Program Input and
the Software
Design Process
ROBERT REAVES
File Input and Output

File is a named area in a secondary storage that holds a collection
of information.

Why would we want to read from a file instead of a keyboard?


Little mistakes.

Easier to enter in large amounts of data multiple times.
Why would we want to write to a file?

Allows us to look at the output over and over without a rerun.

Output of one program can be used as input to another.
Using Files

If we want file I/O in program we have to do:

Request the preprocessor to include the header file fstream.

#include<fstream>

Use declaration statements to declare the file streams we will use.

Prepare each file for reaching and writing by using a function named
open.

Specify the name of the file stream in each input or output statement.
Using Files

Through the header file fstream, the C++ standard library defines
two data types, ifstream and ostream.

ifstream represents a stream of characters coming from an input file.

ostream represents a stream of characters going to an output file.

ALL the operators we have learned about cout and cin are valid
with these data types.

ifstream uses (>>) operator, get function, ignore function.

ostream uses (<<) operator, endl, setw, setprecision, etc..
Declaring File Streams


Just like we declared int, char, float, etc.. We declare file streams:

int x;

float y;

ifstream myIn;

ostream myOut;
NOTE: Cannot read and write from the same file using the ifstream
and the ostream.
Opening Files

Opening a file causes the computer’s operating system to perform
certain actions that allow us to proceed with file I/O.


Example of opening a file for reading and writing.

myIn.open(“input.txt”); //Name of the file is irrelevant.

myOut.open(“output.txt”); //Name of the file is irrelevant.
Each of these are a function call to the open function. Both are
different open functions, one is associated with the ifstream while
the other the ostream.
Open function

Associates a stream variable used in your program with a physical
file on disk.

Open, with an input file, sets the file’s reading marker to the first
piece of data in the file.

Open, with an output file, checks whether the file already exists.

If it does:


If it doesn’t:


Erases all the old contents of the file. Sets the writing marker to the beginning.
Creates a new, empty file for you. Sets the writing marker to the beginning.
NOTE: Want to open files before any kind of I/O attempts are made.
Close Function

Closing a file causes the operating system to perform certain wrapup activities on it and to break the connection between the stream
variable and the file.

Close function associated with both the ifstream and ostream.

Do we always been to close files?
Run-Time Input of File Names

Open function associated with the ifstream data type requires an
argument that specifies the name of the actual data file.


Program will only work with this particular file, we don’t we want! We
can to be able to run with any file. =)
Command technique is to prompt the user for the name of the file
to read.

ifstream myIn;

string fileName;

cout << “Enter the input file name: “;

cin >> fileName;

myIn.open(fileName); // Compile-time error
Run-Time of File Names
Why do we get a compile-time error?
 Open function does not expect an argument of type string, but instead
it expects a C string.
 We will talk more about C strings later, just KNOW that open doesn’t
take string types. =)
 Lucky for us our string type does provide a function for us!



c_str, value returning function.
c_str function returns a C string that is equivalent to the one contained
in the variable used to call it.

fileName.c_str();

This function is to allow programmers to call library functions that expect C
strings and not just string strings. =)
Input Failure

Things can go wrong whenever you input something to a program.\



Prompt user for int value, but we give it some char.
cin will enter a fail state.

Any further I/O operations using that stream are considered null
operations.

Computer doesn’t halt program execution or display an error.
Invalid data is the most common reason for input failure.