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Saturday March 07, 2009

Introduction to Programming

Lecture 9

What is a File?

• Combination of characters, words, sentences and paragraph are called as file.

• Two Types of files– Text File (readable English characters)– Executable program file (Dos Command)

File Properties

• Name of the file• Path of the file (Absolute & Relative)• Length of the file• Date of Creation • File Access

Why do we need files?

• Information of the computer is volatile• Execute the program and the data is saved on

the hard disk.• e.g Payroll system for SEECS Employees• Store information once and reuse it• Read/write/manipulate the data is File

handling

File Handling

• Two ways to Access the Files– Sequential Access File– Random Access File

Text File Handling

• Basic steps for File handling

– Open the file– Read and Write – Close the File

Cont…

• Previously we worked with “cin” and “cout”.• Concept of Streams• During File Handling a header file called

“<fstream.h>” is to be included.• <fstream.h> is a header file for file streams.

9

Read and Write file

• When we read the file, it means that file is used as an input for the program.

• We need to have a stream for input file• ifstream ( input file stream)

• In order to read and write from the same file , ofstream.

Declaring File Stream (Step 1)

ifstream inFile; //object for reading from file

ofstream outFile; //object for writing into file

NOTE: The variable/ objects inFile and outFile are used as

“handle to refer” files.

Cont…..(Step 2)

• So far we didn’t attach the file with a handle.– ifstream myFile;

• We must open a file. • For this we will use a function “Open”.

– myFile.open(Filename);Note: Absolute / Relative Path for Filename

Cont…..(Step 3)

• Tell the compiler what you want to do with file i.e read, write or modify .

• Once the file is opened we can read it. myFile>>c;

/* The first word of the file will be read in c, where c is a character array, same as using cin. */

Cont…

myFile>>c1>>c2>>c3;

// The first word will be read in c1,2nd in c2, 3rd in c3.

Cont….(Step 4)

• File must be closed and it is the responsibility of programmer to close the file.

• myFile.close();• No argument in close function.

Think…….

• What is the File that we are trying to open does not exist on the disk?????????

Error Checking Mechanism

• It is very important• ifstream myFile; myFile.open(“mytext.txt”);

/*If this file does not exists on the disk, the variable myFile will not be associated with any file. We must make sure that the file opening process is successful. */

Cont…

If(!myFile) { cout<<“There is some error opening the

file”<<endl; cout<<“ File cannot be opened”<<endl; } else cout<<“File opened successfully”<<endl;

Example:#include <fstream.h>void main(){ char name[50]; char sal[10]; char dept[30]; ifstream inFile; inFile.open("myfile.txt"); if(!inFile) { cout<<"Can’t open input file"<<endl; } while(!inFile.eof()) { inFile>>name>>sal>>dept; cout<<name<<"\t“<<sal<<“\t”<<dept<<endl; } inFile.close();} //end of main

Output of the Program

Name Salary Department Omer 20000 Exam Ali 40000 Faculty Rashid 60000 Faculty

Output File Handling

• Creation of a new file on the disk and writing data in it. or • Open an existing file and overwrite it in such a manner that

all old information is lost from it and new information is stored.

or • Open an existing file and append it in the end or• Open an existing file and modify it from anywhere.

Cont…

• We can use the previous options when opening the file.

• open(filename,mode)

• The first argument is the name of the file, second argument will the mode of the file.

Cont…..

