• ofstream – output to the file
• fstream -input and output to the file
To use these objects, you should include header
file, <fstream.h>

Opening a File
• Before data can be written to, or read from a
file, before that the file must be opened.
ifstream inputFile;
inputFile.open(“customer.txt”);
Open function opens a file if
it exist previously, otherwise
it will create that file (in the
current directory)

Example (File opening)
#include <iostream.h>
#include <fstream.h>
void main()
{
fstream dataFile; //Declare file stream object
char fileName[81];
cout<<"Enter name of file to open or create”;
cin.getline(fileName, 81);
dataFile.open(fileName, ios::out );
cout << "The file “<<fileName<<" was opened.";
}
File modes

Default File Opening Modes
File Types Default Open Modes
ofstream
File is opened for output only. (Information may be
written to file, but not read from the file.)
If the file does not exist, it is created. If the file
already exists, its contents are deleted.
ifstream
The file is opened for input only. (Information may
be read from the file, but not written to it.) File’s
contents will be read from its beginning.
If the file does not exist, the open function fails.

File Mode
Flag
Meaning
ios::app
Append mode.
If the file already exists, its contents are
preserved and all output is written to the end of
the file. By default, this flag causes the file to be
created if it does not exist.
ios::ate
If the file already exists, the program goes directly
to the end of it. Output may be written anywhere
in the file.
ios::binary
Binary mode. When a file is opened in binary
mode, information is written to or read from it in
pure binary format. (The default mode is text.)
ios::noreplace
If the file does not already exist, this flag will cause
the open function to fail. (The file will not be
created.)

File Mode
Flag
Meaning
ios::in
Input mode. Information will be read from the
file. If the file does not exist, it will not be created
and the open function will fail.
ios::nocreate
If the file does not already exist, this flag will
cause the open function to fail. (The file will
not be created.)
ios::out
Output mode. Information will be written to the
file. By default, the file’s contents will be deleted if
it already exists.
ios::trunc
If the file already exists, its contents will be deleted
(truncated). This is the default mode used by
ios::out.

Opening a File at Declaration
fstream dataFile(“names.dat”, ios::in | ios::out);
void main( )
{
fstream dataFile("names.txt", ios::in | ios::out);
cout << "The file names.txt was opened.n";
}

Testing for Open Errors
fstream dataFile;
dataFile.open(“cust.dat”, ios::in);
if (!dataFile)
{
cout << “Error opening file.n”;
}

Another way to Test for Open Errors
fstream dataFile;
dataFile.open(“cust.dat”, ios::in);
if (dataFile.fail())
{
cout << “Error opening file.n”;
}

Closing a File
• A file should be closed when a program is finished
using it.
void main(void)
{
fstream dataFile;
dataFile.open("testfile.txt", ios::out);
if (!dataFile)
{
cout << "File open error!”<<endl;
return;
}
cout << "File was created successfullyn";
cout << "Now closing the filen";
dataFile.close();
}

Using << to Write Information to a File
• The stream insertion operator (<<) may be used to
write information to a file.
outputFile<<“I love C++ programming !”;
fstream object

Example – File writing using <<
void main(void)
{
fstream dataFile;
dataFile.open("demofile.txt", ios::out);
if (!dataFile)
{
cout << "File open error!" << endl;
return;
}
cout << "File opened successfully.n";
cout << "Now writing information to the file.n";
dataFile << "Jonesn";
dataFile << "Smithn";
dataFile << "Willisn";
dataFile << "Davisn";
dataFile.close();
cout << "Done.n";
}

Program Screen Output
File opened successfully.
Now writing information to the file.
Done.
Output to File demofile.txt
Jones
Smith
Willis
Davis

Example – Append
void main(void)
{
fstream dataFile;
dataFile.open("demofile.txt", ios::out);
dataFile << "Jonesn";
dataFile << "Smithn";
dataFile.close();
dataFile.open("demofile.txt", ios::app);
dataFile << "Willisn";
dataFile << "Davisn";
dataFile.close();
}

Jones
Smith
Willis
Davis
Example – Append (Output)

File Output Formatting
• File output may be formatted the same way as
screen output.

