Kapil Sehgal PGT Computer. Science Ankleshwar Gujarat Ch - 7. Data File Handling

Transcription

1 Introduction Data File Handling The fstream.h Header file Data Files Opening and Closing File Steps to process a File in your Program Changing the behavior of Stream Sequential I/O With Files Detecting EOF File Pointers and Random Access Basic Operation on Files Error Handling During I/O

2 Introduction of File Most Computer Programs work with files. Word Processor creates documents file, Database creates files of Information, Compilers read source file and generates executable files. So, File itself is a bunch of bytes stored on some storage device like tape, or Magnetic disk etc. In C++, File Input/Output facility are implemented through a component header file of C++ standard library. This header file is fstream.h

3 fstream.h Header File The C++ input/output operation are very much similar to the console input and output operation. The file operations also make use of streams as an interface between the programs and the files. What is stream? A stream is a sequence of Bytes. A stream is a general name given to a flow of data. Different streams are used to represent different kind of data flow. Each streams is associated with a particular class.

4 Continue Each stream is associated with particular class, which contains member function and definitions for dealing with that particular kind of data flow. The istream class represent input disk file. The streams that supplies data to the program is known as input stream. It read the data from the file and hands it over to the program. The streams that received data from program is known as output stream. It writes the received data to the file.

5 Text File A text file stores information in ASCII characters. In text files, each line of text is terminated, with special characters known as EOL (End of Line) characters. In text files some internal translations take place when this EOL characters is read or written.

6 Binary Files A Binary file just a file that contains information in the same way as it store in memory. In binary file, there is no delimiters for a line. In binary file, no translation occurs. Binary files are faster and easier for a program to read and write than a text file. Binary files are best way to store information In C++, the default file system is Text file.

7 Opening the Files In C++, for open a file, you must first obtains a stream. There are three types of Stream; Input, Output and Input- Output. To create an Input stream, declare the stream to be of class istream. To create an Output stream, declare the stream to be of class ostream. Stream that will perform both input and output operation must be declared as fstream. Opening a file can be achieved in two ways. (1) Using the constructor function of the stream class. (2) Using the function open()

8 When we open a file with Constructor This method is preferred, when a single file is used with a stream. When we open a file by open() method This method is preferred, when we want to manage multiple files with same stream.

9 Opening file using Constructor To open a file, Data file as an input file ( it means we will read from it, but can not modifying it. And no other operation like writing and modifying is possible). We shall create a file stream object of input type. ifstream type. Syntax : ifstream inputfile( Datafile ); Here inputfile is the object of ifstream class and Datafile is the file name to open in input mode. After creating the object, datafile is open and attach with object inputfile.

10 How to read data from file To read the data from this file, object of inputfile is used and by using getfrom operator >> Syntax char ch; inputfile >>ch; float amt; inputfile >>amt;

11 How to write into the file Similarly, when we want to write onto the file, we must open the file in output mode. This is accomplished by. Creating ofstream object to manage the output stream. Associating tat object with a particular file Syntax.. ofstream outputfile( datafile ); Here outputfile is the object and datafile is file.

12 Extra Notation about file object The connection with a file are closed automatically, when the input and output stream object expire. Also you can close connection with a file explicitly by using close() method. inputfile.close(); outputfile.close(); Closing such connection does not eliminate the stream. It just disconnect it from file.

13 Opening a file using open() function The are many situations requiring a program to open more than one file. So we need create a separate stream for each of the file. First of all, declares the stream object without initializing it, then use the second statement and associated a file name. Syntax.. ifstream infileobject; Infileobject.open( filename.dat ); To close a file. We can use infileobject.close();

14 Concept of file mode The filemode describe how a file is to be used. To read from it, to write to it, to append it and so on. When you associate a stream with file, either by constructor or by the open() function, the second argument is the mode of the file, in which it open. Syntax.. Stream_object.open ( FileName,Filemode); The second argument of open method is of int type and you can choose one of the mode of file.

