C++ File Input Output

8/6/2019 C++ File Input Output

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BITG 1113:

Files : Input Output

LECTURE 10


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To understand the fundamentals of files

and streams input/output

Able to write programs that use data files

for input and output.

Objectives:


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FILES

File an external collection of related data treated as a

unit.

Purpose to keep a record of data.

record of data


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FILES

Why use files?

In normal way, it is difficult to handle large amounts of

input data and output data. The data would be lost as

soon as we turned the computer off. By using files, data can be stored in a more

permanent form.

Data can be created by one program, stored on

secondary devices, and then accessed or modifiedbyother programs when necessary.


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FILES

Two major classes of files:

Text files

Binary files

Files are stored in secondary storage devices. Two of itsmost common forms are:

Disk (Hard disk, CD, and DVD)

Tape


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Files in a secondary storage can be both read andwritten. These processes are also known as I/O orinput/output.

Files exist separately from the computer and ourprograms, so we must be able to create a linkagebetween the external file and its usage in our program.

A stream is this linkage. Stream is an abstractrepresentation of input data source, or output datadestination, used in our program.

FILES & STREAM


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FILES & STREAMS

To use files, input streams and/or output streams have tobe created depending on the required file operations.

Stream classes have been included in a

header file. This header file also automatically includesthe header file, so we no longer need toinclude it.

The type of stream class determines if it is an input file or

an output file orboth.


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Input file are of type ifstream(Stream class to read from

files).

Output files are of type ofstream(Stream class to writeon files).

Input & output files are of type fstream(Stream class to

both read and write from/to files).

FILES & STREAMS


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We can use our file streams the same way we use cin andcout, with the only difference that we have to associatethese streams with physical files.

// basic file operations#include

#include using namespace std;

int main ()

{ ofstream myfile("example.txt");//open file example.txt andassociate output stream object myfile with it.

myfile


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This code creates a file called example.txt and inserts a

sentence into it in the same way we are used to do with

cout, but using the stream object myfile instead.

The content of file example.txt:Writing this to file example.txt.

The output displayed on the computers monitor is:Writing this to the monitor.

FILES & STREAMS


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INPUT/OUTPUT FILES

A program that prints to an output file OR reads data

from an input file should make sure the file :

1. Has an acceptable file name.2. Is opened before it is used.

3. Is closed after it is used.


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There are a few modes to open a file. They are listed asfollows:Mode Description

ios::in - Open a file for input operations.

ios::out - Open a file for output operations.ios::ate - Open a file for output operations. Set the initial

position at the end of the file.

ios::app - Open a file for output operations. All outputoperations are performed at the end of the file,

appending the content to the current content of thefile.

ios::trunc - If the file opened for output operations already existedbefore, its previous content is deleted and replacedby the new one.

INPUT/OUTPUT FILES : Openinga file


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All these flags can be combined using the bitwise operator

OR (|).

For example, if we want to open the file example.bin toadd data we could do it by the following :

ofstream myfile("example.bin", ios::out | ios::app);



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For all stream classes, default mode/s is/areautomatically assumed, even if a mode that does notinclude them is passed as second argument to open thefile.

Streamclass Defaultmodeparameter

ofstream ios::out

ifstream ios::in

fstream ios::in | ios::out

The default value is only applied if the file is openedwithout specifying any value for the mode parameter.



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When we are finished with our input and outputoperations on a file we shall close it so that its resourcesbecome available again.

In order to do that we have to call the stream's functionclose(). This function takes no parameters, and what itdoes is to flush the associated buffers and close the file:

myfile.close();

Once this member function is called, the stream object(above eg: myfile) can be used again to open anotherfile.

INPUT/OUTPUT FILES : Closinga file


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OUTPUT FILES

Writing output to a text file : Example 1

# include

# include

using namespace std;

int main(){

double income = 123.45, expenses = 987.65;

int week = 7, year = 2006;

ofstream outfile("L4_2.out");// default mode is ios::out

outfile


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OUTPUT FILES

Content of L4_2.out text file :

You can find the L4_2.out file in the same folder as theprogram that wrote the file. Open it using the notepad.


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Writing output to a text file : Example 2

#include

#include


int main (){

ofstream myfile ("example.txt");

if (myfile.is_open())

{ myfile


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Input files can be created using any text editor. Notepad

is an example of text editor you can use to save your

data.

To create an input file, open the notepad and type the

data on it. Save the file in the same folder with the

program that will use these data. Example of the file is

shown below:

INPUT FILES


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Reading data from a text file: Eg 1# include # include #include using namespace std;

int main(){

double x;int i,j;

ifstream infile1("L4_3A.dat");ifstream infile2("L4_3B.dat");

infile1>>i;infile1>>j>>x;infile1.close();

cout


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Reading data from a text file: Eg 2#include #include

#include


int main ()

{ string line;

ifstream myfile ("example.txt");

if (myfile.is_open())

{ while (! myfile.eof() )

{ getline (myfile,line);

cout

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C++ File Input Output