• Syntax myFile.open(“myFile.txt”, ios::in);

or myFile.open(“myFile.txt”);

/* The second argument associated myFile stream object with “myFile.txt” for input. */

// There are different modes available

File output: Appending#include <fstream>#include<iostream>

using namespace std;

void main()

{ cout<<"creating a file"<<endl; ofstream afile("test1.dat"); cout<<"writing to file.."<<endl; afile<<"these are test data";

} //end of file

ios :: in open for reading (default for ifstream)ios :: out open for writing (default for ofstream)ios :: app start writing at end of file (APPend)ios :: ate start reading or writing at EOF of file (ATEnd)ios :: trunc truncate file to zero length if it exists (TRUNCate)ios :: nocreate error when opening if file does not already existios :: noreplace error when opening for output if file already existsios :: binary open file in binary (not text) mode

List of File Handling Modes

25

Cont…

Mode Meaning

in Open a file/stream for extraction (input)

out Open a file/stream for insertion

(output)

App Append rather than truncate an existing file. Each insertion (output) will be written to end of the file.

trunc Discard the file contents if exists (similar to default behavior

ate Opens the file without truncating, but allows data to be written anywhere in the file.

binary Treat the file as binary rather than text. A binary file has data stored in internal formats, rather than readable text.

Important……

• If a file is opened with ios::out mode, a new file is created. However if the file already exists, its contents will be deleted and get empty unless you write something to it.

Example 2

main()

{

ofstream outFile;

outFile.open(“myFileOut.txt”,ios::out);

if(!outFile)

{

cout<<“ cannot open file”<<endl;

}

outFile.close();

}

Try yourself…..

Write a program, which reads an input file of employee’s i.e. employeein.txt”. Add the salary of each employee by 200, and write the result in new file “ employeeout.txt”??

get( )

• “>>” sign is not enough• get( ) function is used to get a character from the file.• The last character in the file is EOF, defined in header file.• E.g char c; while((c=inFile.get( ) !=EOF) { //do all the processing outFile.put(c); }

Why get( )

• >> does not read the new line character where as the get( ) function reads each character as it is typed.

• If we need to make copy of the file then get( ) function must be used.

put( )

• put( ) function writes a character including new line feed to the file.

Think…….

What if we want to write/read a whole line to file instead of writing character or words. After all the processing speed now is day is quite high?

getLine()

• Two functions– getLine() for input file stream– putLine() for output file stream

// char name[100]; int maxChar=100; int stopchar=‘o’; inFile.getLine(name,maxChar,stopchar);

myfile.getline (char *s, int n, char delim);

Character array delimiter

Numbers of characters to

be read

Try yourself…..

• Write a program which reads a file using the getLine() function and display it on the screen?

fail()

• A call to open can be unsuccessful for a number of reasons.

• A member function of ifstream and ofstream named fail can be used to test whether or not a stream operation has failed.

• It take no arguments and returns a bool.• Syntax:

– in_stream.fail( );

Cont…Example: in_stream.open(“stuff.dat”); if (in_stream.fail()){ cout<<“input file opening failed.\n”; exit();

//to use this function include #include <cstdlib>

}

isspace()

• The function isspace() returns true if its argument is a whitespace character else returns false.

• Whitespace are all the characters that are displayed as blank space on the screen, including the blank character, the tab character, and the new-line character ’\n’.

Example:The following program will read the sentence terminatewith the period and echo the string with all whitespacecharacters replace with the symbol “_”.

char next;do{ cin.get(next); if(isspace(next)) cout<<‘_’; else cout<<next; } while (next !=‘.’);

Try it yourself !!!!!!

Write a program that creates the output file “myoutput.txt” that is identical to the file“myoriginal.txt” except that all theoccurrences of ‘C’ are replace by “C++”.

Random Access Files

• Discuss how to access the files randomly, forward and backward.

• Current position inside the file.• Concept of file position ( Pointer into the file)• tellg( ) and tellp( )----functions for

determining the file pointer position.

Position in a File

• Assume that a file stream myfile is opened for reading .

• Myfile.tellg( ) gives us the current get position of the file pointer.

• It returns a whole number of type long, which is the position of the next character to be read from that file.

• tellp() function is used to determine the next position to write a character while writing into a file. It also returns a long number.

Setting the Position

• How to move forward and backward within the file????