15 S. No Constant Meaning Stream Type 1 ios::in It opens file for reading, in input mode ifstream 2 ios::out It opens file for writing, in output mode. This also opens the file in ios::trunk mode by default. This means an existing file is truncated when opened. It means the previous contents are discarded. 3 ios::ate This seeks to end-of-file upon opening the file, I/O operation can still occur anywhere within the file. 4 ios::app This causes all output to that file to be appended to the end. This value can be used only with files capable of output 5 ios::trunk This value causes the contents of a pre-existing file by the same name to be destroyed and truncated the file to zero length. 6 ios::noncr eate 7 ios::norepl ace This cause the open() function to fail if the file does not already exist. It will not create a new file with that name. This causes open() function to fail if the file already exist. This is used when you want to create a new file and at the same time. 8 ios::binary This causes a file to be opened in binary mode. By default, files are opened in text mode. When a file opened in text mode, various characters translations take place. Such as carriage return to new line. However no such character translation occurs in files opened in binary mode. ofstream Ifstream Ofstrea m ofstream ofstream ofstream ofstream ofstream ifstream

16 Extra Notations about file mode The ifstream and ofstream provide default mode of the file is ios::in(input mode) The fstream class does not provide the default mode. It specify explicitly. You can combine more than one file mode using bitwise (OR) operator. Ex ofstream fout; fout.open ( Master,ios::app ios::nocreate); To open a binary file add ios::binary with file mode. fout.open ( Master, ios::nocreate ios::binary);

17 Steps to process a file in your Program Determine the type of link required. Declare a stream for the desired type of link. Attach the desired file to the stream. Now process as required. Close the file-link with stream.

18 Determine the type of link required. This step is required the type of link, if the data is to be brought in from a file to memory, then the link can be said to be File-to-Memory. Similarly fif the data is sent from memory to file then the link can be said as Memory-to-file. If both side need data transfer then two-way link is created. File-to-Memory Input type Memory-to-file Output type File-to-Memory & Memory-to-file // I/O type

19 Declaring the stream for type of link After determining the type of the link, next step is create a stream for it. In order to create the file stream, the header file <fstream.h> is included in the program, and if you have <fstream.h> then no need to include <iostream.h.> as cin, cout. These are also available in <fstream.h>. istream fi; // here stream name is fi; ofstream fo; // here stream name is fo;

20 Attach desired file to declare stream In this step we want to attach a file with the created stream in second step. This can be done in two ways Using Constructor. Using open() method ifstream fin( sample.dat ); // sample.dat is a file. fin.open( sample.dat,ios::in) // ios::in file mode

21 Process as required. Under this step, whatever process is required for the given problem, is performed for instance. Close the file link with stream This is the final step where we de-link the file with stream. This is performed through close () function ifstream fin; fin.close();

22 EX #include<fstream.h> void main() ofstream filout; filout.open("marks.dat",i os::out); char ans='y'; int follno; float marks; while(ans=='y' ans=='y') cout<<endl<<"enter rollno "; cin>>rollno; cout<<endl<<"etner Marks "; cin>>marks; filout<<rollno<<"\n"<<marks<<"\n"; cout<<'\n Do you want to add more record "; cin>>ans; filout.close(); After the Executing the program, we can open the file marks.dat in notepad and see the data is reached in the file or not

23 Q. Write a C++ program to wite 5 students name in student file and again read it from file #include<fstream.h> #include<conio.h> void main() int i; ofstream fout("student"); char name[25],ch; float marks=0.0; clrscr(); for(i=0;i<5;i++) cout<<"student "<<i+1 <<":\t Name "; cin.get(name,25); cout<<"\t Marks "; cin>>marks; cin.get(ch); fout<<name<<"\n"<<marks<<"\n";

24 fout.close(); ifstream fin("student"); fin.seekg(0); cout<<"\n"; for(i=0;i<5;i++) fin.get(name,25); fin.get(ch); fin>>marks; fin.get(ch); cout<<"student Name "<<name; cout<<"\t Marks : "<<marks<< "\n "; fin.close(); getch();

25 Changing the behavior of Stream When you open a file, it means when you link a file with stream using constructor, the stream is activated in its default mode. The default mode in input type stream is ios::in and for output type stream is ios::out. The default behavior of ifstream type stream allow user to read content from the file. If the file mode is ios::in only (fin.open( abc.dat,ios::in)) then reading is performed in text file only. If the file mode is ios::in ios::binary then reading is performed on binary file.

26 Write a C++ program to append data in already stored file marks.dat #include<fstream.h> void main() ofstream fout; fout.open("marks.dat",ios::app); char ans='y'; int rollno; float marks; while(ans=='y' ans=='y') cout<<endl<<"enter Rollno "; cin>>rollno; cout<<endl<<"etner Marks "; cin>>marks; fout<<rollno<<"\n"<<marks<<"\n"; cout<<'\n Do you want to add more record "; cin>>ans; fout.close();

27 get(), getline() and put() function The function get() and put() are byte oriented. get() will read byte of data and put() will write byte of data. The get() have many forms and used as.. istream & get(char & ch); ostream & put(char ch); The get() function reads a single characters from the associated stream and put the value in ch. It returns the reference to the stream. The put write the value of ch to the stream and returns the reference to the stream.

