FILE
INPUT/OUTPUT
Let’s say we wish to find the average of 100
temperatures. We design a C++ program
containing a while loop whose execution is regulated with a sentinel. What would happen if we typed in the 75th
temperature incorrectly?
A file is a collection of related data items stored
on an external storage device such as a hard disk.
A text file is a sequence of ASCII coded
characters.
Input and output using text files in a C++ program
is done using the C++ stream class. The
data type of the stream object for input is ifstream, and the data type of the stream object for output is
ofstream. These stream variables are in the fstream header file.
To use a file for input we need to have the
operating system open the file. When we
open a file, the operating system designates whether the file is to be for
input (is read from) or output (written to) and then sets a file pointer to the
beginning of the file. The file pointer
keeps track of what portion of the file has been processed and which portion
hasn’t.
The open member function of a stream variable opens
the file as either input or output depending on whether the stream variable is
an input or output stream variable. The
file pointer is positioned at the beginning of the file. If the file didn’t previously exist then the
operating system creates one. If it did
exist, then its previous contents are erased and overwritten,
The extraction operator >> is used in the same manner with the file input
as cin is used for standard input. The data is extracted from the file and
placed into the variable’s memory location.
At the end of a program we must close the file. The close member function is used. The closed member function sends any data in
the stream to the external file, saves the external file, and detaches the file
from ofstream so that no more
output is allowed.
Assume we create a data file called temps.txt that
contains the following data:
10
34
67
95
32
212
40
50
60
70
88
#include
<fstream>
#include
<iomanip>
#include
<iostream>
using
namespace std;
int
main()
{
float temp, totaltemp = 0, averagetemp;
int
num_temps, count = 0;
ifstream tempin; //declare an
input stream variable
tempin.open("temps.txt"); //open the file temps.txt for input
tempin
>> num_temps;
//read from temps.txt
cout
<< "The number of records is: " << num_temps
<< endl;
cout
<< fixed << showpoint << setprecision(2);
while(count < num_temps)
{
tempin >> temp;
cout << temp << " in Fahrenheit is
equivalent to " << 5.0/9 * (temp - 32)
<< "
Celsius" << endl;
totaltemp += temp;
count++;
}
cout
<< "The average temperature is: " << totaltemp/count
<< " Fahrenheit." << endl;
tempin.close(); //close the input
file
return 0;
}
The
number of records is: 10
34.00
in Fahrenheit is equivalent to 1.11 Celsius
67.00
in Fahrenheit is equivalent to 19.44 Celsius
95.00
in Fahrenheit is equivalent to 35.00 Celsius
32.00
in Fahrenheit is equivalent to 0.00 Celsius
212.00
in Fahrenheit is equivalent to 100.00 Celsius
40.00
in Fahrenheit is equivalent to 4.44 Celsius
50.00
in Fahrenheit is equivalent to 10.00 Celsius
60.00
in Fahrenheit is equivalent to 15.56 Celsius
70.00
in Fahrenheit is equivalent to 21.11 Celsius
88.00
in Fahrenheit is equivalent to 31.11 Celsius
The
average temperature is: 74.80 Fahrenhet.
Press
any key to continue
#include
<fstream>
#include
<iomanip>
#include
<iostream>
using
namespace std;
int
main()
{
float temp, totaltemp = 0;
int
count = 0;
ifstream tempin;
tempin.open("temps.txt");
cout
<< fixed << showpoint << setprecision(2);
while(!tempin.eof())
{
tempin >> temp;
cout << temp << " in Fahrenheit is
equivalent to " << 5.0/9 * (temp - 32)
<< "
Celsius" << endl;
totaltemp += temp;
count++;
}
cout
<< "The average temperature is: " << totaltemp/count
<< " Fahrenheit." << endl;
tempin.close();
return 0;
}
10.00
in Fahrenheit is equivalent to -12.22 Celsius
34.00
in Fahrenheit is equivalent to 1.11 Celsius
67.00
in Fahrenheit is equivalent to 19.44 Celsius
95.00
in Fahrenheit is equivalent to 35.00 Celsius
32.00
in Fahrenheit is equivalent to 0.00 Celsius
212.00
in Fahrenheit is equivalent to 100.00 Celsius
40.00
in Fahrenheit is equivalent to 4.44 Celsius
50.00
in Fahrenheit is equivalent to 10.00 Celsius
60.00
in Fahrenheit is equivalent to 15.56 Celsius
70.00
in Fahrenheit is equivalent to 21.11 Celsius
88.00
in Fahrenheit is equivalent to 31.11 Celsius
The
average temperature is: 68.91 Fahrenheit.