File Output Formatting - Example
void main(void)
{
fstream dataFile;
float num = 123.456;
dataFile.open("numfile.txt", ios::out);
if (!dataFile)
{
return;
}
dataFile << num << endl;
dataFile.precision(5);
}

123.456
123.45600
123.4560
123.456
File Output Formatting Example -Output

Using >> to Read Information from a File
• The stream extraction operator (>>) may be
used to read information from a file.

#include < fstream.h>
void main(void)
{
fstream dataFile;
dataFile.open("demofile.txt", ios::in);
if (!dataFile)
{
return;
}
cout << "Now reading information from the file.nn";
for (int count = 0; count < 4; count++)
{
dataFile >> name;
cout << name << endl;
}
dataFile.close();
cout << "nDone.n"; }
Using >> to Read Information from a File

Now reading information from the file.
Jones
Smith
Willis
Davis
Done.
Using >> to Read Information from a File (output)

Detecting the End of a File
• The eof() member function reports when the
end of a file has been encountered.
if (inFile.eof())
inFile.close();

void main(void)
{
fstream dataFile;
dataFile.open("demofile.txt", ios::in);
if (!dataFile)
{
return;
}
cout << "Now reading information from the file.nn";
dataFile >> name; // Read first name from the file
while (!dataFile.eof())
{
cout << name << endl;
dataFile >> name;
}
dataFile.close();
cout << "nDone.n";
}
Detecting the End of a File

Now reading information from the file.
Jones
Smith
Willis
Davis
Done.
Detecting the End of a File - Output

Note on eof()
• 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.

getline Member Function
• dataFile.getline(str, 81, ‘n’);
str – Name of a character array. Information read
from file will be stored here.
81 – This number is one greater than the maximum
number of characters to be read. In this example, a
maximum of 80 characters will be read.
‘n’ – This is a delimiter character of your choice. If
this delimiter is encountered, it will cause the
function to stop reading. This argument is optional If
it’s left out, ‘n’ is the default.)

Example Program
void main(void)
{
fstream nameFile;
char input[81];
nameFile.open(“Address.txt", ios::in);
if (!nameFile)
{
return;
}
nameFile.getline(input, 81); // use n as a delimiter
while (!nameFile.eof())
{
cout << input << endl;
nameFile.getline(input, 81); // use n as a delimiter
}
nameFile.close();
}

Program Screen Output
Mohammad Ai Jinnah University
Islamabad Expressway, Zone V, Kahuta Road
Islamabad Pakistan

Program Example
// This file shows the getline function with a user-
// specified delimiter.
void main(void)
{
fstream dataFile("names2.txt", ios::in);
char input[81];
dataFile.getline(input, 81, '$');
while (!dataFile.eof())
{
cout << input << endl;
dataFile.getline(input, 81, '$');
}
dataFile.close();
}

Program Output
Jayne Murphy
47 Jones Circle
Almond, NC 28702
Bobbie Smith
217 Halifax Drive
Canton, NC 28716
Bill Hammet
PO Box 121
Springfield, NC 28357

The get Member Function
inFile.get(ch);
//one character at a time

Example Program
// This program asks the user for a file name. The file is
// opened and its contents are displayed on the screen.
void main(void)
{
fstream file;
char ch, fileName[51];
cout << "Enter a file name: ";
cin >> fileName;
file.open(fileName, ios::in);
if (!file)
{
cout << fileName << “ could not be opened.n";
return;
}
file.get(ch); // Get a character
while (!file.eof())
{
cout << ch;
file.get(ch); // Get another character
}
file.close();
}

The put Member Function
outFile.put(ch);
//Writes or outputs one character at a
time to the file

Example Program
// This program demonstrates the put member function.
void main(void)
{
fstream dataFile("sentence.txt", ios::out);
char ch;
cout<< "Type a sentence and be sure to end it with a ";
cout<< "period.n";
while (1)
{
cin.get(ch);
dataFile.put(ch);
if (ch == '.')
break;
}
dataFile.close();
}

Cs1123 10 file operations

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