28 Write a program to display the content of file #include<fstream.h> #include<conio.h> int main() clrscr(); char ch; ifstream fin; fin.open("c:\\ab.txt",ios::in); if(!fin) cout<<"\n Cannot open the file "; return(0); while(fin) fin.get(ch); cout<<ch; fin.close(); getch(); return 0;

29 Write a program to create a file using put() #include<fstream.h> #include<conio.h> int main() clrscr(); ofstream fout; fout.open("c:\\abc.txt",ios::out); if(!fout) cout<<"\n Cannot open the file "; return(0); for(int i=65;i<=90;i++) fout.put((char)i); fout.close(); getch(); return 0;

30 Another forms of get() Other forms of get() as follows istream & get(char *buf, int num, char delt) int get(); The first form read the characters into a character array pointed to by buf until either num character have been read or the character specified by delt has been occur. For Example char line[40]; cin.get(line,40, $ ); This will read characters into line either 40 characters or $ is encountered which ever is occur first. The second form of get() returns the next character from stream, it returns EOF if end of file encountered. Ex.. int i; char ch; ch = i = fin.get(); If A is read then value of i will 65 and ch will A

31 getline() function. This function read a line into the characters array. istream & getline(char *buf, int num, char delm) This function virtually identical with get() function This function also reads the characters from input stream and placed it onto the characters array, either number of characters specified or the delimiter character is encountered. cin.getline(a,40, $ );

32 Difference between getline() & get() The difference between get(buf,num,delt) and getline() is that getline() read and remove the delimeter new-line character from the input stream if it is encountered which is not done in get(). EX void main() char nm[40],n[40]; int i; clrscr(); cout<<"enter First String "; cin.getline(nm,40,'$'); cout<<"enter Second String "; cin.get(n,40,'%'); cout<<endl<<nm<<strlen(nm ); cout<<endl<<n<<strlen(n); getch();

33 Difference between getline() & get()

34 Programming diff. b/n get and getline #include<fstream.h> #include<conio.h> #include<string.h> void main() char nm[40],n[40],ch; int i; clrscr(); cout<<"enter First String "; cin.get(nm,40); //cin.get(ch); cout<<"enter Second String "; cin.getline(n,40); cout<<endl<<nm<<strlen(nm); cout<<endl<<n<<strlen(n); getch(); Without cin.get(ch) output is With cin.get(ch) output is

35 Read a text file and create a file that stores the text line by line along with line nos. #include<fstream.h> #include<conio.h> int main() char line[75]; int lc=0; ofstream outfile("f11.cpp",ios::app); ifstream infile("f1.cpp",ios::in); clrscr(); if (!infile) cout<<"failed to open input file "; return 0; while(!infile.eof()) // while(1) infile.getline(line,75); //if (infile.eof()) break; lc++; outfile<<lc<<":"<<line<<"\n"; infile.close(); outfile.close(); cout<<"output "<<lc<<" Records"<<endl; getch(); return 0;

36 The read() and write() function To manipulate binary data into the file we use two another function read() and write() as follow istream & read( (char *) & buf, int sizeof(buf)); ostream & write((char *) & buf,int sizeof(buf)); The data written to a file using write can only be read accurately using read().

37 Read and write a binary file using struct #include<fstream.h> #include<conio.h> #include<string.h> struct customer char name[25]; float bal;; int main() customer savec; clrscr(); strcpy(savec.name,"123546"); savec.bal= ; ofstream fout("saving",ios::out ios::binary); if (!fout) cout<<"file can't be open "; return 0; fout.write((char *) & savec, sizeof(customer)); fout.close(); ifstream fin("saving",ios::in ios::binary); fin.read((char *) & savec, sizeof(customer)); cout<<savec.name; cout<<savec.bal; getch(); return 0;

38 Read and write a binary file using class #include<fstream.h> #include<conio.h> class student char name[25]; int rollno; public: void readdata(); void printdata(); ; void student :: readdata() cout<<endl<<"enter Name "; cin>>name; cout<<endl<<"enter Rollno "; cin>>rollno; void student :: printdata() cout<<endl<<"name = "<<name; cout<<endl<<"rollno = "<<rollno; void main() fstream file; student s,t ; file.open("file.txt",ios::out ios::in ios::binary); clrscr(); s.readdata(); file.write((char *) &s, sizeof(s)); s.readdata(); file.write((char *) &s, sizeof(s)); file.seekg(0); file.read((char *) &t, sizeof(t)); t.printdata(); file.read((char *) &t, sizeof(t)); t.printdata(); getch();

