Post on 28-Oct-2014
description
Data File Handling in C++
Data File Handling in C++
Introduction Introduction • Computer programs are associated to
work with files as it helps in storing data & information permanently.
• File - itself a bunch of bytes stored on some storage devices.
• In C++ this is achieved through a component header file called fstream.hfstream.h
• The I/O library manages two aspects- as interface and for transfer of data.
• The library predefine a set of operations for all file related handling through certain classes.
• C++ provides a new technique for handling I/O operations through mechanism known as streams.
• A stream refers to a flow of data.• Classified in 2 categories:1. Output stream2. Input streamIn output stream flow of data is from program to the
output device.In input stream the flow of data is from input device to
a program in main memory.
The fstream.h header file
Streams act as an interface between files and programs.
They represent as a sequence of bytes and deals with the flow of data.
Every stream is associated with a class having member functions and operations for a particular kind of data flow.
File Program ( Input stream) - reads
Program File (Output stream) – write
All designed into fstream.h and hence needs to be included in all file handling programs.
Diagrammatically as shown in next slide
Stream Class Hierarchy
ios
ostreamistream
fstream
iostream
ofstreamifstream
User-Defined Streams
System-Defined Streams
Why to use Files:•Convenient way to deal large quantities of data.•Store data permanently (until file is deleted).•Avoid typing data into program multiple times.•Share data between programs. We need to know:
how to "connect" file to programhow to tell the program to read data
how to tell the program to write dataerror checking and handling EOF
Stream Insertion Operators• Are defined in the ostream class• The operator “<<” is called the inserter
Stream Extraction Operators• Are defined in the istream class and are used to receive data from the input device• The operator “>>”, called the extractor, accepts any built-in data type passed as arguments
1: To access file handling routines:#include <fstream.h>#include <fstream.h>
2: To declare variables that can be used to access file:ifstream in_stream;ifstream in_stream;
ofstream out_stream;ofstream out_stream; 3: To connect your program's variable (its internal name) to an external file (i.e., on the Unix file system):
in_stream.open("infile.dat");in_stream.open("infile.dat");out_stream.open("outfile.dat");out_stream.open("outfile.dat");
4: To see if the file opened successfully:if (in_stream.fail())if (in_stream.fail())
{ cout << "Input file open failed\n";{ cout << "Input file open failed\n"; exit(1);exit(1); // requires <stdlib.h>}// requires <stdlib.h>}
5: To get data from a file (one option), must declare a variable to hold the data and then read it using the extraction operator:
int num;int num;in_stream >> num;in_stream >> num;
[Compare: cin >> num;][Compare: cin >> num;] 6: To put data into a file, use insertion operator:
out_stream << num;out_stream << num;[Compare: cout << num;][Compare: cout << num;]
NOTE: Streams are sequential – data is read and written in order – generally can't back up.
7: When done with the file:in_stream.close();in_stream.close();
out_stream.close();out_stream.close();
Example (creating file)
#include<fstream.h>
#include<process.h>
Void main()
{
int rollno
char name[10];
ofstream ofil(“stud.txt”);
cout<<“enter details”;
cin>>rollno>>name;
cout<<“writing student details into file…..”;
ofil<<rollno<<endl<<name;
getch();
}
Reading a file
#include<fstream.h>
#include<process.h>
Void main()
{
int rollno
char name[10];
Ifstream ifil(“stud.txt”);
If(!ifil)
{
Cout<<“can not open file”;
Exit(-1);
}
cout<<“reading data from file…”;
ifil>>rollno>>name;
cout<<“roll no is”<<rollno<<endl;
cout<<“name is”<<name<<endl;
getch();
}
ios:: app - (append) write all output to the end of file ios:: ate - data can be written anywhere in the file ios:: binary - read/write data in binary format ios:: in - (input) open a file for input ios::out - (output) open a file for output ios:: trunc -(truncate) discard the files’ contents if it exists
File Open Modes
Stream state member functions
•In C++, file stream classes inherit a stream state member from the ios class, which gives out
the information regarding the status of the stream.