• seekg( ) and seekp( ) functions• seekg( )--- takes us to certain position to start

reading.• seekp( )---> leads the position to write into.• Both functions require an argument of type long to

let them know the number of bytes to move forward or backward. (positive and negative number)

filePtr.seekg ( long Num , ios :: origin ) ;filePtr.seekg ( long Num , ios :: origin ) ;

Number of characters to move to

Starting point

Setting the Position

seekg ( 10L , ios :: beg ) ;seekg ( 10L , ios :: beg ) ;seekg (10L , ios :: cur ) ;seekg (10L , ios :: cur ) ;

seekg ( 10L , ios :: end ) ;seekg ( 10L , ios :: end ) ;

Example:--)• aFile.seekg(10L, ios::beg) //move 10 bytes forward from the beginning of the file.• aFile.seekg(20L, ios::cur) // move 20 bytes in the forward direction starting from current

position (where current position can be obtained by using tellg( ) function.

• aFile.seekg(-10L,ios::cur) // move 10 bytes in the backward direction from the current

position• aFile.seekg(-100, ios::end) // move 100 bytes in backward direction from the end of the

file.

Think and Apply…..

• Try to move the file pointer beyond the end of the file and before the beginning of the file and observe the behavior…….

Example 1#include<fstream.h>main ( ){

int length ; ifstream inFile ( “myFile.txt” ) ; inFile.seekg ( 0L , ios :: end ) ; length = inFile.tellg ( ) ;

}It will tell you Length of the file

Name city Date-of-Birth : : :

Jamil Ahmed Sukkur 10-10-1982 : : :

File

Rawalpindi

Very tricky if you want to write at the middle of the file

Merge Method

This is a text data And needs

Original file Empty file

To be replaced NOT

For insertion in the middle of the file

1. The language does not impose any structure of the file, the file is just a bits and bytes for C++

2. For easiness1. Use serial number2. Use sorted files

get ( ) and put ( ) character in a file

myInputFile.get ( c ) ;myOutputFile.put ( c ) ;

read ( ) and write ( ) Functions

read ( char *buff , int count ) ;

write ( char *buff , int count ) ;

Area in memoryNumber of bytes to

be read

Area in memoryNumber of bytes to

be written

char str [ 10000 ] ;myInputFile.read ( str , 10000 ) ;myOuputFile.write ( str , 10000 ) ;

Example 3

1. Read 10000 bytes from the file into str2. Write 10000 bytes to a file from str

Example 2This is an Apple

This is a Sample

Example 2:This program firstly writes a string into a file and then replaces it partially.

main(){ long pos; ofstream outfile; outfile.open(“test.txt”); outfile.write(“this is an apple”,16);

//write the string in the file pos=outfile.tellp();

//get the file pointer position outfile.seekp(pos-7);

// move 7 positions backward outfile.close(); }

Try it !!!!!!!!

• Write a program that read the contents from “myfile1.txt” and copies the contents in reverse order in “outputfile.txt”……..

(Hint:---Make use of seekg function)

HINTS:-)afile.seekg(0L, ios::end);afile.seekg(-1L, ios::end);//set the position of the cursor to the last

character of the file

while(afile){ cout<<afile.tellg()<<endl; afile.get(c); Ofile.put(c); afile.seekg(-2L,ios::cur);

//set the position of the cursor to the one character before the current position

}

fstream

fstream myFile ( “Sample.txt” , ios :: in | ios :: out ) ;

Example:- Try to understand by yourself

fstream rfile;main(){char rchar;rfile.open(“myfile.txt”,ios::in||ios::out)if(!rfile){ cout<<“error opening file”;}for(rchar=‘A’;rchar<=‘Z’;rchar++){ rfile<<rchar; }rfile.seekg(8L, ios::beg);rfile>>rchar;cout<<“the 8th char is”<<rchar;rfile.seekg(-16L, ios::end);rfile>>rchar;cout<<“ the 16th character from the end”<<rchar;rfile.close();}