39 Detecting EOF We can detect, when the end of file is reached by using the member function eof(); Syntax int eof(); ifstream fin; fin.open("master",ios::in); while(!fin.eof()) if (fin.eof()) cout<<"end of file is reached ";

40 File pointers and Random Access Every file maintain two pointers called get_pointer (in input mode file) and put_pointer (in output mode file) which is tell the current position in the file where writing and reading take place. File pointer not a pointer in c++, but it is like a book mark for the file. By these pointers we can maintain a file in random access way. This means we can read from any where and write onto any where in the file.

41 seekg(), seekp() & tellg(), tellp(). In C++ random accessing is achieved by four function called seekg(), seekp() and tellg(), tellp(). seekg() and tellg() functions are used to set the get pointers. Similarly seekp() and tellp() are used to set the put_pointers. The seekg() and tellg() functions are used in ifstream and seekp() and tellp() functions are used in ofstream If we use these function with fstream object then the tellg and tellp returns the same value and seekg and seekp works in the same way.

42 SYNTAX Ex seekg() : istream & seekg(long); // form 1; istream & seekg(long,seek_dir); // form 2; seekp() : ostream & seekp(long); // form 1; ostream & seekp(long,seek_dir); // form 2; tellg() - returns long get_pointers; tellp() - returns long put_pointers; Seek_dir fin.seekg(30); will move the get_pointer to byte no 30; fin.seekp(30); will move the put_pointer to byte no 30; ios::beg // refers to beginning of the file. ios::cur // refers to current of the file. ios::end // refers to end of file.

43 EXAMPLES fin.seekg(30,ios::beg); // goto byte no 30 from the beginning of the file; fin.seekg(-5,ios::cur); // goto back 5 bytes from the current position; fin.seekg(0,ios::end);; // goto the end of the file; fin.seekg(-5,ios::end); // goto 5 byte back from end of file

44 Basic operations on Binary Files There are some operations performs on binary files like.. Searching Appending Data Inserting Data on Sorted File Deleting a Record Modifying Data

45 Search Data from File using class #include<fstream.h> #include<conio.h> class student char name[25]; int rollno; public: void readdata(); void printdata(); int getrollno() return(rollno); ; void student :: readdata() cout<<endl<<"enter Name "; cin>>name; cout<<endl<<"enter Rollno "; cin>>rollno; void student :: printdata() cout<<endl<<"name = "<<name; cout<<endl<<"rollno = "<<rollno; void main() char ch; int rno; fstream file; student s,t ; file.open("file.txt",ios::out ios::in ios::binary); clrscr(); while(1) s.readdata(); file.write((char *) &s, sizeof(s)); cout<<"do you Continue (Yes for Y or y) ";

46 CONTINUE cin>>ch; if (ch!='y' && ch!='y') break; cout<<endl<<"enter Rollno to be search : "; cin>>rno; ch='n'; file.seekg(0); while(!file.eof()) file.read((char *) &t, sizeof(t)); if (t.getrollno()==rno) cout<<endl<<"data Found "<<endl; t.printdata(); ch='y'; if (ch=='n') cout<<endl<<"data Not Found "; getch();

47 Appending Data to the File. To append data in the file, the opened file in two mode. File is opened in output mode ios::out File is opened in append mode ios::app Once the file get opened in app mode, the old data will retained and new data get appended to the file

48 Append Data in the File using class #include<fstream.h> #include<conio.h> class student char name[25]; int rollno; public: void readdata(); ; void student :: readdata() cout<<endl<<"enter Name "; cin>>name; cout<<endl<<"enter Rollno "; cin>>rollno; void main() char ch; int rno; fstream file; student s,t ; file.open("file.txt",ios::app ios::binary); clrscr(); while(1) s.readdata(); file.write((char *) &s, sizeof(s)); cout<<"do you Continue (Yes for Y or y) "; cin>>ch; if (ch!='y' && ch!='y') break; getch();

49 Inserting Data in Sorted File To inserted data in a sorted file, firstly its appropriate position is determined and then records in the file prior to this determined position are copied to temporary file, followed by the new record to be inserted and then the rest of the records from the file also copied. After that old file is delete and new file rename as old file. So inserting in sorted file is done with following steps.