For e.g.:eof() –used to check the end of file eof() –used to check the end of file charactercharacterfail()- used to check the status of file at fail()- used to check the status of file at opening for I/Oopening for I/Obad()- used to check whether invalid file bad()- used to check whether invalid file operations or unrecoverable error .operations or unrecoverable error .good()- used to check whether the good()- used to check whether the previous file operation has been previous file operation has been successfulsuccessful
File Input and Output Using ObjectsExample:#include<fstream.h>class student
{
private:
int iReg_no;
char name[20];
public:
void setRegno()
{
Cout<<“registration no”;
Cin>>iReg_no;
}
void setName()
{
Cout<<“enter name”;
Cin>>name;
}
int getRegno()
{
Cout<<iReg_no;
}
char getName()
{
Cout<<name;
}
};
File Input and Output Using Objects (Contd.)void main(){
ofstream Sfil(“studfile.dat”);
char ch;
student Svar;
Svar.setRegno();
Svar.setName();
Sfil<<Svar.getRegno()
<<“ ”<<Svar.getName();
Sfil.close(); //Closes the open file
File Input and Output Using Objects (Contd.)cout<< “\n Do you want to view the contents of a file (y/n)?”;
cin>>ch;if(ch== ‘y’){ifstream Sfil(“studfile.dat”);char ireg;char nam[20];Sfil>>ireg>>nam;cout<<“\n Registration Number is ” <<ireg;cout<<“\n Student Name is ” <<nam;}
}
// Initial experience reading and writing files#include <fstream.h>#include <iostream.h>#include <stdlib.h>int main(){ ifstream in_stream; ofstream out_stream; int num; in_stream.open("numbers.dat"); if (in_stream.fail()) { cout << "Input file could not be opened.\n";
exit(1); } out_stream.open("squares.dat"); if (out_stream.fail()) { cout <<"Output file could not opened.\n";
exit(1); } in_stream >> num; out_stream << "The square of " << num << " is " <<num * num; in_stream.close(); out_stream.close();}
• INPUT AND OUTPUT OPERATION• put() and get() function
the function put() writes a single character to the associated stream. Similarly, the function get() reads a single character form the associated stream.example :file.get(ch);file.put(ch);
• write() and read() functionwrite() and read() functions write and read blocks of data.example:file.read((char *)&obj, sizeof(obj));file.write((char *)&obj, sizeof(obj));
int main()
{
ifstream mys;
char c;
mys.open("new.txt");
if(mys.fail())
{
cout<<"file not found";
}
while (!mys.eof())
{
cout << c;
mys.get(c);
}
mys.close();
getch();
return 0;
}
class Person
{
private:
char name[40];
int age;
public:
void getData()
{
cout << " Enter name:"; cin >> name;
cout << "\n Enter age:"; cin >> age;
}} ;
int main()
{
Person per ;
per.getData();
ofstream outfile("Person.txt");
outfile.write((char*)&per, sizeof(per));
return 0;
getch();}
struct record
{
char player[20];
int age;
int runs;
}emp;
void add()
{
ofstream fp("jk.txt",ios::out);
if(!fp)
{
cout<<"not exits";
exit(1);
}
cout<<"enter name and age and runs";
cin>>emp.player>>emp.age>>emp.runs;
fp.write((char *)&emp,sizeof(emp));
fp.close();
}
void display()
{
ifstream fp("jk.txt",ios::in);
if(!fp)
{
printf("file cannot be opened");
exit(1);
}
printf("records entered");
fp.read((char *)&emp,sizeof(emp));
cout<<emp.player<<" "<<emp.age<<" "<<emp.runs;
fp.close();
}
int main()
{
add();
display();
getch();
return 0;
}
deletionvoid del()
{ int r;
student obj;
ifstream fp1;
ofstream fp2,fp3;
fp1.open("student.dat",ios::in);
fp2.open("temp1.txt",ios::app);
fp3.open("temp2.dat",ios::app);
puts("Enter the deleting roll no.");
cin>>r;
while(fp1.read((char *)&obj,sizeof(obj)))
{
if(r==obj.roll())
{
fp2.write((char *)&obj,sizeof(obj));
}
else
{
fp3.write((char *)&obj,sizeof(obj));
}
}
remove("student.txt");
rename("temp2.txt","student.txt");
getch();}
updatingvoid update()
{
student obj;
int r;
ifstream fp1;
fp1.open("student.txt");
ofstream fp2;
fp2.open("temp.txt",ios::app);
puts("Enter the rollno. to be modified");
cin>>r;
while(fp1.read((char*)&obj,sizeof(obj)))
{
if(obj.roll()==r)
{
puts("Enter new roll no. and name");
obj.getdata();
fp2.write((char*)&obj,sizeof(obj));
}
else
{
fp1.write((char *)&obj,sizeof(obj));
}
}
remove("student.txt");
rename("temp.txt","student.txt");
getch();
}
File pointers
The get Pointer
• Specifies the location in the file where the next read operation will occur
The put Pointer
• Specifies the location in the file where the next write operation will occur
The seekg() Function
• Helps to control the get pointer
• Moves the get pointer to an absolute address within the file or to a certain number of bytes from a particular position
• Takes two arguments:
– The number of bytes to move–The reference in the file from where the pointer has to be repositioned
Example:
ifstream iFil;
iFil.seekg(10,ios::beg);
The tellg() Function
• Helps to control the get pointer
• Can be used to find the current position of the get file pointer in a file
• Does not take any arguments
Example:
int iPosition=iFil.tellg();
The seekp() Function
• Helps to control the put pointer
• Moves the put pointer to an absolute address within the file or to a certain number of bytes from a particular position
The tellp() Function
• Helps to control the put pointer
• Can be used to find the current position of the put file pointer in a file
Example To find the size of the file#include<fstream.h>#include<process.h>Void main(){Ifstream ifil(“abc.txt”);If(!ifill){Cout<<“file cannot open correctly”;exit(-1);}
ifill.seekg(0,ios::end);
int x=ifil.tellg();
Cout<<“size of file is”<<x;
}