Input Failure Loops:

A very common usage of loops is the repetitive reading of data .  This reading may be from the keyboard or from an existing data file which contains a long list of numbers.   In the case of reading from a data file, an  approach is to let the program read to the end of the file or until an improper data field is encountered.


There are some functions, which help us decide if the end of the file has been reached or if an improper data has been encountered.  Say that the input stream we are using is mydata,  the functions are :

 
mydata.eof( ) will evaluate to TRUE when we reach the end of file
                                                   ( immediately after mydata >> ..)

mydata.fail( ) 		will evaluate to TRUE when improper data is encountered
                                                    ( immediately after mydata >> ..)

 

The following is an annotated example for several numbers, which are read from a file and outputted to the screen, while their sum is calculated.

 

Initial comments 
	
/*   input file failure example         file: 3ex3.cpp   
    
 
FALL 2001
___________________________________
Jacob Y. Kazakia   jyk0
May 2, 2001
Programming example  3 of unit #3
Recitation Instructor: J.Y.Kazakia
Recitation Section  01
___________________________________
 
Purpose: This program reads a sequence of  numbers,
         which are written in a  file called 3ex3data.txt  
         The file looks like this:
 
   1.00345       4.567       1.89e-03
   2.345         1.78
   2.897e-6    123.678      34
 
        We assume that the user does NOT know, ahead of time,
        how many numbers are listed.  The task of the program 
        is to output the numbers to the screen and calculate
        the sum.
 
Algorithm: A stream named ex3input is established. A while loop
           provides repeated reading of numbers.  The condition
           of the loop is set to:
                                      
                    while ( ! ex3input.eof( )  )
            
           When the stream 3ex3input reaches the END OF FILE
           the ex3input.eof( ) evaluates to TRUE. As long as
           we are not at the end it evaluates to FALSE and
           consequently the prefixed  !  is needed.              
                                  */
The fstream.h header can support both file and default I/O
 
The stream report is created		 
#include          // header needed for file input output
 
void main()
 {
 
ifstream ex3input ( "3ex3data.txt", ios :: in);
Variables are declared and initialized
 
The first number is read
 
The heading of the list is created		 
float number ;     
float sum = 0;
 
ex3input >> number; 
 
cout << "\n\n This is the list of numbers read \n\n";
 
The while loop. It will run as long as there are numbers to be read
 
Note that the reading of the next number is done as the last statement		 
while ( ! ex3input.eof( ) )  // read the numbers and add them up
  {
    
                cout << number << endl;
 
                sum = sum + number;
 
                ex3input >> number;
 
   }
When the end of file is reached, the while loop terminates and the statements beyond the bracket are executed 
cout << "\n\n  End of file reached. The sum is : " ;
cout << sum;
 
 
cout << " \n\n  Enter e(exit) to exit    ...";
char hold;
cin >> hold;
 
}
Note that the numbers are read as the file is scanned from left to right in the first line and then the second line and the third and so on until the end of file is reached.
 
We will discuss selective reading later in the course.		 
/*     THE OUTPUT :
 
 
  
 
 This is the list of numbers read
 
1.00345
4.567
0.00189
2.345
1.78
2.897e-006
123.678
34
 
 
End of file reached. The sum is : 167.375
 
Enter e(exit) to exit    ...
 
 
*/