|
So, just how much data is in that file? The
exact contents of a file may not be precisely known.
Usually the general format style of the file and the type of data
contained within the file are known. |
| The amount of data stored in the
file, however, is often unknown. So, do we spend our time counting data
in a text file by hand, or do we let the computer deal with the amount of
data? Of course, we let the computer do the counting. |
 |
C++ provides a special function, eof(
), that returns nonzero (meaning TRUE) when there are
no more data to be read from an input file stream, and zero (meaning FALSE)
otherwise.
Rules for using
end-of-file (eof( )):
1. Always test for the
end-of-file condition before processing
data read from an input file stream.
a. use a priming input statement before
starting the loop
b. repeat the input statement at the bottom
of the loop body
2. Use a while loop
for getting data from an input file stream. A for
loop is desirable only when you
know the exact number of data items in the file, which we do not know. |
#include <iostream.h>
#include <fstream.h>
#include <assert.h>
int main(void)
{
int data;
//
file contains an undermined number of integer values
ifstream fin;
// declare stream variable name
fin.open("myfile",ios::in); //
open file
assert (!fin.fail( ));
fin >>
data; //
get first number from the file (priming the input statement)
// You must attempt to read info prior to an eof( ) test.
while (!fin.eof( ))
//if
not at end of file, continue reading numbers
{
cout<<data<<endl;
//print numbers to screen
fin >>
data;
//get next number from file
}
fin.close( );
//close
file
assert(!fin.fail( ));
return 0;
}
The eof( ) function has been
known to be persnickety under certain conditions. If you experience
problems with the function, you may want to consider this alternate approach to check for
end of file:
//This example creates a file of apstrings
//then opens the new file and prints the info to the screen
#include<iostream.h>
#include<fstream.h>
#include<stdlib.h>
//for exit()
#include "apstring.cpp"
int main(void)
{
ofstream fout;
ifstream fin;
apstring sentences, sent;
fout.open("sentences.dat");
//creating
the file
if (!fout)
{
cerr<<"Unable to open file"<<endl;
exit(1);
}
for(int i = 0; i < 5; i++)
//file will contain 5 apstring variables
fout<<"This is sentence #"<<i+1<<endl;
fout.close( );
//open file and read from file
fin.open("sentences.dat");
//open file to access information
while (getline(fin,sent))
//The test condition is TRUE
// only while there is
something to read.
{
//Works nicely as an end of file check.
cout<<sent<<endl;
}
fin.close( );
return 0;
}
|