Press
any key to continue
What do you do when you don’t know the
number of data items in your file? The eof member function allows us to test for the end of the file. The function returns a true value when the
next character of the output stream is the end of file character and a false
value otherwise.
#include
<fstream>
#include
<iomanip>
#include
<cstdlib>
#include
<iostream>
using
namespace std;
int
main()
{
float temp, totaltemp = 0;
int
count = 0;
ifstream tempin;
tempin.open("temps.txt");
cout
<< fixed << showpoint << setprecision(2);
while(!tempin.eof())
{
tempin >> temp;
cout << temp << " in Fahrenheit is
equivalent to " << 5.0/9 * (temp - 32)
<< "
Celsius" << endl;
totaltemp += temp;
count++;
}
cout
<< "The average temperature is: " << totaltemp/count
<< " Fahrenheit." << endl;
tempin.close();
return 0;
}
What happens when we try to open a file
that doesn’t exist? Use the fail member function to test for an open failure.
#include <fstream>
#include
<iomanip>
#include
<cstdlib>
#include
<iostream>
using
namespace std;
int
main()
{
float temp, totaltemp = 0;
int
count = 0;
ifstream tempin;
tempin.open("temps.txt");
if (tempin.fail())
//using fail member function to test for faliure
{
cerr << "File failed to open";
abort(); //program is terminated
}
cout
<< fixed << showpoint << setprecision(2);
while(!tempin.eof())
{
tempin >> temp;
cout << temp << " in Fahrenheit is
equivalent to " << 5.0/9 * (temp - 32)
<< "
Celsius" << endl;
totaltemp += temp;
count++;
}
cout
<< "The average temperature is: " << totaltemp/count
<< " Fahrenheit." << endl;
tempin.close();
return 0;
}
10.00
in Fahrenheit is equivalent to -12.22 Celsius
34.00
in Fahrenheit is equivalent to 1.11 Celsius
67.00
in Fahrenheit is equivalent to 19.44 Celsius
95.00
in Fahrenheit is equivalent to 35.00 Celsius
32.00
in Fahrenheit is equivalent to 0.00 Celsius
212.00
in Fahrenheit is equivalent to 100.00 Celsius
40.00
in Fahrenheit is equivalent to 4.44 Celsius
50.00
in Fahrenheit is equivalent to 10.00 Celsius
60.00
in Fahrenheit is equivalent to 15.56 Celsius
70.00
in Fahrenheit is equivalent to 21.11 Celsius
88.00
in Fahrenheit is equivalent to 31.11 Celsius
The
average temperature is: 68.91 Fahrenheit.
Press
any key to continue
We can write to an output file instead of
the standard output (monitor) using an ofstream variable and the member
functions just discussed.
#include
<fstream>
#include
<cstdlib>
#include
<iomanip>
#include
<iostream>
using
namespace std;
void
main()
{
float temp;
ifstream tempin;
ofstream tempout;
tempout.open("output.txt");
if (tempout.fail())
{
cerr << "File failed to
open:output.txt";
abort();
}
tempin.open("temps.txt");
if (tempin.fail())
{
cerr << "File failed to open:
temps.txt";
abort();
}
tempout
<< fixed << showpoint << setprecision(2);
while(!tempin.eof())
{
tempin >> temp;
tempout << temp << " in Fahrenheit is
equivalent to " << 5.0/9 * (temp - 32)
<< "
Celsius" << endl;
}
tempin.close();
tempout.close();
}
