Data File - File Handling in C++

File Handling in C++

• get ( ) function:

Prototypes are :

istream & get (char * buf, int num, char delim = '\n') ;

The above first form reads characters into a character array pointed to by buf until either num characters have been read, or the character specified by delim has been encountered. For instance,

char line ; cin.get (line, 40, '$') ;

The above statements will read characters into line until either 40 characters are read or '$' character is encountered, whichever occurs earlier. If the input given in rtesponse to above statements is as follows :

Value is $ 177.5

Then line will be storing

Value is

And if the input given is as follows :

The amount is 17.5.

The contents of line will be

The amount is 17.5.

The array pointed to by buf will be null-terminated by get ( ). If no delim character is specified, by default a newline character acts as a delimiter. If the delimiter character is encountered in the input stream the get ( ) function does not extract it. Rather, the delimiter character remains in the stream until the next input operation.

int get ( ) ;

The above second form of get ( ) returns the next character from the stream. It returns EOF if the end-of-file is encountered. For instance, the following code fragment illustrates it :

int i ; char ch ; ch = i = fin.get ( ) ;

If the input given is A, then the value of i will be 65(ASCII value of A) and the value of ch will be A.

• getline ( ) function

Prototype is :

istream & getline (char * buf, int num, char delim = '\n') ;

This function is virtually identical to get(buf, num, delim) version of get ( ). The difference between get(buf, num, delim) and getline ( ) is that getline ( ) reads and removes the delimiter newline character from the input stream if it is encountered which is not done by the get ( ) function. Following figure explains the difference between get ( ) and getline ( ) functions :

• read ( ) and write ( ) functions :

Reading and writing blocks of binary data is to use C++'s read ( ) and write ( ) functions. Their prototypes are :

istream & read ( (char *) & buf, int sizeof (buf)) ; ostream & write ( (char *) & buf, int sizeof (buf)) ;

The read ( ) function reads sizeof(buf) bytes from the associated stream and puts them in the buffer pointed to by buf. The write ( ) function writes sizeof(buf) bytes to the associated stream from the buffer pointed to by buf. The data written to a file using write ( ) can only be read accurately using read ( ). The following program writes a structure to the disk and then reads it back using write ( ) and read ( ) functions.

#include #include #include // for clrscr ( ) struct customer { char name ; float balance ; }; int main ( ) { clrscr ( ) ; customer savac; strcpy(savac.name, "Tina Marshall") ; // copy value to structure savac.balance = 21310.75 ; // variable savac ofstream fout ; fout.open("Saving", ios :: out | ios :: binary) ; // open output file if(!fout) { cout << "File can't be opened \n" ; return 1; } fout.write((char *) & savac, sizeof(customer)) ; // write to file fout.close ; // close connection // read it back now ifstream fin ; fin.open("Saving", ios :: out | ios :: binary) ; // open input file fin.read((char *) & savac, sizeof(customer)) ; // read structure cout << savac.name ; // display structure now cout << "has the balance amount of Rs." << savac.balance << "\n" ; fin.close( ); }

As you can see, only a single call to read ( ) or write ( ) is necessary to read or write the entire structure. Each individual field need not be read or written separately. If the end-of-file is reached before the specified number of characters have been read, the read ( ) simply stops, and the buffer contains as many characters as were available.