50 Continue.. Copy the records prior to determined position to a temporary file say.. temp.dat Append the new record in temp.dat Now append the rest of record of main file into the temp.dat Delete the main file like student.dat remove( student.dat ); rename the temp.dat as student.dat as rename( temp.dat, student.dat );

51 Insert Data into Sorted file #include<fstream.h> #include<stdio.h> #include<conio.h> class student char name[25]; int rollno; public: void readdata(); int getrollno() return(rollno); ; void student :: readdata() cout<<endl<<"enter Name "; cin>>name; cout<<endl<<"enter Rollno "; cin>>rollno; int main() fstream file,outfile; student prev,next,nrec; file.open("file.txt",ios::in ios::binary); outfile.open("fil.txt",ios::out ios::binary); nrec.readdata(); clrscr();

52 CONTINUE while(1) file.read((char *) &next, sizeof(next)); if (next.getrollno()==nrec.getrollno()) cout<<endl<<"record already exitst ";getch();return 0; else if (next.getrollno()>nrec.getrollno()) outfile.write((char *) &nrec, sizeof(nrec));break; else prev=next; outfile.write((char *) &prev, sizeof(prev)); file.seekg(0); while(!file.eof()) file.read((char *) &next, sizeof(next)); if (next.getrollno()>nrec.getrollno()) outfile.write((char *) &next, sizeof(next)); remove("file.txt"); rename("fil.txt","file.txt"); getch(); return 0;

53 Deleting a Record Firstly determine the position of the record to be deleted by performing search operation. Keep copying the record other than the record to be deleted in a temporary file called temp.dat Do not copy the deleted record into the temp.dat Delete the orginal file as.. remove( student.dat ); rename the temp.dat as student.dat as rename( temp.dat, student.dat );

54 PROGRAM #include<fstream.h> #include<stdio.h> #include<conio.h> class student char name[25]; int rollno; public: void readdata(); int getrollno() return(rollno); ; void student :: readdata() cout<<endl<<"enter Rollno "; cin>>rollno; int main() fstream file,outfile; student prev,next,drec; file.open("file.txt",ios::in ios::binary); outfile.open("fi.txt",ios::out ios::binary); drec.readdata(); clrscr(); while(!file.eof()) file.read((char *) &next, sizeof(next)); if (next.getrollno()!=drec.getrollno()) outfile.write((char *) &next, sizeof(next)); file.close(); outfile.close(); remove("file.txt"); rename("fi.txt","file.txt"); getch(); return 0;

55 Modifying a Record To modify a record, the file is opened in I/O mode and an important step is performed that gives the beginning address of record being modified. After the record is modified in memory, the file pointer is once again is placed in at beginning position of this record and record is re-written.

56 PROGRAM #include<fstream.h> #include<stdio.h> #include<conio.h> class student char name[25]; int rollno; public: void modifydata(); int getrollno() return(rollno); ; void student :: modifydata() cout<<endl<<"enter Name "; cin>>name; cout<<endl<<"enter Rollno "; cin>>rollno; int main() int rno; long pos; fstream file; student mrec; file.open("file.txt",ios::in ios::out ios::binary); clrscr(); cout<<endl<<"enter Roll No to be modify "; cin>>rno; //mrec.readdata(); clrscr(); while(!file.eof()) pos=file.tellg(); file.read((char *) &mrec, sizeof(mrec)); if (mrec.getrollno()==rno) mrec.modifydata(); file.seekg(pos); file.write((char *) &mrec, sizeof(mrec)); file.close(); getch(); return 0;

57 Error Handling during File I/O During file operation, errors may occurs such as File not exist. for reading file File already exist. for writing file. End of file occurs File can not be create for less disk space File can not be open as corrupt file To check for such errors and to ensure smooth processing, C++ file stream inherits Stream-state members from ios class, that stores the information on the status of the file being currently used.

58 Flag bits eofbit 1 when end of file is encounter, 0 otherwise failbit 1 when a not fatal error I/O error has encounter 0 otherwise badbit 1 when a fatal i/o error has occured, 0 otherwise goodbit 0 value. There are several error handling functions supported by the class ios that help you read and process the status recorded in a file stream. Some error handling function are as follows

59 Error Handling Functions int bad() int eof() int fail() int good() clear() Returns non-zero value, if an invalid operation is attempted or any unrecoverable error has occured. if it is zero then possible to recover the error Returns non-zero (true value) if end of file is encountered while reading otherwise return zero (false) Returns non-zero (true) when input and output operation is failed. Returns non-zero (true) if no error has occured, This means all the above function are false Reset the error state so that further operation can be attempted.